Add coverage corpus

conformance/SURFACE.md

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+# SURFACE — what `js/mpl.js` actually implements
+
+Audited from the source of `js/mpl.js` (Stage 2, C3) and pinned by the
+corpus. This file describes the implemented surface — exactly, no more.
+Nothing here is ratified; where behavior looks accidental it is flagged in
+`conformance/JUDGMENT_CALLS.md`, but it is recorded as observed.
+
+## Lexer
+
+- Whitespace: space, tab, CR, LF.
+- Comments: line `-- …`; block `{- … -}`, nesting tracked (unterminated →
+  `err_comment`).
+- Strings: `"…"`, escape `\n` → newline, `\t` → tab, any other `\x` → `x`
+  (the backslash is silently dropped — `\q` becomes `q`); unterminated →
+  `err_string`.
+- ASCII escapes: `\word` for word ∈ {lambda, forall, in, coloneq,
+  leftarrow, implies, and, or, neq, leq, geq, times, div, trace, bot,
+  parallel, circ, ast}; unknown word → `err_escape`.
+- Numbers: `[0-9]+(.[0-9]+)?` via `parseFloat` (all numbers are JS
+  doubles; no scientific notation, no leading `.`).
+- Identifiers: `[a-zA-Z][a-zA-Z0-9_]*`; `true`/`false` are boolean
+  literals. A leading `_` is not an identifier start (`err_char`).
+- Type symbols `ℕ ℤ ℚ ℝ ℂ 𝔹` lex as identifiers (code-point-safe; 𝔹 is
+  supplementary-plane).
+- Single-char tokens: `✎ λ ∀ ∈ ≜ ← ⟹ ∧ ∨ ≠ ≤ ≥ × ÷ ∗ ∘ ‖ ⊥ + - / = < > |
+  ; : , ( ) [ ] { }`.
+- Anything else → `err_char`.
+
+## Parser (loosest to tightest)
+
+`;` sequence → `‖` (desugars to sequence!) → `≜` (right-assoc, id target
+only) → `←` (right-assoc, id target only) → `|` (left-assoc) → `⟹`
+(right-assoc) → `∨` → `∧` → comparison (`= ≠ < > ≤ ≥`, non-associative —
+`1 < 2 < 3` is a parse error) → `+ -` → `× ∗ ÷ /` → unary (`✎`, `-`) →
+call `f(a, …)` (postfix, nullary `f()` parses) → atom.
+
+Atoms: number/string/bool literals, `⊥`, identifiers,
+`λ params [∈ constraint] : body` (≥ 1 parameter; the constraint is parsed
+and **discarded unevaluated**), `∀ id ∈ expr : body`, list `[…]`, parens
+`( expr )` (a single expression — `;` inside parens is a parse error),
+braces `{ seq }` (`{}` evaluates to `⊥`).
+
+Parse-class error keys: `err_char`, `err_escape`, `err_string`,
+`err_comment`, `err_expect`, `err_unexpected`, `err_def_target`,
+`err_assign_target`.
+
+Lexed but unusable: `∘` (compose) has a token and an escape but **no
+parser rule** — any use is `err_expect`.
+
+## Evaluator
+
+- Values: number (double), string, boolean, list, closure, `⊥`.
+- `✎ e` prints `show(value)` and returns the value.
+- `show`: `⊥` → `⊥`; strings bare at top level but **quoted inside
+  lists**; lists `[a, b]`; closures → `λ`; booleans `true`/`false`;
+  numbers via JS `String()` (integral doubles print without `.0`).
+- `+` is numeric addition unless either side is a string — then it is
+  concatenation of `show`-rendered operands. `- × ∗ ÷ /` are numeric only
+  (`err_num`), `÷ 0` → `err_div0`.
+- Guards: `c ⟹ e` fires iff `c` is `true` **or a non-zero number**;
+  strings/lists/`⊥` never fire. A non-firing guard yields NOMATCH, which
+  `|` catches; NOMATCH surfacing anywhere else becomes `⊥`.
+- `∧ ∨`: short-circuit; operands are tested with `=== true`, so non-boolean
+  operands behave as false (`1 ∧ true` → `false` — contrast with `⟹`).
+- Comparison: `=`/`≠` via JSON serialization (structural for lists,
+  cross-type → `false`, `⊥ = ⊥` → `true`, comparing a self-referential
+  closure crashes keyless); `< > ≤ ≥` are raw JS comparisons (mixed-type
+  coercion applies).
+- `≜` binds in the **current** scope; `←` mutates the nearest enclosing
+  binding, creating one in the current scope if none exists. Both return
+  the value; both may rebind.
+- Braces `{…}` do **not** create a scope (bindings leak out). λ bodies and
+  each `∀` iteration do (the `∀` variable shadows and is restored).
+- Closures capture the defining **environment** (later mutations are
+  visible), enabling recursion via `≜`.
+- `∀ x ∈ list : body` requires a list (`err_iter`), evaluates the body per
+  element, and yields the last body value (`⊥` for an empty list).
+- Errors carry a key + 1-based line:col. Runtime keys: `err_undef`,
+  `err_num`, `err_div0`, `err_notfn`, `err_arity`, `err_iter`,
+  `err_steps` (evaluation-step budget: 500 000).
+- Deep recursion overflows the host stack **before** the step budget —
+  a keyless RangeError, unpinnable by the corpus (see JUDGMENT_CALLS).
+
+## Explicitly out of corpus scope
+
+- `‖` — parses, and the evaluator runs it as plain sequencing, but its
+  semantics are an M1 design question (locked decision 6): no corpus
+  entry pins it.
+- Everything the grammar has that `js/mpl.js` does not implement (modules,
+  resources, channels, exceptions, metaprogramming, records, sets, choice
+  types, …) — Stage 4/5 artifacts, not Stage 2 ones.

