diff --git a/expr_parser.js b/expr_parser.js
new file mode 100644
index 0000000..8304af7
--- /dev/null
+++ b/expr_parser.js
@@ -0,0 +1,818 @@
+/**
+ * See `expr_parser.md` for notes on the implementation of this parser!
+ */
+const ExprParser = (function() {
+  // buffer containing a list of tokens and a movable pointer into the list
+  class TokenBuf {
+    constructor(tokens) {
+      this.tokens = tokens;
+      this.index = 0;
+    }
+
+    get peek() {
+      return this.tokens[this.index];
+    }
+
+    consume(termName) {
+      if (this.peek.type !== termName) {
+        throw new Error(`Expected a ${termName}, but got a ${this.peek.type}!`)
+      }
+      const node = { type: 'term', token: this.peek };
+      ++this.index;
+      return node;
+    }
+  }
+
+  // tokenize an expression string
+  function tokenize(exprStr) {
+    exprStr = exprStr.trim();
+    const tokens = [];
+    let col = 0;
+
+    function pushSymbol(sym) {
+      tokens.push({ type: sym });
+      col += sym.length;
+    }
+
+    while (col < exprStr.length) {
+      // parse fixed symbols, like operators and stuff
+      switch (exprStr[col]) {
+        case '(':
+        case ')':
+        case ',':
+        case '&':
+        case '|':
+        case '+':
+        case '-':
+        case '*':
+        case '/':
+        case '^':
+        case '=':
+          pushSymbol(exprStr[col]);
+          continue;
+        case '>':
+          pushSymbol(exprStr[col + 1] === '=' ? '>=' : '>');
+          continue;
+        case '<':
+          pushSymbol(exprStr[col + 1] === '=' ? '<=' : '<');
+          continue;
+        case '!':
+          pushSymbol(exprStr[col + 1] === '=' ? '!=' : '!');
+          continue;
+        case ' ': // ignore extra whitespace
+          ++col;
+          continue;
+      }
+      if (exprStr.slice(col, col + 2) === "?=") {
+        pushSymbol("?=");
+        continue;
+      }
+      // parse a numeric literal
+      let m;
+      if ((m = /^\d+(?:\.\d*)?/.exec(exprStr.substring(col))) !== null) {
+        tokens.push({ type: 'nLit', value: parseFloat(m[0]) });
+        col += m[0].length;
+        continue;
+      }
+      // parse a string literal
+      if ((m = /^"([^"]+)"/.exec(exprStr.substring(col))) !== null) { // with double-quotes
+        tokens.push({ type: 'sLit', value: m[1] });
+        col += m[0].length;
+        continue;
+      }
+      if ((m = /^'([^']+)'/.exec(exprStr.substring(col))) !== null) { // with single-quotes
+        tokens.push({ type: 'sLit', value: m[1] });
+        col += m[0].length;
+        continue;
+      }
+      // parse an identifier or boolean literal
+      if ((m = /^\w[\w\d%]*/.exec(exprStr.substring(col))) !== null) {
+        switch (m[0]) {
+          case 'true':
+            tokens.push({ type: 'bLit', value: true });
+            col += 4;
+            continue;
+          case 'false':
+            tokens.push({ type: 'bLit', value: false });
+            col += 5;
+            continue;
+        }
+        tokens.push({ type: 'ident', id: m[0] });
+        col += m[0].length;
+        continue;
+      }
+      // if we reach here without successfully parsing a token, it's an error
+      throw new Error(`Could not parse character "${exprStr[col]}" at position ${col}`);
+    }
+    tokens.push({ type: 'eof' });
+    return new TokenBuf(tokens);
+  }
+
+  // parse an AST from a sequence of tokens
+  function takeExpr(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case '!':
+      case '(':
+      case '-':
+      case 'bLit':
+      case 'ident':
+      case 'nLit':
+      case 'sLit':
+        children.push(takeConj(tokens));
+        return { type: 'nonterm', name: 'expr', prod: 0, children };
+      default:
+        throw new Error('Could not parse expr!');
+    }
+  }
+
+  function takeExprList(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case '!':
+      case '(':
+      case '-':
+      case 'bLit':
+      case 'ident':
+      case 'nLit':
+      case 'sLit':
+        children.push(takeExpr(tokens));
+        children.push(takeExprList0(tokens));
+        return { type: 'nonterm', name: 'exprList', prod: 0, children };
+      default:
+        throw new Error('Could not parse exprList!');
+    }
+  }
+
+  function takeExprList0(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case ')':
+        return { type: 'nonterm', name: 'exprList\'', prod: 1, children };
+      case ',':
+        children.push(tokens.consume(','));
+        children.push(takeExpr(tokens));
+        children.push(takeExprList0(tokens));
+        return { type: 'nonterm', name: 'exprList\'', prod: 0, children };
+      case 'eof':
+        return { type: 'nonterm', name: 'exprList\'', prod: 1, children };
+      default:
+        throw new Error('Could not parse exprList\'!');
+    }
+  }
+
+  function takeConj(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case '!':
+      case '(':
+      case '-':
+      case 'bLit':
+      case 'ident':
+      case 'nLit':
+      case 'sLit':
+        children.push(takeDisj(tokens));
+        children.push(takeConj0(tokens));
+        return { type: 'nonterm', name: 'conj', prod: 0, children };
+      default:
+        throw new Error('Could not parse conj!');
+    }
+  }
+
+  function takeConj0(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case '&':
+        children.push(tokens.consume('&'));
+        children.push(takeDisj(tokens));
+        children.push(takeConj0(tokens));
+        return { type: 'nonterm', name: 'conj\'', prod: 0, children };
+      case ')':
+      case ',':
+      case 'eof':
+        return { type: 'nonterm', name: 'conj\'', prod: 1, children };
+      default:
+        throw new Error('Could not parse conj\'!');
+    }
+  }
+
+  function takeDisj(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case '!':
+      case '(':
+      case '-':
+      case 'bLit':
+      case 'ident':
+      case 'nLit':
+      case 'sLit':
+        children.push(takeCmpEq(tokens));
+        children.push(takeDisj0(tokens));
+        return { type: 'nonterm', name: 'disj', prod: 0, children };
+      default:
+        throw new Error('Could not parse disj!');
+    }
+  }
+
+  function takeDisj0(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case '&':
+      case ')':
+      case ',':
+        return { type: 'nonterm', name: 'disj\'', prod: 1, children };
+      case '|':
+        children.push(tokens.consume('|'));
+        children.push(takeCmpEq(tokens));
+        children.push(takeDisj0(tokens));
+        return { type: 'nonterm', name: 'disj\'', prod: 0, children };
+      case 'eof':
+        return { type: 'nonterm', name: 'disj\'', prod: 1, children };
+      default:
+        throw new Error('Could not parse disj\'!');
+    }
+  }
+
+  function takeCmpEq(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case '!':
+      case '(':
+      case '-':
+      case 'bLit':
+      case 'ident':
+      case 'nLit':
+      case 'sLit':
+        children.push(takeCmpRel(tokens));
+        children.push(takeCmpEq0(tokens));
+        return { type: 'nonterm', name: 'cmpEq', prod: 0, children };
+      default:
+        throw new Error('Could not parse cmpEq!');
+    }
+  }
+
+  function takeCmpEq0(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case '!=':
+        children.push(tokens.consume('!='));
+        children.push(takeCmpRel(tokens));
+        children.push(takeCmpEq0(tokens));
+        return { type: 'nonterm', name: 'cmpEq\'', prod: 1, children };
+      case '&':
+      case ')':
+      case ',':
+        return { type: 'nonterm', name: 'cmpEq\'', prod: 3, children };
+      case '=':
+        children.push(tokens.consume('='));
+        children.push(takeCmpRel(tokens));
+        children.push(takeCmpEq0(tokens));
+        return { type: 'nonterm', name: 'cmpEq\'', prod: 0, children };
+      case '?=':
+        children.push(tokens.consume('?='));
+        children.push(takeCmpRel(tokens));
+        children.push(takeCmpEq0(tokens));
+        return { type: 'nonterm', name: 'cmpEq\'', prod: 2, children };
+      case '|':
+      case 'eof':
+        return { type: 'nonterm', name: 'cmpEq\'', prod: 3, children };
+      default:
+        throw new Error('Could not parse cmpEq\'!');
+    }
+  }
+
+  function takeCmpRel(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case '!':
+      case '(':
+      case '-':
+      case 'bLit':
+      case 'ident':
+      case 'nLit':
+      case 'sLit':
+        children.push(takeSum(tokens));
+        children.push(takeCmpRel0(tokens));
+        return { type: 'nonterm', name: 'cmpRel', prod: 0, children };
+      default:
+        throw new Error('Could not parse cmpRel!');
+    }
+  }
+
+  function takeCmpRel0(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case '!=':
+      case '&':
+      case ')':
+      case ',':
+        return { type: 'nonterm', name: 'cmpRel\'', prod: 4, children };
+      case '<':
+        children.push(tokens.consume('<'));
+        children.push(takeSum(tokens));
+        children.push(takeCmpRel0(tokens));
+        return { type: 'nonterm', name: 'cmpRel\'', prod: 1, children };
+      case '<=':
+        children.push(tokens.consume('<='));
+        children.push(takeSum(tokens));
+        children.push(takeCmpRel0(tokens));
+        return { type: 'nonterm', name: 'cmpRel\'', prod: 0, children };
+      case '=':
+        return { type: 'nonterm', name: 'cmpRel\'', prod: 4, children };
+      case '>':
+        children.push(tokens.consume('>'));
+        children.push(takeSum(tokens));
+        children.push(takeCmpRel0(tokens));
+        return { type: 'nonterm', name: 'cmpRel\'', prod: 2, children };
+      case '>=':
+        children.push(tokens.consume('>='));
+        children.push(takeSum(tokens));
+        children.push(takeCmpRel0(tokens));
+        return { type: 'nonterm', name: 'cmpRel\'', prod: 3, children };
+      case '?=':
+      case '|':
+      case 'eof':
+        return { type: 'nonterm', name: 'cmpRel\'', prod: 4, children };
+      default:
+        throw new Error('Could not parse cmpRel\'!');
+    }
+  }
+
+  function takeSum(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case '!':
+      case '(':
+      case '-':
+      case 'bLit':
+      case 'ident':
+      case 'nLit':
+      case 'sLit':
+        children.push(takeProd(tokens));
+        children.push(takeSum0(tokens));
+        return { type: 'nonterm', name: 'sum', prod: 0, children };
+      default:
+        throw new Error('Could not parse sum!');
+    }
+  }
+
+  function takeSum0(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case '!=':
+      case '&':
+      case ')':
+        return { type: 'nonterm', name: 'sum\'', prod: 2, children };
+      case '+':
+        children.push(tokens.consume('+'));
+        children.push(takeProd(tokens));
+        children.push(takeSum0(tokens));
+        return { type: 'nonterm', name: 'sum\'', prod: 0, children };
+      case ',':
+        return { type: 'nonterm', name: 'sum\'', prod: 2, children };
+      case '-':
+        children.push(tokens.consume('-'));
+        children.push(takeProd(tokens));
+        children.push(takeSum0(tokens));
+        return { type: 'nonterm', name: 'sum\'', prod: 1, children };
+      case '<':
+      case '<=':
+      case '=':
+      case '>':
+      case '>=':
+      case '?=':
+      case '|':
+      case 'eof':
+        return { type: 'nonterm', name: 'sum\'', prod: 2, children };
+      default:
+        throw new Error('Could not parse sum\'!');
+    }
+  }
+
+  function takeProd(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case '!':
+      case '(':
