IO predicates: term_to_atom/2, term_string/2, with_output_to/2, format/1,2, writeln/1

Adds 6 new built-in predicates to the Prolog runtime and 24 tests covering
term<->atom conversion (bidirectional), output capture, format directives (~w/~a/~d/~n/~~).
456/456 tests passing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/prolog/tests/io_predicates.sx

@@ -0,0 +1,326 @@
+;; lib/prolog/tests/io_predicates.sx — term_to_atom/2, term_string/2,
+;;   with_output_to/2, writeln/1, format/1, format/2
+
+(define pl-io-test-count 0)
+(define pl-io-test-pass 0)
+(define pl-io-test-fail 0)
+(define pl-io-test-failures (list))
+
+(define
+  pl-io-test!
+  (fn
+    (name got expected)
+    (begin
+      (set! pl-io-test-count (+ pl-io-test-count 1))
+      (if
+        (= got expected)
+        (set! pl-io-test-pass (+ pl-io-test-pass 1))
+        (begin
+          (set! pl-io-test-fail (+ pl-io-test-fail 1))
+          (append!
+            pl-io-test-failures
+            (str name "\n    expected: " expected "\n    got:      " got)))))))
+
+(define
+  pl-io-goal
+  (fn
+    (src env)
+    (pl-instantiate (nth (first (pl-parse (str "g :- " src "."))) 2) env)))
+
+(define pl-io-db (pl-mk-db))
+
+;; helper: get output buffer after running a goal
+(define
+  pl-io-capture!
+  (fn
+    (goal)
+    (do
+      (pl-output-clear!)
+      (pl-solve-once! pl-io-db goal (pl-mk-trail))
+      pl-output-buffer)))
+
+;; ─── term_to_atom/2 — bound Term direction ─────────────────────────────────
+
+(pl-io-test!
+  "term_to_atom(foo(a,b), A) — compound"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "term_to_atom(foo(a,b), A)" env)
+      (pl-mk-trail))
+    (pl-atom-name (pl-walk-deep (dict-get env "A"))))
+  "foo(a, b)")
+
+(pl-io-test!
+  "term_to_atom(hello, A) — atom"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "term_to_atom(hello, A)" env)
+      (pl-mk-trail))
+    (pl-atom-name (pl-walk-deep (dict-get env "A"))))
+  "hello")
+
+(pl-io-test!
+  "term_to_atom(42, A) — number"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "term_to_atom(42, A)" env)
+      (pl-mk-trail))
+    (pl-atom-name (pl-walk-deep (dict-get env "A"))))
+  "42")
+
+(pl-io-test!
+  "term_to_atom(foo(a,b), 'foo(a, b)') — succeeds when Atom matches"
+  (pl-solve-once!
+    pl-io-db
+    (pl-io-goal "term_to_atom(foo(a,b), 'foo(a, b)')" {})
+    (pl-mk-trail))
+  true)
+
+(pl-io-test!
+  "term_to_atom(hello, world) — fails on mismatch"
+  (pl-solve-once!
+    pl-io-db
+    (pl-io-goal "term_to_atom(hello, world)" {})
+    (pl-mk-trail))
+  false)
+
+;; ─── term_to_atom/2 — parse direction (Atom bound, Term unbound) ───────────
+
+(pl-io-test!
+  "term_to_atom(T, 'foo(a)') — parse direction gives compound"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "term_to_atom(T, 'foo(a)')" env)
+      (pl-mk-trail))
+    (let
+      ((t (pl-walk-deep (dict-get env "T"))))
+      (and (pl-compound? t) (= (pl-fun t) "foo"))))
+  true)
+
+(pl-io-test!
+  "term_to_atom(T, hello) — parse direction gives atom"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "term_to_atom(T, hello)" env)
+      (pl-mk-trail))
+    (let
+      ((t (pl-walk-deep (dict-get env "T"))))
+      (and (pl-atom? t) (= (pl-atom-name t) "hello"))))
+  true)
+
+;; ─── term_string/2 — alias ──────────────────────────────────────────────────
+
+(pl-io-test!
+  "term_string(bar(x), A) — same as term_to_atom"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "term_string(bar(x), A)" env)
+      (pl-mk-trail))
+    (pl-atom-name (pl-walk-deep (dict-get env "A"))))
+  "bar(x)")
+
+(pl-io-test!
+  "term_string(42, A) — number to string"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "term_string(42, A)" env)
+      (pl-mk-trail))
+    (pl-atom-name (pl-walk-deep (dict-get env "A"))))
+  "42")
+
+;; ─── writeln/1 ─────────────────────────────────────────────────────────────
+
+(pl-io-test!
