From efbab24cb203528db1deeec907d43b8acbbb5812 Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Fri, 24 Apr 2026 17:03:00 +0000
Subject: [PATCH] erlang: sequential eval (+54 tests)

---
 lib/erlang/tests/eval.sx | 128 +++++++++++++++
 lib/erlang/transpile.sx  | 338 +++++++++++++++++++++++++++++++++++++++
 plans/erlang-on-sx.md    |   3 +-
 3 files changed, 468 insertions(+), 1 deletion(-)
 create mode 100644 lib/erlang/tests/eval.sx
 create mode 100644 lib/erlang/transpile.sx

diff --git a/lib/erlang/tests/eval.sx b/lib/erlang/tests/eval.sx
new file mode 100644
index 00000000..5fc30e4c
--- /dev/null
+++ b/lib/erlang/tests/eval.sx
@@ -0,0 +1,128 @@
+;; Erlang evaluator tests — sequential expressions.
+
+(define er-eval-test-count 0)
+(define er-eval-test-pass 0)
+(define er-eval-test-fails (list))
+
+(define
+  eev-deep=
+  (fn
+    (a b)
+    (cond
+      (and (= (type-of a) "dict") (= (type-of b) "dict"))
+      (let
+        ((ka (sort (keys a))) (kb (sort (keys b))))
+        (and (= ka kb) (every? (fn (k) (eev-deep= (get a k) (get b k))) ka)))
+      (and (= (type-of a) "list") (= (type-of b) "list"))
+      (and
+        (= (len a) (len b))
+        (every? (fn (i) (eev-deep= (nth a i) (nth b i))) (range 0 (len a))))
+      :else (= a b))))
+
+(define
+  er-eval-test
+  (fn
+    (name actual expected)
+    (set! er-eval-test-count (+ er-eval-test-count 1))
+    (if
+      (eev-deep= actual expected)
+      (set! er-eval-test-pass (+ er-eval-test-pass 1))
+      (append! er-eval-test-fails {:actual actual :expected expected :name name}))))
+
+(define ev erlang-eval-ast)
+(define nm (fn (v) (get v :name)))
+
+;; ── literals ──────────────────────────────────────────────────────
+(er-eval-test "int" (ev "42") 42)
+(er-eval-test "zero" (ev "0") 0)
+(er-eval-test "float" (ev "3.14") 3.14)
+(er-eval-test "string" (ev "\"hi\"") "hi")
+(er-eval-test "atom" (nm (ev "ok")) "ok")
+(er-eval-test "atom true" (nm (ev "true")) "true")
+(er-eval-test "atom false" (nm (ev "false")) "false")
+
+;; ── arithmetic ────────────────────────────────────────────────────
+(er-eval-test "add" (ev "1 + 2") 3)
+(er-eval-test "sub" (ev "5 - 3") 2)
+(er-eval-test "mul" (ev "4 * 3") 12)
+(er-eval-test "div-real" (ev "10 / 4") 2.5)
+(er-eval-test "div-int" (ev "10 div 3") 3)
+(er-eval-test "rem" (ev "10 rem 3") 1)
+(er-eval-test "div-neg" (ev "-10 div 3") -3)
+(er-eval-test "precedence" (ev "1 + 2 * 3") 7)
+(er-eval-test "parens" (ev "(1 + 2) * 3") 9)
+(er-eval-test "unary-neg" (ev "-(1 + 2)") -3)
+(er-eval-test "unary-neg int" (ev "-7") -7)
+
+;; ── comparison ────────────────────────────────────────────────────
+(er-eval-test "lt true" (nm (ev "1 < 2")) "true")
+(er-eval-test "gt false" (nm (ev "1 > 2")) "false")
+(er-eval-test "le equal" (nm (ev "2 =< 2")) "true")
+(er-eval-test "ge equal" (nm (ev "2 >= 2")) "true")
+(er-eval-test "eq" (nm (ev "2 == 2")) "true")
+(er-eval-test "neq" (nm (ev "1 /= 2")) "true")
+(er-eval-test "exact-eq same" (nm (ev "1 =:= 1")) "true")
+(er-eval-test "exact-neq int" (nm (ev "1 =:= 2")) "false")
+(er-eval-test "=/= true" (nm (ev "1 =/= 2")) "true")
+(er-eval-test "atom-eq" (nm (ev "ok == ok")) "true")
+(er-eval-test "atom-neq" (nm (ev "ok == error")) "false")
+
+;; ── logical ───────────────────────────────────────────────────────
+(er-eval-test "and tt" (nm (ev "true and true")) "true")
+(er-eval-test "and tf" (nm (ev "true and false")) "false")
+(er-eval-test "or tf" (nm (ev "true or false")) "true")
+(er-eval-test