+      case '-':
+      case 'bLit':
+      case 'ident':
+      case 'nLit':
+      case 'sLit':
+        children.push(takeExp(tokens));
+        children.push(takeProd0(tokens));
+        return { type: 'nonterm', name: 'prod', prod: 0, children };
+      default:
+        throw new Error('Could not parse prod!');
+    }
+  }
+
+  function takeProd0(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case '!=':
+      case '&':
+      case ')':
+        return { type: 'nonterm', name: 'prod\'', prod: 2, children };
+      case '*':
+        children.push(tokens.consume('*'));
+        children.push(takeExp(tokens));
+        children.push(takeProd0(tokens));
+        return { type: 'nonterm', name: 'prod\'', prod: 0, children };
+      case '+':
+      case ',':
+      case '-':
+        return { type: 'nonterm', name: 'prod\'', prod: 2, children };
+      case '/':
+        children.push(tokens.consume('/'));
+        children.push(takeExp(tokens));
+        children.push(takeProd0(tokens));
+        return { type: 'nonterm', name: 'prod\'', prod: 1, children };
+      case '<':
+      case '<=':
+      case '=':
+      case '>':
+      case '>=':
+      case '?=':
+      case '|':
+      case 'eof':
+        return { type: 'nonterm', name: 'prod\'', prod: 2, children };
+      default:
+        throw new Error('Could not parse prod\'!');
+    }
+  }
+
+  function takeExp(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case '!':
+      case '(':
+      case '-':
+      case 'bLit':
+      case 'ident':
+      case 'nLit':
+      case 'sLit':
+        children.push(takeUnary(tokens));
+        children.push(takeExp0(tokens));
+        return { type: 'nonterm', name: 'exp', prod: 0, children };
+      default:
+        throw new Error('Could not parse exp!');
+    }
+  }
+
+  function takeExp0(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case '!=':
+      case '&':
+      case ')':
+      case '*':
+      case '+':
+      case ',':
+      case '-':
+      case '/':
+      case '<':
+      case '<=':
+      case '=':
+      case '>':
+      case '>=':
+      case '?=':
+        return { type: 'nonterm', name: 'exp\'', prod: 1, children };
+      case '^':
+        children.push(tokens.consume('^'));
+        children.push(takeUnary(tokens));
+        children.push(takeExp0(tokens));
+        return { type: 'nonterm', name: 'exp\'', prod: 0, children };
+      case '|':
+      case 'eof':
+        return { type: 'nonterm', name: 'exp\'', prod: 1, children };
+      default:
+        throw new Error('Could not parse exp\'!');
+    }
+  }
+
+  function takeUnary(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case '!':
+        children.push(tokens.consume('!'));
+        children.push(takeUnary(tokens));
+        return { type: 'nonterm', name: 'unary', prod: 1, children };
+      case '(':
+        children.push(takePrim(tokens));
+        return { type: 'nonterm', name: 'unary', prod: 2, children };
+      case '-':
+        children.push(tokens.consume('-'));
+        children.push(takeUnary(tokens));
+        return { type: 'nonterm', name: 'unary', prod: 0, children };
+      case 'bLit':
+      case 'ident':
+      case 'nLit':
+      case 'sLit':
+        children.push(takePrim(tokens));
+        return { type: 'nonterm', name: 'unary', prod: 2, children };
+      default:
+        throw new Error('Could not parse unary!');
+    }
+  }
+
+  function takePrim(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case '(':
+        children.push(tokens.consume('('));
+        children.push(takeExpr(tokens));
+        children.push(tokens.consume(')'));
+        return { type: 'nonterm', name: 'prim', prod: 4, children };
+      case 'bLit':
+        children.push(tokens.consume('bLit'));
+        return { type: 'nonterm', name: 'prim', prod: 1, children };
+      case 'ident':
+        children.push(tokens.consume('ident'));
+        children.push(takeIdentTail(tokens));
+        return { type: 'nonterm', name: 'prim', prod: 3, children };
+      case 'nLit':
+        children.push(tokens.consume('nLit'));
+        return { type: 'nonterm', name: 'prim', prod: 0, children };
+      case 'sLit':
+        children.push(tokens.consume('sLit'));
+        return { type: 'nonterm', name: 'prim', prod: 2, children };
+      default:
+        throw new Error('Could not parse prim!');
+    }
+  }
+
+  function takeIdentTail(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case '!=':
+      case '&':
+        return { type: 'nonterm', name: 'identTail', prod: 1, children };
+      case '(':
+        children.push(tokens.consume('('));
+        children.push(takeArgs(tokens));
+        children.push(tokens.consume(')'));
+        return { type: 'nonterm', name: 'identTail', prod: 0, children };
+      case ')':
+      case '*':
+      case '+':
+      case ',':
+      case '-':
+      case '/':
+      case '<':
+      case '<=':
+      case '=':
+      case '>':
+      case '>=':
+      case '?=':
+      case '^':
+      case '|':
+      case 'eof':
+        return { type: 'nonterm', name: 'identTail', prod: 1, children };
+      default:
+        throw new Error('Could not parse identTail!');
+    }
+  }
+
+  function takeArgs(tokens) {
+    const children = [];
+    switch (tokens.peek.type) {
+      case '!':
+      case '(':
+        children.push(takeExprList(tokens));
+        return { type: 'nonterm', name: 'args', prod: 0, children };
+      case ')':
+        return { type: 'nonterm', name: 'args', prod: 1, children };
+      case '-':
+      case 'bLit':
+      case 'ident':
+      case 'nLit':
+      case 'sLit':
+        children.push(takeExprList(tokens));
+        return { type: 'nonterm', name: 'args', prod: 0, children };
+      case 'eof':
+        return { type: 'nonterm', name: 'args', prod: 1, children };
+      default:
+        throw new Error('Could not parse args!');
+    }
+  }
+
+  // apply tree transformations to recover the nominal-grammar AST
+  function fixUp(ast) {
+    if (ast.type === 'term') {
+      return ast;
+    }
+    switch (ast.name) {
+      case 'exprList': // recover left-recursive structures
+      case 'conj':
+      case 'disj':
+      case 'exp':
+        return fixUpRightRecurse(ast, 1);
+      case 'sum':
+      case 'prod':
+        return fixUpRightRecurse(ast, 2);
+      case 'cmpEq':
+        return fixUpRightRecurse(ast, 3);
+      case 'cmpRel':
+        return fixUpRightRecurse(ast, 4);
+      case 'prim': // recover left-factored identifier things
+        switch (ast.prod) {
+          case 3:
+            switch (ast.children[1].prod) {
+              case 0: // function call
+                return {
+                  type: 'nonterm', name: 'prim', prod: 3,
+                  children: [fixUp(ast.children[0]), ...mapFixUp(ast.children[1].children)]
+                };
+              case 1: // just an identifier
+                return { type: 'nonterm', name: 'prim', prod: 4, children: [fixUp(ast.children[0])] };
+            }
+            break;
+          case 4:
+            return { ...ast, prod: 5, children: mapFixUp(ast.children) };
+        }
+        break;
+    }
+    return { ...ast, children: mapFixUp(ast.children) };
+  }
+  function mapFixUp(nodes) {
+    return nodes.map(chAst => fixUp(chAst));
+  }
+  function fixUpRightRecurse(ast, maxProd) {
+    let nomAst = { type: 'nonterm', name: ast.name, prod: maxProd, children: [fixUp(ast.children[0])] };
+    let tree = ast.children[1];
+    while (tree.prod < maxProd) {
+      nomAst = {
+        type: 'nonterm', name: ast.name, prod: tree.prod,
+        children: [nomAst, ...mapFixUp(tree.children.slice(0, tree.children.length - 1))]
+      };
+      tree = tree.children[tree.children.length - 1];
+    }
+    return nomAst;
+  }
+
+  // compile nominal AST into an item builder expression function
+  function translate(ast, builtInProps, builtInFuncs) {
+    function trans(ast) {
+      return translate(ast, builtInProps, builtInFuncs);
+    }
+    if (ast.type === 'term') {
+      switch (ast.token.type) {
+        case 'nLit':
+          return ast.token.value;
+        case 'bLit':
+          return ast.token.value;
+        case 'sLit':
+          return ast.token.value;
+        case 'ident':
+          return ast.token.value;
+      }
+    } else if (ast.type === 'nonterm') {
+      switch (ast.name) {
+        case 'expr':
+          return trans(ast.children[0]);
+        case 'exprList':
+          switch (ast.prod) {
+            case 0:
+              return [...ast.children[0], trans(ast.children[2])];
+            case 1:
+              return [trans(ast.children[0])];
+          }
+          break;
+        case 'conj':
+          switch (ast.prod) {
+            case 0:
+              return new ConjTerm(trans(ast.children[0]), trans(ast.children[2]));
+            case 1:
+              return trans(ast.children[0]);
+          }
+          break;
+        case 'disj':
+          switch (ast.prod) {
+            case 0:
+              return new DisjTerm(trans(ast.children[0]), trans(ast.children[2]));
+            case 1:
+              return trans(ast.children[0]);
+          }
+          break;
+        case 'cmpEq':
+          switch (ast.prod) {
+            case 0:
+              return new EqTerm(trans(ast.children[0]), trans(ast.children[2]));
+            case 1:
+              return new NeqTerm(trans(ast.children[0]), trans(ast.children[2]));
+            case 2:
+              return new ContainsTerm(trans(ast.children[0]), trans(ast.children[2]));
+            case 3:
+              return trans(ast.children[0]);
+          }
+          break;
+        case 'cmpRel':
+          switch (ast.prod) {
+            case 0:
+              return new LeqTerm(trans(ast.children[0]), trans(ast.children[2]));
+            case 1:
+              return new LtTerm(trans(ast.children[0]), trans(ast.children[2]));
+            case 2:
+              return new GtTerm(trans(ast.children[0]), trans(ast.children[2]));
+            case 3:
+              return new GeqTerm(trans(ast.children[0]), trans(ast.children[2]));
+            case 4:
+              return trans(ast.children[0]);
+          }
+          break;
+        case 'sum':
+          switch (ast.prod) {
+            case 0:
+              return new AddTerm(trans(ast.children[0]), trans(ast.children[2]));
+            case 1:
+              return new SubTerm(trans(ast.children[0]), trans(ast.children[2]));
+            case 2:
+              return trans(ast.children[0]);
+          }
+          break;
+        case 'prod':
+          switch (ast.prod) {
+            case 0:
+              return new MulTerm(trans(ast.children[0]), trans(ast.children[2]));
+            case 1:
+              return new DivTerm(trans(ast.children[0]), trans(ast.children[2]));
+            case 2:
+              return trans(ast.children[0]);
+          }
+          break;
+        case 'exp':
+          switch (ast.prod) {
+            case 0:
+              return new ExpTerm(trans(ast.children[0]), trans(ast.children[2]));
+            case 1:
+              return trans(ast.children[0]);
+          }
+          break;
+        case 'unary':
+          switch (ast.prod) {
+            case 0:
+              return new NegTerm(trans(ast.children[1]));
+            case 1:
+              return new InvTerm(trans(ast.children[1]));
+            case 2:
+              return trans(ast.children[0]);
+          }
+          break;
+        case 'prim':
+          switch (ast.prod) {
+            case 0:
+              return new NumLitTerm(ast.children[0].token.value);
