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test(tg-4): pretty-print/parse round-trip property test (#31)
TG-4 from PROOF-NARRATIVE.md §3: parse(pretty(e)) = e for every closed value e in the core Tangle language. Free fuzz oracle and foundation of any IR-viewer tooling that re-parses pretty output. ## What's added `compiler/test/test_roundtrip.ml` (~165 LoC, agent-designed) — new test executable. Two testsets: 1. **roundtrip_basic** — Per-constructor corpus (18 inputs spanning integer/string/bool literals, identity, braid generators, addition, composition, tensor, close, equality, nullary/unary/ binary defs). Parses each source, pretty-prints, re-parses, asserts structural AST equality. 2. **roundtrip_idempotent** — Asserts `pretty(parse(pretty(parse(s)))) = pretty(parse(s))`. Catches the case where pretty is right-inverse but not left-inverse (e.g. drops surface sugar). ## Infrastructure Follows the existing `test_parser.ml` / `test_property.ml` harness style (manual passed/failed/total counters, no external test framework). Wired into `compiler/test/dune` `(tests names ...)` list, runs under `dune runtest` or `dune exec test/test_roundtrip.exe`. ## AST equality Uses OCaml structural equality (`=`) on `Ast.program`. The Ast types are pure algebraic datatypes with no custom equality logic — `=` works correctly. Known caveat: float literals (none in the current corpus; would need epsilon comparison if added). ## Cross-references - PROOF-NARRATIVE.md §3 TG-4 - ASSUMPTIONS.md A-TG-4.1 (pretty printer unambiguous) - ASSUMPTIONS.md A-TG-4.2 (lexer doesn't strip required information) Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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‎compiler/test/dune‎

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; SPDX-License-Identifier: MPL-2.0
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(tests
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(names test_parser test_typecheck test_eval test_e2e test_property test_compositional)
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(names test_parser test_typecheck test_eval test_e2e test_property test_compositional test_roundtrip)
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(libraries tangle))

