haskell: expression parser + precedence climbing (+42 tests, 100/100)

plans/haskell-on-sx.md

@@ -56,7 +56,14 @@ Key mappings:
 ### Phase 1 — tokenizer + parser + layout rule
 - [x] Tokenizer: reserved words, qualified names, operators, numbers (int, float, Rational later), chars/strings, comments (`--` and `{-` nested)
 - [x] Layout algorithm: turn indentation into virtual `{`, `;`, `}` tokens per Haskell 98 §10.3
-- [ ] Parser: modules, imports (stub), top-level decls, type sigs, function clauses with patterns + guards + where-clauses, expressions with operator precedence, lambdas, `let`, `if`, `case`, `do`, list comp, sections
+- Parser (split into sub-items — implement one per iteration):
+  - [x] Expressions: atoms, parens, tuples, lists, ranges, application, infix with full Haskell-98 precedence table, unary `-`, backtick operators, lambdas, `if`, `let`
+  - [ ] `case … of` and `do`-notation expressions
+  - [ ] Patterns (var, wildcard, literal, constructor, as, nested) — consumed by lambdas, let, case, and function clauses
+  - [ ] Top-level decls: function clauses, type signatures, `data`, `type`, `newtype`, fixity decls
+  - [ ] `where` clauses + guards
+  - [ ] Module header + imports (stub)
+  - [ ] List comprehensions + operator sections
 - [ ] AST design modelled on GHC's HsSyn at a surface level
 - [x] Unit tests in `lib/haskell/tests/parse.sx` (43 tokenizer tests, all green)
 
@@ -107,6 +114,25 @@ Key mappings:
 
 _Newest first._
 
+- **2026-04-24** — Phase 1: expression parser (`lib/haskell/parser.sx`, ~380 lines).
+  Pratt-style precedence climbing against a Haskell-98-default op table (24
+  operators across precedence 0–9, left/right/non assoc, default infixl 9 for
+  anything unlisted). Supports literals (int/float/string/char), varid/conid
+  (qualified variants folded into `:var` / `:con`), parens / unit / tuples,
+  list literals, ranges `[a..b]` and `[a,b..c]`, left-associative application,
+  unary `-`, backtick operators (`x \`mod\` 3`), lambdas, `if-then-else`, and
+  `let … in` consuming both virtual and explicit braces. AST uses keyword
+  tags (`:var`, `:op`, `:lambda`, `:let`, `:bind`, `:tuple`, `:range`,
+  `:range-step`, `:app`, `:neg`, `:if`, `:list`, `:int`, `:float`, `:string`,
+  `:char`, `:con`). The parser skips a leading `vlbrace` / `lbrace` so it can
+  be called on full post-layout output, and uses a `raise`-based error channel
+  with location-lite messages. 42 new tests in `lib/haskell/tests/parser-expr.sx`
+  cover literals, identifiers, parens/tuple/unit, list + range, app associativity,
+  operator precedence (mul over add, cons right-assoc, function-composition
+  right-assoc, `$` lowest), backtick ops, unary `-`, lambda multi-param,
+  `if` with infix condition, single- and multi-binding `let` (both implicit
+  and explicit braces), plus a few mixed nestings. 100/100 green.
+
 - **2026-04-24** — Phase 1: layout algorithm (`lib/haskell/layout.sx`, ~260 lines)
   implementing Haskell 98 §10.3. Two-pass design: a pre-pass augments the raw
   token stream with explicit `layout-open` / `layout-indent` markers (suppressing