+            case 1:
+              return new BoolLitTerm(ast.children[0].token.value);
+            case 2:
+              return new StrLitTerm(ast.children[0].token.value);
+            case 3:
+              const fn = builtInFuncs[ast.children[0].token.id];
+              if (!fn) {
+                throw new Error(`Unknown function: ${ast.children[0].token.id}`);
+              }
+              return new FnCallTerm(fn, trans(ast.children[2]));
+            case 4: {
+              const prop = builtInProps[ast.children[0].token.id];
+              if (!prop) {
+                throw new Error(`Unknown property: ${ast.children[0].token.id}`);
+              }
+              return new PropTerm(prop);
+            }
+            case 5:
+              return trans(ast.children[1]);
+          }
+          break;
+        case 'args':
+          switch (ast.prod) {
+            case 0:
+              return trans(ast.children[0]);
+            case 1:
+              return [];
+          }
+          break;
+      }
+    }
+  }
+
+  return class ExprParser {
+    constructor(builtInProps, builtInFuncs) {
+      this.builtInProps = builtInProps;
+      this.builtInFuncs = builtInFuncs;
+    }
+
+    parse(exprStr) {
+      const tokens = tokenize(exprStr);
+      if (tokens.tokens.length <= 1) {
+        return null;
+      }
+      const transAst = takeExpr(tokens, 0);
+      if (tokens.peek.type !== 'eof') {
+        throw new Error('Could not parse entire expression!');
+      }
+      const nomAst = fixUp(transAst);
+      return translate(nomAst, this.builtInProps, this.builtInFuncs);
+    }
+  };
+})();
diff --git a/expr_parser.md b/expr_parser.md
new file mode 100644
index 0000000..f7e3ffc
--- /dev/null
+++ b/expr_parser.md
@@ -0,0 +1,197 @@
+This file contains notes on how the parser implemented in `expr_parser.js` is designed.
+The nominal grammar is as follows:
+
+```
+expr := conj
+
+exprList := exprList "," expr
+          | expr
+
+conj := conj "&" disj
+      | disj
+
+disj := disj "|" cmpEq
+      | cmpEq
+
+cmpEq := cmpEq "=" cmpRel
+       | cmpEq "!=" cmpRel
+       | cmpEq "?=" cmpRel
+       | cmpRel
+
+cmpRel := cmpRel "<=" sum
+        | cmpRel "<" sum
+        | cmpRel ">" sum
+        | cmpRel ">=" sum
+        | sum
+
+sum := sum "+" prod
+     | sum "-" prod
+     | prod
+
+prod := prod "*" exp
+      | prod "/" exp
+      | exp
+
+exp := exp "^" unary
+     | unary
+
+unary := "-" unary
+       | "!" unary
+       | prim
+
+prim := nLit
+      | bLit
+      | sLit
+      | ident "(" args ")"
+      | ident
+      | "(" expr ")"
+
+args := exprList
+      | ε
+```
+
+In order to eliminate left-recursion, the above grammar is transformed into the following:
+
+```
+expr := conj
+
+exprList := expr exprListTail
+
+exprList' := "," expr exprList'
+           | ε
+
+conj := disj conj'
+
+conj' := "&" disj conj'
+       | ε
+
+disj := cmpEq disj'
+
+disj' := "|" cmpEq disj'
+       | ε
+
+cmpEq := cmpRel cmpEq'
+
+cmpEq' := "=" cmpRel cmpEq'
+        | "!=" cmpRel cmpEq'
+        | "?=" cmpRel cmpEq'
+        | ε
+
+cmpRel := sum cmpRel'
+
+cmpRel' := "<=" sum cmpRel'
+         | "<" sum cmpRel'
+         | ">" sum cmpRel'
+         | ">=" sum cmpRel'
+         | ε
+
+sum := prod sum'
+
+sum' := "+" prod sum'
+      | "-" prod sum'
+      | ε
+
+prod := exp prod'
+
+prod' := "*" exp prod'
+       | "/" exp prod'
+       | ε
+
+exp := unary exp'
+
+exp' := "^" unary exp'
+      | ε
+
+unary := "-" unary
+       | "!" unary
+       | prim
+
+prim := nLit
+      | bLit
+      | sLit
+      | ident identTail
+      | "(" expr ")"
+
+identTail := "(" args ")"
+           | ε
+
+args := exprList
+      | ε
+```
+
+The parser itself is a recursive-descent LL(1) parser.
+To build the parsing rules, the following `FIRST` and `FOLLOW` sets are computed for the above grammar:
+
+Nonterminal | FIRST
+----------- | -----
+`expr`      | `"!"`, `"("`, `"-"`, `bLit`, `ident`, `nLit`, `sLit`
+`exprList`  | `"!"`, `"("`, `"-"`, `bLit`, `ident`, `nLit`, `sLit`
+`exprList'` | `","`, `ε`
+`conj`      | `"!"`, `"("`, `"-"`, `bLit`, `ident`, `nLit`, `sLit`
+`conj'`     | `"&"`, `ε`
+`disj`      | `"!"`, `"("`, `"-"`, `bLit`, `ident`, `nLit`, `sLit`
+`disj'`     | `"\|"`, `ε`
+`cmpEq`     | `"!"`, `"("`, `"-"`, `bLit`, `ident`, `nLit`, `sLit`
+`cmpEq'`    | `"!="`, `"="`, `"?="`, `ε`
+`cmpRel`    | `"!"`, `"("`, `"-"`, `bLit`, `ident`, `nLit`, `sLit`
+`cmpRel'`   | `"<"`, `"<="`, `">"`, `">="`, `ε`
+`sum`       | `"!"`, `"("`, `"-"`, `bLit`, `ident`, `nLit`, `sLit`
+`sum'`      | `"+"`, `"-"`, `ε`
+`prod`      | `"!"`, `"("`, `"-"`, `bLit`, `ident`, `nLit`, `sLit`
+`prod'`     | `"*"`, `"/"`, `ε`
+`exp`       | `"!"`, `"("`, `"-"`, `bLit`, `ident`, `nLit`, `sLit`
+`exp'`      | `"^"`, `ε`
+`unary`     | `"!"`, `"("`, `"-"`, `bLit`, `ident`, `nLit`, `sLit`
+`prim`      | `"("`, `bLit`, `ident`, `nLit`, `sLit`
+`identTail` | `"("`, `ε`
+`args`      | `"!"`, `"("`, `"-"`, `bLit`, `ident`, `nLit`, `sLit`, `ε`
+
+Nonterminal | FOLLOW
+----------- | ------
+`expr`      | `")"`, `","`, `$`
+`exprList`  | `")"`
+`exprList'` | `")"`
+`conj`      | `")"`, `","`
+`conj'`     | `")"`, `","`
+`disj`      | `"&"`, `")"`, `","`
+`disj'`     | `"&"`, `")"`, `","`
+`cmpEq`     | `"&"`, `")"`, `","`, `"\|"`
+`cmpEq'`    | `"&"`, `")"`, `","`, `"\|"`
+`cmpRel`    | `"!="`, `"&"`, `")"`, `","`, `"="`, `"?="`, `"\|"`
+`cmpRel'`   | `"!="`, `"&"`, `")"`, `","`, `"="`, `"?="`, `"\|"`
+`sum`       | `"!="`, `"&"`, `")"`, `","`, `"<"`, `"<="`, `"="`, `">"`, `">="`, `"?="`, `"\|"`
+`sum'`      | `"!="`, `"&"`, `")"`, `","`, `"<"`, `"<="`, `"="`, `">"`, `">="`, `"?="`, `"\|"`
+`prod`      | `"!="`, `"&"`, `")"`, `"+"`, `","`, `"-"`, `"<"`, `"<="`, `"="`, `">"`, `">="`, `"?="`, `"\|"`
+`prod'`     | `"!="`, `"&"`, `")"`, `"+"`, `","`, `"-"`, `"<"`, `"<="`, `"="`, `">"`, `">="`, `"?="`, `"\|"`
+`exp`       | `"!="`, `"&"`, `")"`, `"*"`, `"+"`, `","`, `"-"`, `"/"`, `"<"`, `"<="`, `"="`, `">"`, `">="`, `"?="`, `"\|"`
+`exp'`      | `"!="`, `"&"`, `")"`, `"*"`, `"+"`, `","`, `"-"`, `"/"`, `"<"`, `"<="`, `"="`, `">"`, `">="`, `"?="`, `"\|"`
+`unary`     | `"!="`, `"&"`, `")"`, `"*"`, `"+"`, `","`, `"-"`, `"/"`, `"<"`, `"<="`, `"="`, `">"`, `">="`, `"?="`, `"^"`, `"\|"`
+`prim`      | `"!="`, `"&"`, `")"`, `"*"`, `"+"`, `","`, `"-"`, `"/"`, `"<"`, `"<="`, `"="`, `">"`, `">="`, `"?="`, `"^"`, `"\|"`
+`identTail` | `"!="`, `"&"`, `")"`, `"*"`, `"+"`, `","`, `"-"`, `"/"`, `"<"`, `"<="`, `"="`, `">"`, `">="`, `"?="`, `"^"`, `"\|"`
+`args`      | `")"`
+
+Then, the parsing rule table is as follows, where each table entry refers to the production number for the given nonterminal:
+
+Nonterminal | "!" | "!=" | "&" | "(" | ")" | "*" | "+" | "," | "-" | "/" | "<" | "<=" | "=" | ">" | ">=" | "?=" | "^" | "&#124;" | bLit | ident | nLit | sLit | $
+----------- | --- | ---- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---- | --- | --- | ---- | ---- | --- | --- | ---- | ----- | ---- | ---- | ---
+expr        | 0   | -    | -   | 0   | -   | -   | -   | -   | 0   | -   | -   | -    | -   | -   | -    | -    | -   | -   | 0    | 0     | 0    | 0    | -
+exprList    | 0   | -    | -   | 0   | -   | -   | -   | -   | 0   | -   | -   | -    | -   | -   | -    | -    | -   | -   | 0    | 0     | 0    | 0    | -
+exprList'   | -   | -    | -   | -   | 1   | -   | -   | 0   | -   | -   | -   | -    | -   | -   | -    | -    | -   | -   | -    | -     | -    | -    | 1
+conj        | 0   | -    | -   | 0   | -   | -   | -   | -   | 0   | -   | -   | -    | -   | -   | -    | -    | -   | -   | 0    | 0     | 0    | 0    | -
+conj'       | -   | -    | 0   | -   | 1   | -   | -   | 1   | -   | -   | -   | -    | -   | -   | -    | -    | -   | -   | -    | -     | -    | -    | 1
+disj        | 0   | -    | -   | 0   | -   | -   | -   | -   | 0   | -   | -   | -    | -   | -   | -    | -    | -   | -   | 0    | 0     | 0    | 0    | -
+disj'       | -   | -    | 1   | -   | 1   | -   | -   | 1   | -   | -   | -   | -    | -   | -   | -    | -    | -   | 0   | -    | -     | -    | -    | 1
+cmpEq       | 0   | -    | -   | 0   | -   | -   | -   | -   | 0   | -   | -   | -    | -   | -   | -    | -    | -   | -   | 0    | 0     | 0    | 0    | -
+cmpEq'      | -   | 1    | 3   | -   | 3   | -   | -   | 3   | -   | -   | -   | -    | 0   | -   | -    | 2    | -   | 3   | -    | -     | -    | -    | 3
+cmpRel      | 0   | -    | -   | 0   | -   | -   | -   | -   | 0   | -   | -   | -    | -   | -   | -    | -    | -   | -   | 0    | 0     | 0    | 0    | -
+cmpRel'     | -   | 4    | 4   | -   | 4   | -   | -   | 4   | -   | -   | 1   | 0    | 4   | 2   | 3    | 4    | -   | 4   | -    | -     | -    | -    | 4
+sum         | 0   | -    | -   | 0   | -   | -   | -   | -   | 0   | -   | -   | -    | -   | -   | -    | -    | -   | -   | 0    | 0     | 0    | 0    | -
+sum'        | -   | 2    | 2   | -   | 2   | -   | 0   | 2   | 1   | -   | 2   | 2    | 2   | 2   | 2    | 2    | -   | 2   | -    | -     | -    | -    | 2
+prod        | 0   | -    | -   | 0   | -   | -   | -   | -   | 0   | -   | -   | -    | -   | -   | -    | -    | -   | -   | 0    | 0     | 0    | 0    | -
+prod'       | -   | 2    | 2   | -   | 2   | 0   | 2   | 2   | 2   | 1   | 2   | 2    | 2   | 2   | 2    | 2    | -   | 2   | -    | -     | -    | -    | 2
+exp         | 0   | -    | -   | 0   | -   | -   | -   | -   | 0   | -   | -   | -    | -   | -   | -    | -    | -   | -   | 0    | 0     | 0    | 0    | -
+exp'        | -   | 1    | 1   | -   | 1   | 1   | 1   | 1   | 1   | 1   | 1   | 1    | 1   | 1   | 1    | 1    | 0   | 1   | -    | -     | -    | -    | 1
+unary       | 1   | -    | -   | 2   | -   | -   | -   | -   | 0   | -   | -   | -    | -   | -   | -    | -    | -   | -   | 2    | 2     | 2    | 2    | -
+prim        | -   | -    | -   | 4   | -   | -   | -   | -   | -   | -   | -   | -    | -   | -   | -    | -    | -   | -   | 1    | 3     | 0    | 2    | -
+identTail   | -   | 1    | 1   | 0   | 1   | 1   | 1   | 1   | 1   | 1   | 1   | 1    | 1   | 1   | 1    | 1    | 1   | 1   | -    | -     | -    | -    | 1
+args        | 0   | -    | -   | 0   | 1   | -   | -   | -   | 0   | -   | -   | -    | -   | -   | -    | -    | -   | -   | 0    | 0     | 0    | 0    | 1
diff --git a/items_2.html b/items_2.html
index 1bb3ff0..256f3bf 100644
--- a/items_2.html
+++ b/items_2.html
@@ -78,6 +78,7 @@
         <script type="text/javascript" src="damage_calc.js"></script>
         <script type="text/javascript" src="display.js"></script>
         <script type="text/javascript" src="query_2.js"></script>
+        <script type="text/javascript" src="expr_parser.js"></script>
         <script type="text/javascript" src="load.js"></script>
         <script type="text/javascript" src="items_2.js"></script>
     </body>
diff --git a/items_2.js b/items_2.js
index bbe96f2..d724cbd 100644
--- a/items_2.js
+++ b/items_2.js
@@ -21,6 +21,7 @@ class ExprField {
         } catch (e) {
             this.errorText.innerText = e.message;
             this.output = null;
+            console.error(e);
         }
         return true;
     }
@@ -38,6 +39,8 @@ function compareLexico(ia, keysA, ib, keysB) {
                 if (aKey > bKey) return 1;
                 break;
             case 'number': // sort numeric stuff in reverse order
+                aKey = isNaN(aKey) ? 0 : aKey;
+                bKey = isNaN(bKey) ? 0 : bKey;
                 if (aKey < bKey) return 1;
                 if (aKey > bKey) return -1;
                 break;
@@ -74,17 +77,23 @@ function init() {
         itemEntries.push(itemElem);
     }
 