conformance/corpus/003_arith_precedence/expected.out

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+14
+20
+5
+2
+4

conformance/corpus/003_arith_precedence/meta.json

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+{"status": "unratified", "source": "coverage", "decision": "", "notes": "× ÷ bind tighter than + -; + - and × ÷ left-associative"}

conformance/corpus/003_arith_precedence/program.mpl

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+✎(2 + 3 × 4);
+✎((2 + 3) × 4);
+✎(10 - 2 - 3);
+✎(20 ÷ 2 ÷ 5);
+✎(2 + 12 ÷ 4 - 1);

conformance/corpus/004_unary_minus/expected.out

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+-5
+5
+5
+-6
+-3

conformance/corpus/004_unary_minus/meta.json

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+{"status": "unratified", "source": "coverage", "decision": "", "notes": "unary minus, including doubled and against binary minus"}

conformance/corpus/004_unary_minus/program.mpl

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+✎(-5);
+✎(- -5);
+✎(3 - -2);
+✎(-2 × 3);
+✎(-(1 + 2));

conformance/corpus/005_slash_div_alias/expected.out

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+2.5
+2.5
+3

conformance/corpus/005_slash_div_alias/meta.json

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+{"status": "unratified", "source": "coverage", "decision": "", "notes": "/ is an ASCII alias of ÷"}

conformance/corpus/005_slash_div_alias/program.mpl

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+✎(10 / 4);
+✎(10 ÷ 4);
+✎(9 / 3);

conformance/corpus/006_number_display/expected.out

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+42
+1.5
+0.5
+2
+7
+0.5

conformance/corpus/006_number_display/meta.json

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+{"status": "unratified", "source": "coverage", "decision": "", "notes": "integral doubles display without decimal point; literals normalize"}

conformance/corpus/006_number_display/program.mpl

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+✎ 42;
+✎ 1.5;
+✎(1 ÷ 2);
+✎(4 ÷ 2);
+✎ 007;
+✎ 0.50;

conformance/corpus/007_float_arithmetic/expected.out

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+3.75
+0.30000000000000004
+6

conformance/corpus/007_float_arithmetic/meta.json

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+{"status": "unratified", "source": "coverage", "decision": "", "notes": "IEEE double arithmetic, including the 0.1+0.2 representation artifact"}

conformance/corpus/007_float_arithmetic/program.mpl

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+✎(1.5 + 2.25);
+✎(0.1 + 0.2);
+✎(3.0 × 2);

conformance/corpus/008_string_escapes/expected.out

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+a
+b
+a	b
+q"q
+b\b

conformance/corpus/008_string_escapes/meta.json

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+{"status": "unratified", "source": "coverage", "decision": "", "notes": "string escapes \\n \\t \\\" \\\\ (unknown escapes like \\q diverge from the grammar — see DIVERGENCES.md, not corpus-pinnable)"}

conformance/corpus/008_string_escapes/program.mpl

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+✎ "a\nb";
+✎ "a\tb";
+✎ "q\"q";
+✎ "b\\b";

conformance/corpus/009_string_concat/expected.out

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+n=5
+5!
+l=[1, 2]
+b=true
+v=⊥
+ab

conformance/corpus/009_string_concat/meta.json

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+{"status": "unratified", "source": "coverage", "decision": "", "notes": "+ concatenates when either operand is a string, rendering the other via show"}

conformance/corpus/009_string_concat/program.mpl

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+✎("n=" + 5);
+✎(5 + "!");
+✎("l=" + [1, 2]);
+✎("b=" + true);
+✎("v=" + ⊥);
+✎("a" + "b");