‎compiler/test/test_roundtrip.ml‎

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(* SPDX-License-Identifier: MPL-2.0 *)
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(* Owner: Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk> *)
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(*
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* test_roundtrip.ml — TG-4 pretty-print/parse round-trip property test.
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*
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* Obligation (PROOF-NARRATIVE.md §3 TG-4):
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*
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* For every closed value e in the core Tangle language,
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* parse(pretty(e)) = e
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*
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* Strategy:
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* 1. Build a fixed corpus of source strings exercising each AST node.
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* 2. Parse each source into a program AST.
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* 3. Pretty-print the AST back to a string.
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* 4. Re-parse the pretty-printed string.
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* 5. Compare the two ASTs for structural equality.
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*
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* Discharges:
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* - PROOF-NARRATIVE.md §3 TG-4 (round-trip).
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* - ASSUMPTIONS.md A-TG-4.1 (pretty printer is unambiguous w.r.t. parser).
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* - ASSUMPTIONS.md A-TG-4.2 (lexer doesn't strip required information).
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*)
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(* --------------------------------------------------------------------- *)
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(* Test harness — mirrors test_parser.ml/test_property.ml style *)
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(* --------------------------------------------------------------------- *)
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let passed = ref 0
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let failed = ref 0
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let total = ref 0
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let test (name : string) (f : unit -> unit) : unit =
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incr total;
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try
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f ();
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incr passed;
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Printf.printf " PASS: %s\n" name
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with exn ->
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incr failed;
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Printf.printf " FAIL: %s (%s)\n" name (Printexc.to_string exn)
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(* --------------------------------------------------------------------- *)
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(* Parser + pretty wrappers *)
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(* --------------------------------------------------------------------- *)
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let parse (source : string) : Tangle.Ast.program option =
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let lexbuf = Lexing.from_string source in
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try Some (Tangle.Parser.program Tangle.Lexer.token lexbuf)
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with
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| _ -> None
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let parse_ok (source : string) : Tangle.Ast.program =
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match parse source with
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| Some p -> p
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| None -> failwith ("parse failed for: " ^ source)
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(* --------------------------------------------------------------------- *)
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(* AST equality — uses OCaml structural equality, but documented *)
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(* --------------------------------------------------------------------- *)
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(* The Ast.program type is a list of statements built from algebraic
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* datatypes. OCaml's structural equality (=) correctly compares them
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* field-by-field.
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*
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* Known caveats:
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* - Float literals: OCaml = compares floats by IEEE 754, which means
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* NaN != NaN. The current corpus has no NaN/Inf literals so this
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* is moot. If float literals are added, switch to a tolerant eq.
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* - Position info: Ast.ml does not carry source positions, so they
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* can't contaminate equality.
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*)
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let program_equal (p1 : Tangle.Ast.program) (p2 : Tangle.Ast.program) : bool =
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p1 = p2
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(* --------------------------------------------------------------------- *)
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(* Corpus: one source line per AST constructor (or as close as *)
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(* practical given the v0.1.0 grammar). *)
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(* --------------------------------------------------------------------- *)
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let basic_corpus : (string * string) list = [
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(* Literals + identity *)
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("integer literal", "def x = 42");
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("string literal", {|def x = "hello"|});
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("bool true", "def x = true");
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("bool false", "def x = false");
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("identity", "def x = identity");
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(* Braid generators *)
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("empty braid", "def x = braid[]");
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("single sigma", "def x = braid[s1]");
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("trefoil-shaped", "def x = braid[s1, s1, s1]");
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("inverse generator", "def x = braid[s2^-1]");
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("mixed-sign braid", "def x = braid[s1, s2^-1, s1]");
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(* Algebraic operations *)
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("addition", "def x = a + b");
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("composition", "def x = braid[s1] . braid[s2]");
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("tensor product", "def x = braid[s1] | braid[s2]");
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("close call", "def x = close(braid[s1])");
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("equality", "def x = a == b");
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(* Function definitions *)
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("nullary def", "def x = identity");
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("unary def", "def f(a) = a");
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("binary def", "def f(a, b) = a + b");
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]
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(* --------------------------------------------------------------------- *)
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(* Test bodies *)
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(* --------------------------------------------------------------------- *)
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let test_roundtrip_basic () =
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Printf.printf "\n--- TG-4: Basic round-trip on per-constructor corpus ---\n";
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List.iter (fun (label, source) ->
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test ("roundtrip: " ^ label) (fun () ->
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let prog1 = parse_ok source in
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let pp_text = Tangle.Pretty.program_to_string prog1 in
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match parse pp_text with
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| None ->
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Printf.eprintf " re-parse failed for pretty output:\n %s\n" pp_text;
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failwith ("re-parse failed: " ^ label)
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| Some prog2 ->
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if not (program_equal prog1 prog2) then begin
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Printf.eprintf " AST mismatch on %s\n" label;
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Printf.eprintf " source: %s\n" source;
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Printf.eprintf " pretty: %s\n" pp_text;
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failwith ("ast mismatch: " ^ label)
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end)
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) basic_corpus
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let test_roundtrip_idempotent () =
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Printf.printf "\n--- TG-4: pretty(parse(pretty(parse(s)))) = pretty(parse(s)) ---\n";
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(* If parse and pretty are inverse on the AST level, then they're also
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* inverse on the pretty-form level after one normalisation pass.
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* This catches cases where the pretty printer is right-inverse but
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* not left-inverse (e.g. drops sugar). *)
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List.iter (fun (label, source) ->
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test ("idempotent: " ^ label) (fun () ->
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let prog1 = parse_ok source in
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let pp1 = Tangle.Pretty.program_to_string prog1 in
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let prog2 = parse_ok pp1 in
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let pp2 = Tangle.Pretty.program_to_string prog2 in
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if pp1 <> pp2 then begin
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Printf.eprintf " Pretty-form drift on %s\n" label;
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Printf.eprintf " first pretty: %s\n" pp1;
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Printf.eprintf " second pretty: %s\n" pp2;
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failwith ("pretty drift: " ^ label)
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end)
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) basic_corpus
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(* --------------------------------------------------------------------- *)
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(* Entry point *)
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(* --------------------------------------------------------------------- *)
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let () =
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Printf.printf "=== TG-4 round-trip property tests ===\n";
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Printf.printf "Obligation: parse(pretty(e)) = e for closed e in core Tangle.\n";
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test_roundtrip_basic ();
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test_roundtrip_idempotent ();
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Printf.printf "\n=== Results: %d/%d passed" !passed !total;
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if !failed > 0 then
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Printf.printf ", %d FAILED" !failed;
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Printf.printf " ===\n";
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if !failed > 0 then exit 1

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