+    // create the expression parser
+    const exprParser = new ExprParser(itemQueryProps, itemQueryFuncs);
+
     // the two search query input boxes
     const searchFilterField = new ExprField('search-filter-field', 'search-filter-error', function(exprStr) {
-        const expr = compileQueryExpr(exprStr);
-        return expr !== null ? expr : (i, ie) => true;
+        const expr = exprParser.parse(exprStr);
+        return expr !== null ? expr : new BoolLitTerm(true);
     });
     const searchSortField = new ExprField('search-sort-field', 'search-sort-error', function(exprStr) {
-        const subExprs = exprStr.split(';').map(compileQueryExpr).filter(f => f != null);
-        return function(i, ie) {
-            const sortKeys = [];
-            for (let k = 0; k < subExprs.length; k++) sortKeys.push(subExprs[k](i, ie));
-            return sortKeys;
+        const subExprs = exprStr.split(';').map(e => exprParser.parse(e)).filter(f => f != null);
+        return {
+            type: 'array',
+            resolve(i, ie) {
+                const sortKeys = [];
+                for (let k = 0; k < subExprs.length; k++) sortKeys.push(subExprs[k].resolve(i, ie));
+                return sortKeys;
+            }
         };
     });
 
@@ -108,8 +117,8 @@ function init() {
         try {
             for (let i = 0; i < searchDb.length; i++) {
                 const item = searchDb[i][0], itemExp = searchDb[i][1];
-                if (checkBool(searchFilterField.output(item, itemExp))) {
-                    searchResults.push({ item, itemExp, sortKeys: searchSortField.output(item, itemExp) });
+                if (checkBool(searchFilterField.output.resolve(item, itemExp))) {
+                    searchResults.push({ item, itemExp, sortKeys: searchSortField.output.resolve(item, itemExp) });
                 }
             }
         } catch (e) {
diff --git a/query_2.js b/query_2.js
index 6f9f149..d0e935f 100644
--- a/query_2.js
+++ b/query_2.js
@@ -1,61 +1,4 @@
-/*
- * disj := conj "|" disj
- *       | conj
- *
- * conj := cmp "&" conj
- *       | cmpEq
- *
- * cmpEq := cmpRel "=" cmpEq
- *        | cmpRel "?=" prim
- *        | cmpRel "!=" cmpEq
- *
- * cmpRel := sum "<=" cmpRel
- *         | sum "<" cmpRel
- *         | sum ">" cmpRel
- *         | sum ">=" cmpRel
- *         | sum
- *
- * sum := prod "+" sum
- *      | prod "-" sum
- *      | prod
- *
- * prod := exp "*" prod
- *       | exp "/" prod
- *       | exp
- *
- * exp := unary "^" exp
- *      | unary
- *
- * unary := "-" unary
- *        | "!" unary
- *        | prim
- *
- * prim := nLit
- *       | bLit
- *       | sLit
- *       | ident "(" [disj ["," disj...]] ")"
- *       | ident
- *       | "(" disj ")"
- */
-
-// a list of tokens indexed by a single pointer
-class TokenList {
-  constructor(tokens) {
-    this.tokens = tokens;
-    this.ptr = 0;
-  }
-
-  get here() {
-    if (this.ptr >= this.tokens.length) throw new Error('Reached end of expression');
-    return this.tokens[this.ptr];
-  }
-
-  advance(steps = 1) {
-    this.ptr = Math.min(this.ptr + steps, this.tokens.length);
-  }
-}
-
-// type casts
+// dynamic type casts
 function checkBool(v) {
   if (typeof v !== 'boolean') throw new Error(`Expected boolean, but got ${typeof v}`);
   return v;
@@ -71,67 +14,79 @@ function checkStr(v) {
   return v;
 }
 