+  "writeln(hello) writes 'hello\n'"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "with_output_to(atom(X), writeln(hello))" env)
+      (pl-mk-trail))
+    (pl-atom-name (pl-walk-deep (dict-get env "X"))))
+  "hello
+")
+
+(pl-io-test!
+  "writeln(42) writes '42\n'"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "with_output_to(atom(X), writeln(42))" env)
+      (pl-mk-trail))
+    (pl-atom-name (pl-walk-deep (dict-get env "X"))))
+  "42
+")
+
+;; ─── with_output_to/2 ──────────────────────────────────────────────────────
+
+(pl-io-test!
+  "with_output_to(atom(X), write(foo)) — captures write output"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "with_output_to(atom(X), write(foo))" env)
+      (pl-mk-trail))
+    (pl-atom-name (pl-walk-deep (dict-get env "X"))))
+  "foo")
+
+(pl-io-test!
+  "with_output_to(atom(X), (write(a), write(b))) — concat output"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "with_output_to(atom(X), (write(a), write(b)))" env)
+      (pl-mk-trail))
+    (pl-atom-name (pl-walk-deep (dict-get env "X"))))
+  "ab")
+
+(pl-io-test!
+  "with_output_to(atom(X), nl) — captures newline"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "with_output_to(atom(X), nl)" env)
+      (pl-mk-trail))
+    (pl-atom-name (pl-walk-deep (dict-get env "X"))))
+  "
+")
+
+(pl-io-test!
+  "with_output_to(atom(X), true) — captures empty string"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "with_output_to(atom(X), true)" env)
+      (pl-mk-trail))
+    (pl-atom-name (pl-walk-deep (dict-get env "X"))))
+  "")
+
+(pl-io-test!
+  "with_output_to(string(X), write(hello)) — string sink works"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "with_output_to(string(X), write(hello))" env)
+      (pl-mk-trail))
+    (pl-atom-name (pl-walk-deep (dict-get env "X"))))
+  "hello")
+
+(pl-io-test!
+  "with_output_to(atom(X), fail) — fails when goal fails"
+  (pl-solve-once!
+    pl-io-db
+    (pl-io-goal "with_output_to(atom(X), fail)" {})
+    (pl-mk-trail))
+  false)
+
+;; ─── format/1 ──────────────────────────────────────────────────────────────
+
+(pl-io-test!
+  "format('hello~n') — tilde-n becomes newline"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "with_output_to(atom(X), format('hello~n'))" env)
+      (pl-mk-trail))
+    (pl-atom-name (pl-walk-deep (dict-get env "X"))))
+  "hello
+")
+
+(pl-io-test!
+  "format('~~') — double tilde becomes single tilde"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "with_output_to(atom(X), format('~~'))" env)
+      (pl-mk-trail))
+    (pl-atom-name (pl-walk-deep (dict-get env "X"))))
+  "~")
+
+(pl-io-test!
+  "format('abc') — plain text passes through"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "with_output_to(atom(X), format(abc))" env)
+      (pl-mk-trail))
+    (pl-atom-name (pl-walk-deep (dict-get env "X"))))
+  "abc")
+
+;; ─── format/2 ──────────────────────────────────────────────────────────────
+
+(pl-io-test!
+  "format('~w+~w', [1,2]) — two ~w args"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "with_output_to(atom(X), format('~w+~w', [1,2]))" env)
+      (pl-mk-trail))
+    (pl-atom-name (pl-walk-deep (dict-get env "X"))))
+  "1+2")
+
+(pl-io-test!
+  "format('hello ~a!', [world]) — ~a with atom arg"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "with_output_to(atom(X), format('hello ~a!', [world]))" env)
+      (pl-mk-trail))
+    (pl-atom-name (pl-walk-deep (dict-get env "X"))))
+  "hello world!")
+
+(pl-io-test!
+  "format('n=~d', [42]) — ~d with integer arg"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "with_output_to(atom(X), format('n=~d', [42]))" env)
+      (pl-mk-trail))
+    (pl-atom-name (pl-walk-deep (dict-get env "X"))))
+  "n=42")
+
+(pl-io-test!
+  "format('~w', [foo(a)]) — ~w with compound"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-io-db
+      (pl-io-goal "with_output_to(atom(X), format('~w', [foo(a)]))" env)
+      (pl-mk-trail))
+    (pl-atom-name (pl-walk-deep (dict-get env "X"))))
+  "foo(a)")
+
+(define
+  pl-io-predicates-tests-run!
+  (fn
+    ()
+    {:failed pl-io-test-fail
+      :passed pl-io-test-pass
+      :total pl-io-test-count
+      :failures pl-io-test-failures}))