+  "andalso short"
+  (nm (ev "false andalso Neverref"))
+  "false")
+(er-eval-test
+  "orelse short"
+  (nm (ev "true orelse Neverref"))
+  "true")
+(er-eval-test "not true" (nm (ev "not true")) "false")
+(er-eval-test "not false" (nm (ev "not false")) "true")
+
+;; ── tuples & lists ────────────────────────────────────────────────
+(er-eval-test "tuple tag" (get (ev "{1, 2, 3}") :tag) "tuple")
+(er-eval-test "tuple len" (len (get (ev "{1, 2, 3}") :elements)) 3)
+(er-eval-test "tuple elem" (nth (get (ev "{10, 20}") :elements) 1) 20)
+(er-eval-test "empty tuple" (len (get (ev "{}") :elements)) 0)
+(er-eval-test "nested tuple"
+  (nm (nth (get (ev "{ok, error}") :elements) 0)) "ok")
+(er-eval-test "nil list" (get (ev "[]") :tag) "nil")
+(er-eval-test "list head" (get (ev "[1, 2, 3]") :head) 1)
+(er-eval-test
+  "list tail tail head"
+  (get (get (get (ev "[1, 2, 3]") :tail) :tail) :head)
+  3)
+
+;; ── list ops ──────────────────────────────────────────────────────
+(er-eval-test "++ head" (get (ev "[1, 2] ++ [3]") :head) 1)
+(er-eval-test "++ last"
+  (get (get (get (ev "[1, 2] ++ [3]") :tail) :tail) :head) 3)
+
+;; ── block ─────────────────────────────────────────────────────────
+(er-eval-test "block last wins" (ev "begin 1, 2, 3 end") 3)
+(er-eval-test "bare body" (ev "1, 2, 99") 99)
+
+;; ── match + var ───────────────────────────────────────────────────
+(er-eval-test "match bind-and-use" (ev "X = 5, X + 1") 6)
+(er-eval-test "match sequential" (ev "X = 1, Y = 2, X + Y") 3)
+(er-eval-test
+  "rebind equal ok"
+  (ev "X = 5, X = 5, X") 5)
+
+;; ── if ────────────────────────────────────────────────────────────
+(er-eval-test "if picks first" (ev "if true -> 1; true -> 2 end") 1)
+(er-eval-test
+  "if picks second"
+  (nm (ev "if 1 > 2 -> bad; true -> good end"))
+  "good")
+(er-eval-test
+  "if with guard"
+  (ev "X = 5, if X > 0 -> 1; true -> 0 end")
+  1)
+
+(define
+  er-eval-test-summary
+  (str "eval " er-eval-test-pass "/" er-eval-test-count))
diff --git a/lib/erlang/transpile.sx b/lib/erlang/transpile.sx
new file mode 100644
index 00000000..db460a45
--- /dev/null
+++ b/lib/erlang/transpile.sx
@@ -0,0 +1,338 @@
+;; Erlang sequential evaluator — tree-walking interpreter over the
+;; parser AST. Phase 2 of plans/erlang-on-sx.md.
+;;
+;; Entry points:
+;;   (erlang-eval-ast SRC)         -- parse body, eval, return last value
+;;   (er-eval-expr NODE ENV)       -- evaluate one AST node
+;;   (er-eval-body NODES ENV)      -- evaluate a comma-sequence, return last
+;;
+;; Runtime values:
+;;   integers / floats      -> SX number
+;;   atoms                  -> {:tag "atom" :name <string>}
+;;   booleans               -> atoms 'true' / 'false'
+;;   strings                -> SX string  (char-list semantics deferred)
+;;   empty list             -> {:tag "nil"}
+;;   cons cell              -> {:tag "cons" :head V :tail V}
+;;   tuple                  -> {:tag "tuple" :elements (list V ...)}
+;;
+;; Environment: mutable dict from variable name (string) to value.
+
+;; ── value constructors / predicates ────────────────────────────────
+(define er-mk-atom (fn (name) {:name name :tag "atom"}))
+(define er-atom-true (er-mk-atom "true"))
+(define er-atom-false (er-mk-atom "false"))
+(define er-mk-nil (fn () {:tag "nil"}))
+(define er-mk-cons (fn (h t) {:tag "cons" :head h :tail t}))
+(define er-mk-tuple (fn (elems) {:tag "tuple" :elements elems}))
+(define er-bool (fn (b) (if b er-atom-true er-atom-false)))
+
+(define
+  er-is-tagged?