+function checkComparable(v) {
+  if (typeof v === 'boolean') throw new Error('Boolean is not comparable');
+  return v;
+}
+
 // properties of items that can be looked up
+// each entry is a function `(item, extended item) -> value`
 const itemQueryProps = (function() {
   const props = {};
-  function prop(names, getProp) {
+
+  function prop(names, type, resolve) {
     if (Array.isArray(names)) {
       for (name of names) {
-        props[name] = getProp;
+        props[name] = { type, resolve };
       }
     } else {
-      props[names] = getProp;
+      props[names] = { type, resolve };
     }
   }
+
   function maxId(names, idKey) {
-    prop(names, (i, ie) => ie.get('maxRolls').get(idKey) || 0);
+    prop(names, 'number', (i, ie) => ie.get('maxRolls').get(idKey) || 0);
   }
+
   function minId(names, idKey) {
-    prop(names, (i, ie) => ie.get('minRolls').get(idKey) || 0);
+    prop(names, 'number', (i, ie) => ie.get('minRolls').get(idKey) || 0);
   }
+
   function rangeAvg(names, getProp) {
-    prop(names, (i, ie) => {
+    prop(names, 'number', (i, ie) => {
       const range = getProp(i, ie);
       if (!range) return 0;
       const ndx = range.indexOf('-');
       return (parseInt(range.substring(0, ndx), 10) + parseInt(range.substring(ndx + 1), 10)) / 2;
     });
   }
-  function map(names, comps, f) {
-    return prop(names, (i, ie) => {
+
+  function map(names, comps, outType, f) {
+    return prop(names, outType, (i, ie) => {
       const args = [];
       for (let k = 0; k < comps.length; k++) args.push(comps[k](i, ie));
       return f.apply(null, args);
     });
   }
+
   function sum(names, ...comps) {
-    return map(names, comps, (...summands) => {
+    return map(names, comps, 'number', (...summands) => {
       let total = 0;
       for (let i = 0; i < summands.length; i++) total += summands[i];
       return total;
     });
   }
 
