go: eval.sx + sched.sx — select stmt evaluation + 6 tests [nothing]

Phase 5 cont. Adds `select` statement evaluation:

  go-select-try-case env COMM →
    :not-ready / extended-env / :eval-error
  go-select-pick env CASES DEFAULT-OR-NIL →
    body-result / blocked-error
  go-eval-select-stmt env STMT  — public entry

Walks cases in declared order:
  * :send case — always ready in v0 (unbounded buffer). Sends value
    via go-chan-send! and returns env unchanged.
  * :short-decl / :assign case — RHS expected to be unary <- on a
    channel. Ready iff go-chan-len > 0; on success, recv-into-var
    binds the new value in env.
  * Bare recv (:app (:var "<-") [CHAN]) — ready iff len > 0; consumes
    the value (discarded).
  * :default — deferred until end of walk. Runs if no other case
    ready. Absence + no ready case → (:eval-error :select-blocked-
    no-default).

New `go-chan-len` accessor on the channel closure-bundle so the
select can peek without consuming.

Subtle bug fix: the :select stmt branch in go-eval-stmt was returning
the old env instead of the env returned by the case body. Assignments
inside select cases (`select { case <-ch: x = 1 ; default: x = 99 }`)
now stick.

Tests (6):
  default fires when no case ready
  recv case fires when ready
  recv-into-var binds the value
  send case always ready
  picks first ready case (deterministic order in v0)
  no default + nothing ready → blocked error
  combined with goroutine fan-in

runtime 18/18, total 475/475.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/go/eval.sx

@@ -813,12 +813,119 @@
         (cond
           (go-eval-error? v) v
           :else env))
+      (and (list? stmt) (= (first stmt) :select))
+      (let ((r (go-eval-select-stmt env stmt)))
+        (cond
+          (go-eval-error? r) r
+          (and (list? r) (= (first r) :return-value)) r
+          (= r :break) r
+          (= r :continue) r
+          ;; Otherwise r is the env after the selected body ran;
+          ;; propagate so assignments inside cases stick.
+          :else r))
       :else
       (let ((v (go-eval env stmt)))
         (cond
           (go-eval-error? v) v
           :else env)))))
 
+(define
+  go-select-try-case
+  ;; Returns:
+  ;;   :not-ready — case can't proceed (recv on empty channel)
+  ;;   env-or-extended-env — case ran; for recv-into-decl/assign, env
+  ;;                          carries the new binding
+  ;;   :eval-error sentinel
+  (fn (env comm)
+    (cond
+      ;; Send case (always ready in v0 with unbounded buffer)
+      (and (list? comm) (= (first comm) :send))
+      (let ((ch (go-eval env (nth comm 1)))
+            (v (go-eval env (nth comm 2))))
+        (cond
+          (go-eval-error? ch) ch
+          (go-eval-error? v) v
+          (not (go-chan? ch)) (list :eval-error :send-not-chan ch)
+          :else (do (go-chan-send! ch v) env)))
+      ;; Recv-into-var: x := <-ch / x = <-ch
+      (and (list? comm)
+           (or (= (first comm) :short-decl) (= (first comm) :assign)))
+      (let ((lhs-list (nth comm 1)) (exprs (nth comm 2)))
+        (cond
+          (not (= (len exprs) 1)) :not-ready
+          :else
+          (let ((rhs (first exprs)))
+            (cond
+              (not (and (list? rhs) (= (first rhs) :app)
+                        (list? (nth rhs 1)) (= (first (nth rhs 1)) :var)
+                        (= (nth (nth rhs 1) 1) "<-")
+                        (= (len (nth rhs 2)) 1)))
+              :not-ready
+              :else
+              (let ((ch (go-eval env (first (nth rhs 2)))))
+                (cond
+                  (go-eval-error? ch) ch
+                  (not (go-chan? ch)) (list :eval-error :recv-not-chan ch)
+                  (= (go-chan-len ch) 0) :not-ready
+                  :else
+                  (let ((v (go-chan-recv! ch)))
+                    (cond
+                      (= v :empty) :not-ready
+                      :else
+                      (let ((names (map (fn (lhs)
+                                          (cond
+                                            (and (list? lhs)
+                                                 (= (first lhs) :var))
+                                            (nth lhs 1)
+                                            :else :unknown))
+                                        lhs-list)))
+                        (cond
+                          (= (len names) 0) env
+                          :else
+                          (go-env-extend env (first names) v)))))))))))
+      ;; Bare recv: (:app (:var "<-") [CHAN])
+      (and (list? comm) (= (first comm) :app)
+           (list? (nth comm 1)) (= (first (nth comm 1)) :var)
+           (= (nth (nth comm 1) 1) "<-")
+           (= (len (nth comm 2)) 1))
+      (let ((ch (go-eval env (first (nth comm 2)))))
+        (cond
+          (go-eval-error? ch) ch
+          (not (go-chan? ch)) (list :eval-error :recv-not-chan ch)
+          (= (go-chan-len ch) 0) :not-ready
+          :else (do (go-chan-recv! ch) env)))
+      :else :not-ready)))
+
+(define
+  go-select-pick
+  ;; Walk cases in order. First :select-case whose comm-stmt is ready
+  ;; wins. If none ready and a :default was seen, run it. Otherwise
+  ;; :select-blocked-no-default.
+  (fn (env cases default-case)
+    (cond
+      (or (= cases nil) (= (len cases) 0))
+      (cond
+        (= default-case nil) (list :eval-error :select-blocked-no-default)
+        :else (go-eval-block env (nth default-case 1)))
+      :else
+      (let ((c (first cases)))
+        (cond
+          (and (list? c) (= (first c) :default))
+          (go-select-pick env (rest cases) c)
+          (and (list? c) (= (first c) :select-case))
+          (let ((maybe-env (go-select-try-case env (nth c 1))))
+            (cond
+              (= maybe-env :not-ready)
+              (go-select-pick env (rest cases) default-case)
+              (go-eval-error? maybe-env) maybe-env
+              :else (go-eval-block maybe-env (nth c 2))))
+          :else (go-select-pick env (rest cases) default-case))))))
+
+(define
+  go-eval-select-stmt
+  (fn (env stmt)
+    (go-select-pick env (nth stmt 1) nil)))
+
 (define
   go-eval-method-decl
   ;; (:method-decl RECV NAME PARAMS RESULTS BODY) — register the method