+  (fn (v tag) (and (= (type-of v) "dict") (= (get v :tag) tag))))
+(define er-atom? (fn (v) (er-is-tagged? v "atom")))
+(define er-nil? (fn (v) (er-is-tagged? v "nil")))
+(define er-cons? (fn (v) (er-is-tagged? v "cons")))
+(define er-tuple? (fn (v) (er-is-tagged? v "tuple")))
+
+(define
+  er-is-atom-named?
+  (fn (v name) (and (er-atom? v) (= (get v :name) name))))
+(define er-truthy? (fn (v) (er-is-atom-named? v "true")))
+
+;; ── environment ───────────────────────────────────────────────────
+(define er-env-new (fn () {}))
+
+(define
+  er-env-lookup
+  (fn
+    (env name)
+    (if
+      (dict-has? env name)
+      (get env name)
+      (error (str "Erlang: unbound variable '" name "'")))))
+
+(define er-env-bind! (fn (env name val) (dict-set! env name val)))
+
+;; ── entry ─────────────────────────────────────────────────────────
+(define
+  erlang-eval-ast
+  (fn
+    (src)
+    (let
+      ((st (er-state-make (er-tokenize src))))
+      (let
+        ((body (er-parse-body st)) (env (er-env-new)))
+        (er-eval-body body env)))))
+
+(define
+  er-eval-body
+  (fn
+    (exprs env)
+    (let
+      ((last (list nil)))
+      (for-each
+        (fn (i) (set-nth! last 0 (er-eval-expr (nth exprs i) env)))
+        (range 0 (len exprs)))
+      (nth last 0))))
+
+;; ── dispatch ──────────────────────────────────────────────────────
+(define
+  er-eval-expr
+  (fn
+    (node env)
+    (let
+      ((ty (get node :type)))
+      (cond
+        (= ty "integer") (parse-number (get node :value))
+        (= ty "float") (parse-number (get node :value))
+        (= ty "atom") (er-mk-atom (get node :value))
+        (= ty "string") (get node :value)
+        (= ty "nil") (er-mk-nil)
+        (= ty "var") (er-eval-var node env)
+        (= ty "tuple") (er-eval-tuple node env)
+        (= ty "cons") (er-eval-cons node env)
+        (= ty "op") (er-eval-op node env)
+        (= ty "unop") (er-eval-unop node env)
+        (= ty "block") (er-eval-body (get node :exprs) env)
+        (= ty "if") (er-eval-if node env)
+        (= ty "match") (er-eval-match node env)
+        :else (error (str "Erlang eval: unsupported node type '" ty "'"))))))
+
+(define
+  er-eval-var
+  (fn
+    (node env)
+    (let
+      ((name (get node :name)))
+      (if
+        (= name "_")
+        (error "Erlang: '_' cannot be used as a value")
+        (er-env-lookup env name)))))
+
+(define
+  er-eval-tuple
+  (fn
+    (node env)
+    (let
+      ((out (list)))
+      (for-each
+        (fn
+          (i)
+          (append! out (er-eval-expr (nth (get node :elements) i) env)))
+        (range 0 (len (get node :elements))))
+      (er-mk-tuple out))))
+
+(define
+  er-eval-cons
+  (fn
+    (node env)
+    (er-mk-cons
+      (er-eval-expr (get node :head) env)
+      (er-eval-expr (get node :tail) env))))
+
+;; ── match (bare-var LHS only; full pattern matching comes next) ────
+(define
+  er-eval-match
+  (fn
+    (node env)
+    (let
+      ((lhs (get node :lhs))
+        (rhs-val (er-eval-expr (get node :rhs) env)))
+      (cond
+        (= (get lhs :type) "var")
+        (let
+          ((name (get lhs :name)))
+          (cond
+            (= name "_") rhs-val
+            (dict-has? env name)
+            (if
+              (er-equal? (get env name) rhs-val)
+              rhs-val
+              (error "Erlang: badmatch (rebind mismatch)"))
+            :else (do (er-env-bind! env name rhs-val) rhs-val)))
+        :else (error
+          "Erlang: pattern matching not yet supported (next Phase 2 step)")))))
+
+;; ── operators ─────────────────────────────────────────────────────
+(define
+  er-eval-op
+  (fn
+    (node env)
+    (let
+      ((op (get node :op)) (args (get node :args)))
+      (cond
+        (= op "andalso") (er-eval-andalso args env)
+        (= op "orelse") (er-eval-orelse args env)
+        :else (er-apply-binop
+          op
+          (er-eval-expr (nth args 0) env)
+          (er-eval-expr (nth args 1) env))))))