-  prop('name', (i, ie) => i.displayName || i.name);
-  prop('type', (i, ie) => i.type);
-  prop(['cat', 'category'], (i, ie) => i.category);
+  prop('name', 'string', (i, ie) => i.displayName || i.name);
+  prop('type', 'string', (i, ie) => i.type);
+  prop(['cat', 'category'], 'string', (i, ie) => i.category);
   const tierIndices = { Normal: 0, Unique: 1, Set: 2, Rare: 3, Legendary: 4, Fabled: 5, Mythic: 6 };
-  prop(['rarityname', 'raritystr', 'tiername', 'tierstr'], (i, ie) => i.tier);
-  prop(['rarity', 'tier'], (i, ie) => tierIndices[i.tier]);
+  prop(['rarityname', 'raritystr', 'tiername', 'tierstr'], 'string', (i, ie) => i.tier);
+  prop(['rarity', 'tier'], 'string', (i, ie) => tierIndices[i.tier]);
 
-  prop(['level', 'lvl', 'combatlevel', 'combatlvl'], (i, ie) => i.lvl);
-  prop(['strmin', 'strreq'], (i, ie) => i.strReq);
-  prop(['dexmin', 'dexreq'], (i, ie) => i.dexReq);
-  prop(['intmin', 'intreq'], (i, ie) => i.intReq);
-  prop(['defmin', 'defreq'], (i, ie) => i.defReq);
-  prop(['agimin', 'agireq'], (i, ie) => i.agiReq);
+  prop(['level', 'lvl', 'combatlevel', 'combatlvl'], 'number', (i, ie) => i.lvl);
+  prop(['strmin', 'strreq'], 'number', (i, ie) => i.strReq);
+  prop(['dexmin', 'dexreq'], 'number', (i, ie) => i.dexReq);
+  prop(['intmin', 'intreq'], 'number', (i, ie) => i.intReq);
+  prop(['defmin', 'defreq'], 'number', (i, ie) => i.defReq);
+  prop(['agimin', 'agireq'], 'number', (i, ie) => i.agiReq);
   sum(['summin', 'sumreq', 'totalmin', 'totalreq'], props.strmin, props.dexmin, props.intmin, props.defmin, props.agimin);
 
-  prop('str', (i, ie) => i.str);
-  prop('dex', (i, ie) => i.dex);
-  prop('int', (i, ie) => i.int);
-  prop('def', (i, ie) => i.def);
-  prop('agi', (i, ie) => i.agi);
+  prop('str', 'number', (i, ie) => i.str);
+  prop('dex', 'number', (i, ie) => i.dex);
+  prop('int', 'number', (i, ie) => i.int);
+  prop('def', 'number', (i, ie) => i.def);
+  prop('agi', 'number', (i, ie) => i.agi);
   sum(['skillpoints', 'skillpts', 'attributes', 'attrs'], props.str, props.dex, props.int, props.def, props.agi);
 
   rangeAvg(['neutraldmg', 'neutraldam', 'ndmg', 'ndam'], (i, ie) => i.nDam);
@@ -155,15 +110,15 @@ const itemQueryProps = (function() {
   maxId(['spellrawdmg', 'spellrawdam', 'spellneutraldmg', 'spellneutraldam', 'sdraw'], 'sdRaw');
 
   const atkSpdIndices = { SUPER_SLOW: -3, VERY_SLOW: -2, SLOW: -1, NORMAL: 0, FAST: 1, VERY_FAST: 2, SUPER_FAST: 3 };
-  prop(['attackspeed', 'atkspd'], (i, ie) => i.atkSpd ? atkSpdIndices[i.atkSpd] : 0);
+  prop(['attackspeed', 'atkspd'], 'string', (i, ie) => i.atkSpd ? atkSpdIndices[i.atkSpd] : 0);
   maxId(['bonusattackspeed', 'bonusatkspd', 'attackspeedid', 'atkspdid', 'atktier'], 'atkTier');
   sum(['sumattackspeed', 'totalattackspeed', 'sumatkspd', 'totalatkspd', 'sumatktier', 'totalatktier'], props.atkspd, props.atktier);
 
-  prop(['earthdef', 'edef'], (i, ie) => i.eDef || 0);
-  prop(['thunderdef', 'tdef'], (i, ie) => i.tDef || 0);
-  prop(['waterdef', 'wdef'], (i, ie) => i.wDef || 0);
-  prop(['firedef', 'fdef'], (i, ie) => i.fDef || 0);
-  prop(['airdef', 'adef'], (i, ie) => i.aDef || 0);
+  prop(['earthdef', 'edef'], 'number', (i, ie) => i.eDef || 0);
+  prop(['thunderdef', 'tdef'], 'number', (i, ie) => i.tDef || 0);
+  prop(['waterdef', 'wdef'], 'number', (i, ie) => i.wDef || 0);
+  prop(['firedef', 'fdef'], 'number', (i, ie) => i.fDef || 0);
+  prop(['airdef', 'adef'], 'number', (i, ie) => i.aDef || 0);
   sum(['sumdef', 'totaldef'], props.edef, props.tdef, props.wdef, props.fdef, props.adef);
 
   maxId(['earthdef%', 'edef%', 'edefpct'], 'eDefPct');
@@ -173,7 +128,7 @@ const itemQueryProps = (function() {
   maxId(['airdef%', 'adef%', 'adefpct'], 'aDefPct');
   sum(['sumdef%', 'totaldef%', 'sumdefpct', 'totaldefpct'], props.edefpct, props.tdefpct, props.wdefpct, props.fdefpct, props.adefpct);
 
-  prop(['health', 'hp'], (i, ie) => i.hp || 0);
+  prop(['health', 'hp'], 'number', (i, ie) => i.hp || 0);
   maxId(['bonushealth', 'healthid', 'bonushp', 'hpid', 'hpbonus'], 'hpBonus');
   sum(['sumhealth', 'sumhp', 'totalhealth', 'totalhp'], props.hp, props.hpid);
 
@@ -207,397 +162,347 @@ const itemQueryProps = (function() {
   maxId(['lootbonus', 'lb'], 'lb');
   maxId(['xpbonus', 'xpb', 'xb'], 'xpb');
   maxId(['stealing', 'esteal'], 'eSteal');
-  prop(['powderslots', 'powders', 'slots', 'sockets'], (i, ie) => i.slots || 0);
+  prop(['powderslots', 'powders', 'slots', 'sockets'], 'number', (i, ie) => i.slots || 0);
 
   return props;
 })();
 
 // functions that can be called in query expressions
 const itemQueryFuncs = {
-  max(args) {
-    if (args.length < 1) throw new Error('Not enough args to max()');
-    let runningMax = -Infinity;
-    for (let i = 0; i < args.length; i++) {
-      if (checkNum(args[i]) > runningMax) runningMax = args[i];
+  max: {
+    type: 'number',
+    fn: function(args) {
+      if (args.length < 1) throw new Error('Not enough args to max()');
+      let runningMax = -Infinity;
+      for (let i = 0; i < args.length; i++) {
+        if (checkNum(args[i]) > runningMax) runningMax = args[i];
+      }
+      return runningMax;
     }
-    return runningMax;
   },
-  min(args) {
-    if (args.length < 1) throw new Error('Not enough args to min()');
-    let runningMin = Infinity;
-    for (let i = 0; i < args.length; i++) {
-      if (checkNum(args[i]) < runningMin) runningMin = args[i];
+  min: {
+    type: 'number',
+    fn: function(args) {
+      if (args.length < 1) throw new Error('Not enough args to min()');
+      let runningMin = Infinity;
+      for (let i = 0; i < args.length; i++) {
+        if (checkNum(args[i]) < runningMin) runningMin = args[i];
+      }
+      return runningMin;
     }
-    return runningMin;
   },
-  floor(args) {
-    if (args.length < 1) throw new Error('Not enough args to floor()');
-    return Math.floor(checkNum(args[0]));
-  },
-  ceil(args) {
-    if (args.length < 1) throw new Error('Not enough args to ceil()');
-    return Math.ceil(checkNum(args[0]));
-  },
-  round(args) {
-    if (args.length < 1) throw new Error('Not enough args to ceil()');
-    return Math.round(checkNum(args[0]));
-  },
-  sqrt(args) {
-    if (args.length < 1) throw new Error('Not enough args to ceil()');
-    return Math.sqrt(checkNum(args[0]));
-  },
-  abs(args) {
-    if (args.length < 1) throw new Error('Not enough args to ceil()');
-    return Math.abs(checkNum(args[0]));
-  },
-  contains(args) {
-    if (args.length < 2) throw new Error('Not enough args to contains()');
-    return checkStr(args[0]).toLowerCase().includes(checkStr(args[1]).toLowerCase());
-  },
-  atkspdmod(args) {
-    if (args.length < 1) throw new Error('Not enough args to atkSpdMod()');
-    switch (checkNum(args[0])) {
-      case 2: return 3.1;
-      case 1: return 2.5;
-      case 0: return 2.05;
-      case -1: return 1.5;
-      case -2: return 0.83;
+  floor: {
+    type: 'number',
+    fn: function(args) {
+      if (args.length < 1) throw new Error('Not enough args to floor()');
+      return Math.floor(checkNum(args[0]));
+    }
+  },
+  ceil: {
+    type: 'number',
+    fn: function(args) {
+      if (args.length < 1) throw new Error('Not enough args to ceil()');
+      return Math.ceil(checkNum(args[0]));
+    }
+  },
+  round: {
+    type: 'number',
+    fn: function(args) {
+      if (args.length < 1) throw new Error('Not enough args to round()');
+      return Math.round(checkNum(args[0]));
+    }
+  },
+  sqrt: {
+    type: 'number',
+    fn: function(args) {
+      if (args.length < 1) throw new Error('Not enough args to sqrt()');
+      return Math.sqrt(checkNum(args[0]));
+    }
+  },
+  abs: {
+    type: 'number',
+    fn: function(args) {
+      if (args.length < 1) throw new Error('Not enough args to abs()');
+      return Math.abs(checkNum(args[0]));
+    }
+  },
+  contains: {
+    type: 'boolean',
+    fn: function(args) {
+      if (args.length < 2) throw new Error('Not enough args to contains()');
+      return checkStr(args[0]).toLowerCase().includes(checkStr(args[1]).toLowerCase());
+    }
+  },
+  atkspdmod: {
+    type: 'number',
+    fn: function(args) {
+      if (args.length < 1) throw new Error('Not enough args to atkSpdMod()');
+      switch (checkNum(args[0])) {
+        case 2:
+          return 3.1;
+        case 1:
+          return 2.5;
+        case 0:
+          return 2.05;
+        case -1:
+          return 1.5;
+        case -2:
+          return 0.83;
+      }
+      if (args[0] <= -3) return 0.51;
+      if (args[0] >= 3) return 4.3;
+      throw new Error('Invalid argument to atkSpdMod()');
     }
-    if (args[0] <= -3) return 0.51;
-    if (args[0] >= 3) return 4.3;
-    throw new Error('Invalid argument to atkSpdMod()');
   }
 };
 