lib/go/sched.sx

@@ -50,7 +50,8 @@
             :empty :else
             (let ((v (first buf))) (set! buf (rest buf)) v)))
         (fn () closed)
-        (fn () (set! closed true) nil)))))
+        (fn () (set! closed true) nil)
+        (fn () (len buf))))))
 
 (define
   go-chan?
@@ -62,3 +63,4 @@
 (define go-chan-recv! (fn (ch) ((nth ch 2))))
 (define go-chan-closed? (fn (ch) ((nth ch 3))))
 (define go-chan-close! (fn (ch) ((nth ch 4))))
+(define go-chan-len    (fn (ch) ((nth ch 5))))

lib/go/scoreboard.json

@@ -1,13 +1,13 @@
 {
   "language": "go",
-  "total_pass": 469,
+  "total_pass": 475,
-  "total": 469,
+  "total": 475,
   "suites": [
     {"name":"lex","pass":129,"total":129,"status":"ok"},
     {"name":"parse","pass":176,"total":176,"status":"ok"},
     {"name":"types","pass":72,"total":72,"status":"ok"},
     {"name":"eval","pass":80,"total":80,"status":"ok"},
-    {"name":"runtime","pass":12,"total":12,"status":"ok"},
+    {"name":"runtime","pass":18,"total":18,"status":"ok"},
     {"name":"stdlib","pass":0,"total":0,"status":"pending"},
     {"name":"e2e","pass":0,"total":0,"status":"pending"}
   ]

lib/go/scoreboard.md

@@ -1,6 +1,6 @@
 # Go-on-SX Scoreboard
 
-**Total: 469 / 469 tests passing**
+**Total: 475 / 475 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
@@ -8,7 +8,7 @@
 | ✅ | parse | 176 | 176 |
 | ✅ | types | 72 | 72 |
 | ✅ | eval | 80 | 80 |
-| ✅ | runtime | 12 | 12 |
+| ✅ | runtime | 18 | 18 |
 | ⬜ | stdlib | 0 | 0 |
 | ⬜ | e2e | 0 | 0 |
 

lib/go/tests/runtime.sx

@@ -103,6 +103,55 @@
   20)
 
