detect qualified ops as single-token lookahead in the scanner

grammar/exp.js

@@ -5,6 +5,16 @@ module.exports = {
   // expression
   // ------------------------------------------------------------------------
 
+  _exp_op: $ => choice(
+    $._sym,
+    $._op_ticked,
+    alias($._prefix_dot, $.operator),
+    seq(
+      $._cond_qualified_op,
+      $._qsym,
+    ),
+  ),
+
   exp_parens: $ => parens($, $._exp),
 
   /**
@@ -44,15 +54,14 @@ module.exports = {
     $,
     $._infixexp,
     choice(
-      $._ops,
+      $._exp_op,
       $._operator_minus,
     ),
   ),
 
   exp_section_right: $ => parens(
     $,
     choice(
-      prec('section-minus-shift', seq(alias('-', $.operator), $._infixexp)),
       seq(alias($._operator_qual_dot_head, $.operator), $._infixexp),
       seq($._ops, $._infixexp),
     )
@@ -301,31 +310,33 @@ module.exports = {
     field('arg', $._exp_apply_arg),
   )),
 
-  exp_negation: $ => prec('negation', seq($._negation, prec('negation-reduce', $._infixexp))),
-
-  _infix_minus: $ => prec('operator-minus', alias('-', $.operator)),
-
-  _exp_op: $ => choice(
-    $._sym,
-    $._op_ticked,
-    alias($._prefix_dot, $.operator),
-  ),
+  exp_negation: $ => prec('negation', seq('-', prec('negation-reduce', $._infixexp))),
 
+  /**
+   * This is a trick that allows negation to bind less tight than application and tighter than infix.
+   * The reduction to `exp_negation` conflicts with shifts for both other rules, and tree-sitter doesn't allow them to
+   * be ordered apply > negation > infix because all shifts must compare identically to the reduction in a
+   * multi-conflict.
+   *
+   * Splitting infix minus from the other variants ensures that apply and infix have separate conflicts with negation.
+   * The reason this works is that the second path is right-associative.
+   * It doesn't need a named shift prec to that, because it shares the `-` lookahead with `exp_negation`, so the shift
+   * is covered by the `exp_apply`/`exp_negation` conflict.
+   *
+   * Unfortunately, this has additional side effects:
+   * Left sections of minus with infix minus in their expression (`2 - 1 -`) are broken due to right-associativity, and
+   * negation in the first operand of `-` binds less tight (`- 2 - 1`).
+   */
   exp_infix: $ => choice(
-    prec.left('infix', seq(
+    prec('infix', seq(
       field('left_operand', $._infixexp),
       field('operator', $._exp_op),
-      field('right_operand', $._infixexp),
+      field('right_operand', prec('infix-reduce', $._infixexp)),
     )),
     seq(
       field('left_operand', $._infixexp),
-      field('operator', $._infix_minus),
-      prec('infix-minus', field('right_operand', $._infixexp)),
-    ),
-    seq(
-      field('left_operand', $._infixexp),
-      field('operator', $._qsym),
-      prec('infix-qualified', field('right_operand', $._infixexp)),
+      field('operator', $._operator_minus),
+      field('right_operand', prec('infix-reduce', $._infixexp)),
     ),
   ),
 

grammar/externals.js

@@ -123,6 +123,8 @@ module.exports = {
     $._cond_prefix_tilde,
     $._cond_prefix_percent,
 
+    $._cond_qualified_op,
+
     // Symbolic operators, producing text nodes.
     // These are very difficult to parse in the grammar, because unlike most languages, Haskell's operators are not a
     // finite set, and therefore cannot be lexically disambiguated from special operators like `->`.

grammar/pat.js

@@ -47,25 +47,24 @@ module.exports = {
     field('arg', $._pat_apply_arg),
   )),
 
-  pat_negation: $ => prec('negation', seq($._negation, $._number)),
+  pat_negation: $ => prec('negation', seq('-', $._number)),
 
   _pat_op: $ => choice(
     $.constructor_operator,
     $._conids_ticked,
   ),
 
-  pat_infix: $ => choice(
-    prec.left('infix', seq(
-      field('left_operand', $._infixpat),
-      field('operator', $._pat_op),
-      field('right_operand', $._infixpat),
+  pat_infix: $ => prec('infix', seq(
+    field('left_operand', $._infixpat),
+    field('operator', choice(
+      $._pat_op,
+      seq(
+        $._cond_qualified_op,
+        $._qconsym,
+      ),
     )),
-    seq(
-      field('left_operand', $._infixpat),
-      field('operator', $._qconsym),
-      prec('infix-qualified', field('right_operand', $._infixpat)),
-    ),
-  ),
+    field('right_operand', prec('infix-reduce', $._infixpat)),
+  )),
 
   _infixpat: $ => choice(
     $.pat_infix,

grammar/precedences.js

@@ -12,8 +12,7 @@ module.exports = {
       'prefix',
       'apply',
       'negation-reduce',
-      'infix-qualified',
-      'infix-minus',
+      'infix-reduce',
       'infix',
       'fun',
       'implicit',
@@ -51,7 +50,6 @@ module.exports = {
     [
       'operator-minus',
       'negation',
-      'section-minus-shift',
     ],
 
     // ------------------------------------------------