+
+(define
+  er-eval-andalso
+  (fn
+    (args env)
+    (let
+      ((a (er-eval-expr (nth args 0) env)))
+      (if (er-truthy? a) (er-eval-expr (nth args 1) env) a))))
+
+(define
+  er-eval-orelse
+  (fn
+    (args env)
+    (let
+      ((a (er-eval-expr (nth args 0) env)))
+      (if (er-truthy? a) a (er-eval-expr (nth args 1) env)))))
+
+(define
+  er-apply-binop
+  (fn
+    (op a b)
+    (cond
+      (= op "+") (+ a b)
+      (= op "-") (- a b)
+      (= op "*") (* a b)
+      (= op "/") (/ a b)
+      (= op "div") (truncate (/ a b))
+      (= op "rem") (remainder a b)
+      (= op "==") (er-bool (er-equal? a b))
+      (= op "/=") (er-bool (not (er-equal? a b)))
+      (= op "=:=") (er-bool (er-exact-equal? a b))
+      (= op "=/=") (er-bool (not (er-exact-equal? a b)))
+      (= op "<") (er-bool (er-lt? a b))
+      (= op ">") (er-bool (er-lt? b a))
+      (= op "=<") (er-bool (not (er-lt? b a)))
+      (= op ">=") (er-bool (not (er-lt? a b)))
+      (= op "++") (er-list-append a b)
+      (= op "and") (er-bool (and (er-truthy? a) (er-truthy? b)))
+      (= op "or") (er-bool (or (er-truthy? a) (er-truthy? b)))
+      :else (error (str "Erlang eval: unsupported operator '" op "'")))))
+
+(define
+  er-eval-unop
+  (fn
+    (node env)
+    (let
+      ((op (get node :op)) (a (er-eval-expr (get node :arg) env)))
+      (cond
+        (= op "-") (- 0 a)
+        (= op "+") a
+        (= op "not") (er-bool (not (er-truthy? a)))
+        :else (error (str "Erlang eval: unsupported unary '" op "'"))))))
+
+;; ── equality / comparison ─────────────────────────────────────────
+(define
+  er-equal?
+  (fn
+    (a b)
+    (cond
+      (and (= (type-of a) "number") (= (type-of b) "number")) (= a b)
+      (and (er-atom? a) (er-atom? b)) (= (get a :name) (get b :name))
+      (and (er-nil? a) (er-nil? b)) true
+      (and (er-cons? a) (er-cons? b))
+      (and
+        (er-equal? (get a :head) (get b :head))
+        (er-equal? (get a :tail) (get b :tail)))
+      (and (er-tuple? a) (er-tuple? b))
+      (let
+        ((ea (get a :elements)) (eb (get b :elements)))
+        (and
+          (= (len ea) (len eb))
+          (every?
+            (fn (i) (er-equal? (nth ea i) (nth eb i)))
+            (range 0 (len ea)))))
+      (and (= (type-of a) "string") (= (type-of b) "string")) (= a b)
+      :else false)))
+
+;; Exact equality: 1 =/= 1.0 in Erlang.
+(define
+  er-exact-equal?
+  (fn
+    (a b)
+    (if
+      (and (= (type-of a) "number") (= (type-of b) "number"))
+      (and (= (integer? a) (integer? b)) (= a b))
+      (er-equal? a b))))
+
+(define
+  er-lt?
+  (fn
+    (a b)
+    (cond
+      (and (= (type-of a) "number") (= (type-of b) "number")) (< a b)
+      (and (er-atom? a) (er-atom? b)) (< (get a :name) (get b :name))
+      (and (= (type-of a) "string") (= (type-of b) "string")) (< a b)
+      :else (< (er-type-order a) (er-type-order b)))))
+
+(define
+  er-type-order
+  (fn
+    (v)
+    (cond
+      (= (type-of v) "number") 0
+      (er-atom? v) 1
+      (er-tuple? v) 2
+      (er-nil? v) 3
+      (er-cons? v) 3
+      (= (type-of v) "string") 4
+      :else 5)))
+
+(define
+  er-list-append
+  (fn
+    (a b)
+    (cond
+      (er-nil? a) b
+      (er-cons? a)
+      (er-mk-cons (get a :head) (er-list-append (get a :tail) b))
+      :else (error "Erlang: ++ left argument is not a proper list"))))
+
+;; ── if ────────────────────────────────────────────────────────────
+(define er-eval-if (fn (node env) (er-eval-if-clauses (get node :clauses) 0 env)))
+
+(define
+  er-eval-if-clauses
+  (fn
+    (clauses i env)
+    (if
+      (>= i (len clauses))
+      (error "Erlang: if: no clause matched")
+      (let
+        ((c (nth clauses i)))
+        (if
+          (er-eval-guards (get c :guards) env)
+          (er-eval-body (get c :body) env)
+          (er-eval-if-clauses clauses (+ i 1) env))))))
+
+;; Guards: outer list = OR, inner list = AND. Empty outer = always pass.