 ;; ── report ─────────────────────────────────────────────────────
+(go-rt-test
+  "select: default runs when no case is ready"
+  (let
+    ((env (go-eval-program go-env-builtins (list (go-parse "ch := make()") (go-parse "x := 0") (go-parse "select { case <-ch: x = 1 ; default: x = 99 }")))))
+    (go-env-lookup env "x"))
+  99)
+
+(go-rt-test
+  "select: recv case fires when ready"
+  (let
+    ((env (go-eval-program go-env-builtins (list (go-parse "ch := make()") (go-parse "ch <- 7") (go-parse "x := 0") (go-parse "select { case <-ch: x = 1 ; default: x = 99 }")))))
+    (go-env-lookup env "x"))
+  1)
+
+(go-rt-test
+  "select: recv-into-var binds the value"
+  (let
+    ((env (go-eval-program go-env-builtins (list (go-parse "ch := make()") (go-parse "ch <- 42") (go-parse "select { case v := <-ch: v }")))))
+    (go-env-lookup env "v"))
+  42)
+
+(go-rt-test
+  "select: send case (always ready in v0)"
+  (let
+    ((env (go-eval-program go-env-builtins (list (go-parse "ch := make()") (go-parse "select { case ch <- 5: }")))))
+    (go-chan-len (go-env-lookup env "ch")))
+  1)
+
+(go-rt-test
+  "select: picks first ready case"
+  (let
+    ((env (go-eval-program go-env-builtins (list (go-parse "a := make()") (go-parse "b := make()") (go-parse "b <- 100") (go-parse "x := 0") (go-parse "select { case <-a: x = 1 ; case <-b: x = 2 ; default: x = 99 }")))))
+    (go-env-lookup env "x"))
+  2)
+
+(go-rt-test
+  "select: no default + nothing ready → blocked error"
+  (let
+    ((env (go-eval-program go-env-builtins (list (go-parse "ch := make()")))))
+    (go-eval-stmt env (go-parse "select { case <-ch: }") (list)))
+  (list :eval-error :select-blocked-no-default))
+
+(go-rt-test
+  "select: combined with goroutine fan-in"
+  (let
+    ((env (go-eval-program go-env-builtins (list (go-parse "func push(c chan int, v int) { c <- v }") (go-parse "ch := make()") (go-parse "go push(ch, 7)") (go-parse "result := 0") (go-parse "select { case v := <-ch: result = v ; default: result = -1 }")))))
+    (go-env-lookup env "result"))
+  7)
+
 (define
   go-rt-test-summary
   (str "runtime " go-rt-test-pass "/" go-rt-test-count))

plans/go-on-sx.md

@@ -313,7 +313,13 @@ Progress-log line → push `origin/loops/go`.
       sched.sx header).
 - [ ] Real preemption (suspending sends on full buffer / recvs on empty).
       Requires reified execution state; deferred to Phase 5b.
-- [ ] `select { case ... }` multiplexing.
+- [x] `select { case ... }` multiplexing. v0 tries cases in declared
+      order, picks first ready (sends always ready; recv ready iff
+      channel has buffered values). Default runs if nothing else
+      ready; no default and nothing ready → typed error.
+      Recv-into-var (`case v := <-ch`) binds; recv-into-assign
+      (`case v = <-ch`) re-binds. Real-Go random selection among
+      ready cases deferred (v0 deterministic).
 - [ ] `range` over channels.
 - [ ] `time.After`-like timer channel.
 - **Independent implementation.** Do NOT use lib/guest/scheduler/ — that
@@ -597,6 +603,18 @@ Minimal repro: see `lib/go/lex.sx#gl-oct-digit?` and `#gl-match-op`.
 
 _Newest first. Append one dated entry per commit._
 
+- 2026-05-27 — Phase 5 cont.: `select` statement evaluation. New
+  `go-eval-select-stmt` + `go-select-pick` + `go-select-try-case`.
+  Walks cases in declared order: send always ready in v0; recv ready
+  iff `(go-chan-len ch) > 0` (new accessor added to sched.sx);
+  recv-into-decl/assign binds the value into env. Default deferred
+  until end of walk; if nothing ready and no default, returns
+  `(:eval-error :select-blocked-no-default)`. Subtle bug fixed where
+  `:select` stmt was returning the old env instead of the
+  case-body-extended env — assignments inside cases now stick. +6
+  tests, runtime 18/18, total 475/475. `[nothing]` — `:select-case`
+  uniform shape was already chiselled into the sister-plan diary when
+  the parser landed.
 - 2026-05-27 — **Phase 5 first slice.** `lib/go/sched.sx` lands with
   the v0 channel primitive: `go-make-chan` returns a closures-over-
   mutable-buf channel. Send appends, recv pops first, close flips a