apl: train/fork notation (f g h) and (g h) (+6 tests, 496/496)

Parser: when a parenthesised subexpression contains only function
segments (>= 2), collect-segments-loop now emits a :train AST node
instead of treating it as a value-producing expression.

Resolver: apl-resolve-{monadic,dyadic} handle :train.
- monadic 2-train (atop):  (g h)⍵ = g (h ⍵)
- monadic 3-train (fork):  (f g h)⍵ = (f ⍵) g (h ⍵)
- dyadic 2-train:          ⍺(g h)⍵ = g (⍺ h ⍵)
- dyadic 3-train:          ⍺(f g h)⍵ = (⍺ f ⍵) g (⍺ h ⍵)

apl-run "(+/÷≢) 1 2 3 4 5"  → 3   (mean)
apl-run "(- ⌊) 5"           → -5  (atop)
apl-run "2 (+ × -) 5"       → -21 (dyadic fork)
apl-run "(⌈/-⌊/) 3 1 4 …"   → 8   (range)

lib/apl/parser.sx

@@ -308,11 +308,23 @@
                   ((inner-tokens (slice tokens (+ i 1) end))
                     (after (+ end 1)))
                   (let
-                    ((br (maybe-bracket (parse-apl-expr inner-tokens) tokens after)))
-                    (collect-segments-loop
-                      tokens
-                      (nth br 1)
-                      (append acc {:kind "val" :node (nth br 0)}))))))
+                    ((inner-segs (collect-segments inner-tokens)))
+                    (if
+                      (and
+                        (>= (len inner-segs) 2)
+                        (every? (fn (s) (= (get s :kind) "fn")) inner-segs))
+                      (let
+                        ((train-node (cons :train (map (fn (s) (get s :node)) inner-segs))))
+                        (collect-segments-loop
+                          tokens
+                          after
+                          (append acc {:kind "fn" :node train-node})))
+                      (let
+                        ((br (maybe-bracket (parse-apl-expr inner-tokens) tokens after)))
+                        (collect-segments-loop
+                          tokens
+                          (nth br 1)
+                          (append acc {:kind "val" :node (nth br 0)}))))))))
             ((= tt :lbrace)
               (let
                 ((end (find-matching-close tokens (+ i 1) :lbrace :rbrace)))

lib/apl/tests/pipeline.sx

@@ -282,3 +282,33 @@
   "multi-axis: select all rows of column 3"
   (mkrv (apl-run "M ← (4 3) ⍴ 1 2 3 4 5 6 7 8 9 10 11 12 ⋄ M[;3]"))
   (list 3 6 9 12))
+
+(apl-test
+  "train: mean = (+/÷≢) on 1..5"
+  (mkrv (apl-run "(+/÷≢) 1 2 3 4 5"))
+  (list 3))
+
+(apl-test
+  "train: mean of 2 4 6 8 10"
+  (mkrv (apl-run "(+/÷≢) 2 4 6 8 10"))
+  (list 6))
+
+(apl-test
+  "train 2-atop: (- ⌊) 5 → -5"
+  (mkrv (apl-run "(- ⌊) 5"))
+  (list -5))
+
+(apl-test
+  "train 3-fork dyadic: 2(+×-)5 → -21"
+  (mkrv (apl-run "2 (+ × -) 5"))
+  (list -21))
+
+(apl-test
+  "train: range = (⌈/-⌊/) on vector"
+  (mkrv (apl-run "(⌈/-⌊/) 3 1 4 1 5 9 2 6"))
+  (list 8))
+
+(apl-test
+  "train: mean of ⍳10 has shape ()"
+  (mksh (apl-run "(+/÷≢) ⍳10"))
+  (list))

lib/apl/transpile.sx

@@ -451,6 +451,24 @@
                   (= (first bound) :dfn))
                 (fn (arg) (apl-call-dfn-m bound arg))
                 (error "apl-resolve-monadic: name not bound to dfn")))))
+        ((= tag :train)
+          (let
+            ((fns (rest fn-node)))
+            (let
+              ((n (len fns)))
+              (cond
+                ((= n 2)
+                  (let
+                    ((g (apl-resolve-monadic (nth fns 0) env))
+                      (h (apl-resolve-monadic (nth fns 1) env)))
+                    (fn (arg) (g (h arg)))))
+                ((= n 3)
+                  (let
+                    ((f (apl-resolve-monadic (nth fns 0) env))
+                      (g (apl-resolve-dyadic (nth fns 1) env))
+                      (h (apl-resolve-monadic (nth fns 2) env)))
+                    (fn (arg) (g (f arg) (h arg)))))
+                (else (error "monadic train arity not 2 or 3"))))))
         (else (error "apl-resolve-monadic: unknown fn-node tag"))))))
 
 (define
@@ -499,6 +517,24 @@
               ((f (apl-resolve-dyadic f-node env))
                 (g (apl-resolve-dyadic g-node env)))
               (fn (a b) (apl-inner f g a b)))))
+        ((= tag :train)
+          (let
+            ((fns (rest fn-node)))
+            (let
+              ((n (len fns)))
+              (cond
+                ((= n 2)
+                  (let
+                    ((g (apl-resolve-monadic (nth fns 0) env))
+                      (h (apl-resolve-dyadic (nth fns 1) env)))
+                    (fn (a b) (g (h a b)))))
+                ((= n 3)
+                  (let
+                    ((f (apl-resolve-dyadic (nth fns 0) env))
+                      (g (apl-resolve-dyadic (nth fns 1) env))
+                      (h (apl-resolve-dyadic (nth fns 2) env)))
+                    (fn (a b) (g (f a b) (h a b)))))
+                (else (error "dyadic train arity not 2 or 3"))))))
         (else (error "apl-resolve-dyadic: unknown fn-node tag"))))))
 
 (define apl-run (fn (src) (apl-eval-ast (parse-apl src) {})))