-// the compiler itself
-const compileQueryExpr = (function() {
-  // tokenize an expression string
-  function tokenize(exprStr) {
-    exprStr = exprStr.trim();
-    const tokens = [];
-    let col = 0;
-    function pushSymbol(sym) {
-      tokens.push({ type: 'sym', sym });
-      col += sym.length;
-    }
-    while (col < exprStr.length) {
-      // parse fixed symbols, like operators and stuff
-      switch (exprStr[col]) {
-        case '(':
-        case ')':
-        case ',':
-        case '&':
-        case '|':
-        case '+':
-        case '-':
-        case '*':
-        case '/':
-        case '^':
-        case '=':
-          pushSymbol(exprStr[col]);
-          continue;
-        case '>':
-          pushSymbol(exprStr[col + 1] === '=' ? '>=' : '>');
-          continue;
-        case '<':
-          pushSymbol(exprStr[col + 1] === '=' ? '<=' : '<');
-          continue;
-        case '!':
-          pushSymbol(exprStr[col + 1] === '=' ? '!=' : '!');
-          continue;
-        case ' ': // ignore extra whitespace
-          ++col;
-          continue;
-      }
-      if (exprStr.slice(col, col+2) === "?=") {
-        pushSymbol("?=");
-        continue;
-      }
-      // parse a numeric literal
-      let m;
-      if ((m = /^\d+(?:\.\d*)?/.exec(exprStr.substring(col))) !== null) {
-        tokens.push({ type: 'num', value: parseFloat(m[0]) });
-        col += m[0].length;
-        continue;
-      }
-      // parse a string literal
-      if ((m = /^"([^"]+)"/.exec(exprStr.substring(col))) !== null) { // with double-quotes
-        tokens.push({ type: 'str', value: m[1] });
-        col += m[0].length;
-        continue;
-      }
-      if ((m = /^'([^']+)'/.exec(exprStr.substring(col))) !== null) { // with single-quotes
-        tokens.push({ type: 'str', value: m[1] });
-        col += m[0].length;
-        continue;
-      }
-      // parse an identifier or boolean literal
-      if ((m = /^\w[\w\d%]*/.exec(exprStr.substring(col))) !== null) {
-        switch (m[0]) {
-          case 'true':
-            tokens.push({ type: 'bool', value: true });
-            col += 4;
-            continue;
-          case 'false':
-            tokens.push({ type: 'bool', value: false });
-            col += 5;
-            continue;
-        }
-        tokens.push({ type: 'id', id: m[0] });
-        col += m[0].length;
-        continue;
-      }
-      // if we reach here without successfully parsing a token, it's an error
-      throw new Error(`Could not parse character "${exprStr[col]}" at position ${col}`);
-    }
-    tokens.push({ type: 'eof' });
-    return new TokenList(tokens);
+// static type check
+function staticCheck(expType, term) {
+  if (expType === 'any' || expType === term.type) {
+    return true;
+  }
+  throw new Error(`Expected ${expType}, but got ${term.type}`);
+}
+
+// expression terms
+class Term {
+  constructor(type) {
+    this.type = type;
   }
 
-  // parse tokens into an ast
-  function takeDisj(tokens) {
-    const left = takeConj(tokens);
-    if (tokens.here.type === 'sym' && tokens.here.sym === '|') {
-      tokens.advance();
-      const right = takeDisj(tokens);
-      return (i, ie) => checkBool(left(i, ie)) || checkBool(right(i, ie));
-    }
-    return left;
+  resolve(item, itemExt) {
+    throw new Error('Abstract method!');
+  }
+}
+
+class LiteralTerm extends Term {
+  constructor(type, value) {
+    super(type);
+    this.value = value;
   }
 
-  function takeConj(tokens) {
-    const left = takeCmpEq(tokens);
-    if (tokens.here.type === 'sym' && tokens.here.sym === '&') {
-      tokens.advance();
-      const right = takeConj(tokens);
-      return (i, ie) => checkBool(left(i, ie)) && checkBool(right(i, ie));
-    }
-    return left;
+  resolve(item, itemExt) {
+    return this.value;
+  }
+}
+
+class BoolLitTerm extends LiteralTerm {
+  constructor(value) {
+    super('boolean', value);
+  }
+}
+
+class NumLitTerm extends LiteralTerm {
+  constructor(value) {
+    super('number', value);
+  }
+}
+
+class StrLitTerm extends LiteralTerm {
+  constructor(value) {
+    super('string', value);
+  }
+}
+
+class BinaryOpTerm extends Term {
+  constructor(type, leftType, left, rightType, right) {
+    super(type);
+    staticCheck(leftType, left);
+    staticCheck(rightType, right);
+    this.left = left;
+    this.right = right;
   }
 
-  function takeCmpEq(tokens) {
-    const left = takeCmpRel(tokens);
-    if (tokens.here.type === 'sym') {
-      switch (tokens.here.sym) {
-        case '=': {
-          tokens.advance();
-          const right = takeCmpEq(tokens);
-          return (i, ie) => {
-            const a = left(i, ie), b = right(i, ie);
-            if (typeof a !== typeof b) return false;
-            switch (typeof a) {
-              case 'number':
-                return Math.abs(left(i, ie) - right(i, ie)) < 1e-4;
-              case 'boolean':
-                return a === b;
-              case 'string':
-                return a.toLowerCase() === b.toLowerCase();
-            }
-            throw new Error('???'); // wut
-          };
-        }
-        case '!=': {
-          tokens.advance();
-          const right = takeCmpEq(tokens);
-          return (i, ie) => {
-            const a = left(i, ie), b = right(i, ie);
-            if (typeof a !== typeof b) return false;
-            switch (typeof a) {
-              case 'number':
-                return Math.abs(left(i, ie) - right(i, ie)) >= 1e-4;
-              case 'boolean':
-                return a !== b;
-              case 'string':
-                return a.toLowerCase() !== b.toLowerCase();
-            }
-            throw new Error('???'); // wtf
-          };
-        }
-        case '?=': {
-          tokens.advance();
-          const right = takePrim(tokens);
-          return (i, ie) => {
-            const a = left(i, ie), b = right(i, ie);
-            if (typeof a !== typeof b) return false;
-            switch (typeof a) {
-              case 'number':
-                return Math.abs(left(i, ie) - right(i, ie)) < 1e-4;
-              case 'boolean':
-                return a === b;
-              case 'string':
-                return a.toLowerCase().includes(b.toLowerCase());
-            }
-            throw new Error('???'); // wtf
-          };
-        }
-      }
-    }
-    return left;
+  resolve(item, itemExt) {
+    return this.apply(this.left.resolve(item, itemExt), this.right.resolve(item, itemExt));
   }
 