+(define
+  er-eval-guards
+  (fn
+    (alts env)
+    (if (= (len alts) 0) true (er-eval-guards-any alts 0 env))))
+
+(define
+  er-eval-guards-any
+  (fn
+    (alts i env)
+    (if
+      (>= i (len alts))
+      false
+      (if
+        (er-eval-guard-conj (nth alts i) env)
+        true
+        (er-eval-guards-any alts (+ i 1) env)))))
+
+(define er-eval-guard-conj (fn (conj env) (er-eval-guard-conj-iter conj 0 env)))
+
+(define
+  er-eval-guard-conj-iter
+  (fn
+    (conj i env)
+    (if
+      (>= i (len conj))
+      true
+      (if
+        (er-truthy? (er-eval-expr (nth conj i) env))
+        (er-eval-guard-conj-iter conj (+ i 1) env)
+        false))))
diff --git a/plans/erlang-on-sx.md b/plans/erlang-on-sx.md
index 0084a46e..3cc97029 100644
--- a/plans/erlang-on-sx.md
+++ b/plans/erlang-on-sx.md
@@ -57,7 +57,7 @@ Core mapping:
 - [x] Unit tests in `lib/erlang/tests/parse.sx`
 
 ### Phase 2 — sequential eval + pattern matching + BIFs
-- [ ] `erlang-eval-ast`: evaluate sequential expressions
+- [x] `erlang-eval-ast`: evaluate sequential expressions — **54/54 tests**
 - [ ] Pattern matching (atoms, numbers, vars, tuples, lists, `[H|T]`, underscore, bound-var re-match)
 - [ ] Guards: `is_integer`, `is_atom`, `is_list`, `is_tuple`, comparisons, arithmetic
 - [ ] BIFs: `length/1`, `hd/1`, `tl/1`, `element/2`, `tuple_size/1`, `atom_to_list/1`, `list_to_atom/1`, `lists:map/2`, `lists:foldl/3`, `lists:reverse/1`, `io:format/1-2`
@@ -99,6 +99,7 @@ Core mapping:
 
 _Newest first._
 
+- **2026-04-24 eval (sequential) green** — `lib/erlang/transpile.sx` (tree-walking interpreter) + `lib/erlang/tests/eval.sx`. 54/54 tests covering literals, arithmetic, comparison, logical (incl. short-circuit `andalso`/`orelse`), tuples, lists with `++`, `begin..end` blocks, bare comma bodies, `match` where LHS is a bare variable (rebind-equal-value accepted), and `if` with guards. Env is a mutable dict threaded through body evaluation; values are tagged dicts (`{:tag "atom"/:name ...}`, `{:tag "nil"}`, `{:tag "cons" :head :tail}`, `{:tag "tuple" :elements}`). Numbers pass through as SX numbers. Gotcha: SX's `parse-number` coerces `"1.0"` → integer `1`, so `=:=` can't distinguish `1` from `1.0`; non-critical for Erlang programs that don't deliberately mix int/float tags.
 - **parser green** — `lib/erlang/parser.sx` + `parser-core.sx` + `parser-expr.sx` + `parser-module.sx`. 52/52 in `tests/parse.sx`. Covers literals, tuples, lists (incl. `[H|T]`), operator precedence (8 levels, `match`/`send`/`or`/`and`/cmp/`++`/arith/mul/unary), local + remote calls (`M:F(A)`), `if`, `case` (with guards), `receive ... after ... end`, `begin..end` blocks, anonymous `fun`, `try..of..catch..after..end` with `Class:Pattern` catch clauses. Module-level: `-module(M).`, `-export([...]).`, multi-clause functions with guards. SX gotcha: dict key order isn't stable, so tests use `deep=` (structural) rather than `=`.
 - **tokenizer green** — `lib/erlang/tokenizer.sx` + `lib/erlang/tests/tokenize.sx`. Covers atoms (bare, quoted, `node@host`), variables, integers (incl. `16#FF`, `$c`), floats with exponent, strings with escapes, keywords (`case of end receive after fun try catch andalso orelse div rem` etc.), punct (`( ) { } [ ] , ; . : :: -> <- <= => << >> | ||`), ops (`+ - * / = == /= =:= =/= < > =< >= ++ -- ! ?`), `%` line comments. 62/62 green.