-  function takeCmpRel(tokens) {
-    const left = takeSum(tokens);
-    if (tokens.here.type === 'sym') {
-      switch (tokens.here.sym) {
-        case '<=': {
-          tokens.advance();
-          const right = takeCmpRel(tokens);
-          return (i, ie) => checkNum(left(i, ie)) <= checkNum(right(i, ie));
-        }
-        case '<': {
-          tokens.advance();
-          const right = takeCmpRel(tokens);
-          return (i, ie) => checkNum(left(i, ie)) < checkNum(right(i, ie));
-        }
-        case '>': {
-          tokens.advance();
-          const right = takeCmpRel(tokens);
-          return (i, ie) => checkNum(left(i, ie)) > checkNum(right(i, ie));
-        }
-        case '>=': {
-          tokens.advance();
-          const right = takeCmpRel(tokens);
-          return (i, ie) => checkNum(left(i, ie)) >= checkNum(right(i, ie));
-        }
-      }
-    }
-    return left;
+  apply(a, b) {
+    throw new Error('Abstract method!');
+  }
+}
+
+class LogicalTerm extends BinaryOpTerm {
+  constructor(left, right) {
+    super('boolean', 'boolean', left, 'boolean', right);
+  }
+}
+
+class ConjTerm extends LogicalTerm {
+  apply(a, b) {
+    return a && b;
+  }
+}
+
+class DisjTerm extends LogicalTerm {
+  apply(a, b) {
+    return a || b;
+  }
+}
+
+class EqualityTerm extends BinaryOpTerm {
+  constructor(left, right) {
+    super('boolean', 'any', left, 'any', right);
+  }
+}
+
+class EqTerm extends EqualityTerm {
+  apply(a, b) {
+    return a === b;
+  }
+}
+
+class NeqTerm extends EqualityTerm {
+  apply(a, b) {
+    return a !== b;
+  }
+}
+
+class ContainsTerm extends BinaryOpTerm {
+  constructor(left, right) {
+    super('boolean', 'string', left, 'string', right);
   }
 
-  function takeSum(tokens) {
-    const left = takeProd(tokens);
-    if (tokens.here.type === 'sym') {
-      switch (tokens.here.sym) {
-        case '+': {
-          tokens.advance();
-          const right = takeSum(tokens);
-          return (i, ie) => checkNum(left(i, ie)) + checkNum(right(i, ie));
-        }
-        case '-': {
-          tokens.advance();
-          const right = takeSum(tokens);
-          return (i, ie) => checkNum(left(i, ie)) - checkNum(right(i, ie));
-        }
-      }
-    }
-    return left;
+  apply(a, b) {
+    return a.toLowerCase().includes(b.toLowerCase());
+  }
+}
+
+class InequalityTerm extends BinaryOpTerm {
+  constructor(left, right) {
+    super('boolean', 'any', left, 'any', right);
   }
 
-  function takeProd(tokens) {
-    const left = takeExp(tokens);
-    if (tokens.here.type === 'sym') {
-      switch (tokens.here.sym) {
-        case '*': {
-          tokens.advance();
-          const right = takeProd(tokens);
-          return (i, ie) => checkNum(left(i, ie)) * checkNum(right(i, ie));
-        }
-        case '/': {
-          tokens.advance();
-          const right = takeProd(tokens);
-          return (i, ie) => checkNum(left(i, ie)) / checkNum(right(i, ie));
-        }
-      }
-    }
-    return left;
+  apply(a, b) {
+    checkComparable(a);
+    checkComparable(b);
+    return this.compare(a, b);
   }
 
-  function takeExp(tokens) {
-    const left = takeUnary(tokens);
-    if (tokens.here.type === 'sym' && tokens.here.sym === '^') {
-      tokens.advance();
-      const right = takeExp(tokens);
-      return (i, ie) => checkNum(left(i, ie)) ** checkNum(right(i, ie));
-    }
-    return left;
+  compare(a, b) {
+    throw new Error('Abstract method!');
+  }
+}
+
+class LeqTerm extends InequalityTerm {
+  compare(a, b) {
+    return a <= b;
+  }
+}
+
+class LtTerm extends InequalityTerm {
+  compare(a, b) {
+    return a < b;
+  }
+}
+
+class GtTerm extends InequalityTerm {
+  compare(a, b) {
+    return a > b;
+  }
+}
+
+class GeqTerm extends InequalityTerm {
+  compare(a, b) {
+    return a >= b;
+  }
+}
+
+class ArithmeticTerm extends BinaryOpTerm {
+  constructor(left, right) {
+    super('number', 'number', left, 'number', right);
+  }
+}
+
+class AddTerm extends ArithmeticTerm {
+  apply(a, b) {
+    return a + b;
+  }
+}
+
+class SubTerm extends ArithmeticTerm {
+  apply(a, b) {
+    return a - b;
+  }
+}
+
+class MulTerm extends ArithmeticTerm {
+  apply(a, b) {
+    return a * b;
+  }
+}
+
+class DivTerm extends ArithmeticTerm {
+  apply(a, b) {
+    return a  / b;
+  }
+}
+
+class ExpTerm extends ArithmeticTerm {
+  apply(a, b) {
+    return a ** b;
+  }
+}
+
+class UnaryOpTerm extends Term {
+  constructor(type, inType, inVal) {
+    super(type);
+    staticCheck(inType, inVal);
+    this.inVal = inVal;
   }
 
-  function takeUnary(tokens) {
-    if (tokens.here.type === 'sym') {
-      switch (tokens.here.sym) {
-        case '-': {
-          tokens.advance();
-          const operand = takeUnary(tokens);
-          return (i, ie) => -checkNum(operand(i, ie));
-        }
-        case '!': {
-          tokens.advance();
-          const operand = takeUnary(tokens);
-          return (i, ie) => !checkBool(operand(i, ie));
-        }
-      }
-    }
-    return takePrim(tokens);
+  resolve(item, itemExt) {
+    return this.apply(this.inVal.resolve(item, itemExt));
   }
 
-  function takePrim(tokens) {
-    switch (tokens.here.type) {
-      case 'num': {
-        const lit = tokens.here.value;
-        tokens.advance();
-        return (i, ie) => lit;
-      }
-      case 'bool': {
-        const lit = tokens.here.value;
-        tokens.advance();
-        return (i, ie) => lit;
-      }
-      case 'str': {
-        const lit = tokens.here.value;
-        tokens.advance();
-        return (i, ie) => lit;
-      }
-      case 'id':
-        const id = tokens.here.id;
-        tokens.advance();
-        if (tokens.here.type === 'sym' && tokens.here.sym === '(') { // it's a function call
-          tokens.advance();
-          const argExprs = [];
-          if (tokens.here.type !== 'sym' || tokens.here.sym !== ')') {
-            arg_iter: // collect arg expressions, if there are any
-            while (true) {
-              argExprs.push(takeDisj(tokens));
-              if (tokens.here.type === 'sym') {
-                switch (tokens.here.sym) {
-                  case ')':
-                    tokens.advance();
-                    break arg_iter;
-                  case ',':
-                    tokens.advance();
-                    continue;
-                }
-              }
-              throw new Error(`Expected "," or ")", but got ${JSON.stringify(tokens.here)}`);
-            }
-          }
-          const func = itemQueryFuncs[id.toLowerCase()];
-          if (!func) throw new Error(`Unknown function: ${id}`);
-          return (i, ie) => {
-            const args = [];
-            for (let k = 0; k < argExprs.length; k++) args.push(argExprs[k](i, ie));
-            return func(args);
-          };
-        } else { // not a function call
-          const prop = itemQueryProps[id.toLowerCase()];
-          if (!prop) throw new Error(`Unknown property: ${id}`);
-          return prop;
-        }
-      case 'sym':
-        if (tokens.here.sym === '(') {
-          tokens.advance();
-          const expr = takeDisj(tokens);
-          if (tokens.here.type !== 'sym' || tokens.here.sym !== ')') throw new Error('Bracket mismatch');
-          tokens.advance();
-          return expr;
-        }
-        break;
-    }
-    throw new Error(tokens.here.type === 'eof' ? 'Reached end of expression' : `Unexpected token: ${JSON.stringify(tokens.here)}`);
+  apply(x) {
+    throw new Error('Abstract method!');
+  }
+}
+
+class NegTerm extends UnaryOpTerm {
+  constructor(inVal) {
+    super('number', 'number', inVal);
   }
 
-  // full compilation function, with extra safety for empty input strings
-  return function(exprStr) {
-    const tokens = tokenize(exprStr);
-    return tokens.tokens.length <= 1 ? null : takeDisj(tokens);
-  };
-})();
+  apply(x) {
+    return -x;
+  }
+}
+
+class InvTerm extends UnaryOpTerm {
+  constructor(inVal) {
+    super('boolean', 'boolean', inVal);
+  }
+
+  apply(x) {
+    return !x;
+  }
+}
+
+class FnCallTerm extends Term {
+  constructor(fn, argExprs) {
+    super(fn.type);
+    this.fn = fn;
+    this.argExprs = argExprs;
+  }
+
+  resolve(item, itemExt) {
+    const argVals = [];
+    for (const argExpr of this.argExprs) {
+      argVals.push(argExpr.resolve(item, itemExt));
+    }
+    return this.fn.fn(argVals);
+  }
+}
+
+class PropTerm extends Term {
+  constructor(prop) {
+    super(prop.type);
+    this.prop = prop;
+  }
+
+  resolve(item, itemExt) {
+    return this.prop.resolve(item, itemExt);
+  }
+}