conformance/corpus/010_multilingual_strings/expected.out

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+مرحبا
+你好
+Salaam

conformance/corpus/010_multilingual_strings/meta.json

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+{"status": "unratified", "source": "coverage", "decision": "", "notes": "non-ASCII string content passes through byte-intact"}

conformance/corpus/010_multilingual_strings/program.mpl

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+✎ "مرحبا";
+✎ "你好";
+✎ "Salaam";

conformance/corpus/011_list_display/expected.out

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+[1, "two", true, [3, 4], ⊥]
+[]
+[λ]

conformance/corpus/011_list_display/meta.json

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+{"status": "unratified", "source": "coverage", "decision": "", "notes": "lists display with strings quoted inside; closures display as λ"}

conformance/corpus/011_list_display/program.mpl

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+id ≜ λx: x;
+✎ [1, "two", true, [3, 4], ⊥];
+✎ [];
+✎ [id];

conformance/corpus/012_lambda_basics/expected.out

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+λ
+7
+1

conformance/corpus/012_lambda_basics/meta.json

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+{"status": "unratified", "source": "coverage", "decision": "", "notes": "λ definition, display, application, multiple parameters"}

conformance/corpus/012_lambda_basics/program.mpl

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+id ≜ λx: x;
+✎ id;
+✎ id(7);
+fst ≜ λa, b: a;
+✎ fst(1, 2);

conformance/corpus/013_closure_capture/expected.out

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+11
+21

conformance/corpus/013_closure_capture/meta.json

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+{"status": "unratified", "source": "coverage", "decision": "", "notes": "closures capture the environment, not values at definition time"}

conformance/corpus/013_closure_capture/program.mpl

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+x ← 10;
+f ≜ λy: x + y;
+✎ f(1);
+x ← 20;
+✎ f(1);

conformance/corpus/014_higher_order/expected.out

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+42
+15

conformance/corpus/014_higher_order/meta.json

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+{"status": "unratified", "source": "coverage", "decision": "", "notes": "curried application and λ literals as call arguments"}

conformance/corpus/014_higher_order/program.mpl

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+twice ≜ λf: λx: f(f(x));
+inc ≜ λn: n + 1;
+✎ twice(inc)(40);
+apply ≜ λf, v: f(v);
+✎ apply(λn: n × 3, 5);

conformance/corpus/015_recursion_fib/expected.out

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+55

conformance/corpus/015_recursion_fib/meta.json

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+{"status": "unratified", "source": "coverage", "decision": "", "notes": "binary recursion through a guarded alternative"}

conformance/corpus/015_recursion_fib/program.mpl

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+fib ≜ λn: (n ≤ 1 ⟹ n) | (fib(n - 1) + fib(n - 2));
+✎ fib(10);

conformance/corpus/016_recursion_sum/expected.out

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+125250

conformance/corpus/016_recursion_sum/meta.json

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+{"status": "unratified", "source": "coverage", "decision": "", "notes": "linear recursion 500 deep (within the host stack)"}

conformance/corpus/016_recursion_sum/program.mpl

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+sum ≜ λn: (n = 0 ⟹ 0) | (n + sum(n - 1));
+✎ sum(500);

conformance/corpus/017_forall_accumulate/expected.out

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+55

conformance/corpus/017_forall_accumulate/meta.json

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+{"status": "unratified", "source": "coverage", "decision": "", "notes": "∀ with an accumulating assignment"}

conformance/corpus/017_forall_accumulate/program.mpl

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+t ← 0;
+∀ n ∈ [1, 2, 3, 4, 5]: t ← t + n × n;
+✎ t;

conformance/corpus/018_forall_value/expected.out

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+6
+⊥

conformance/corpus/018_forall_value/meta.json

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+{"status": "unratified", "source": "coverage", "decision": "", "notes": "a ∀ expression yields the last body value; empty collection yields ⊥"}

conformance/corpus/018_forall_value/program.mpl

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+✎(∀ n ∈ [1, 2, 3]: n × 2);
+✎(∀ n ∈ []: n);

conformance/corpus/019_forall_scope/expected.out

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+1
+2
+100

conformance/corpus/019_forall_scope/meta.json

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+{"status": "unratified", "source": "coverage", "decision": "", "notes": "the ∀ variable shadows per iteration and the outer binding survives"}

conformance/corpus/019_forall_scope/program.mpl

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+n ← 100;
+∀ n ∈ [1, 2]: ✎ n;
+✎ n;

conformance/corpus/020_guarded_alternatives/expected.out

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+A
+B
+C

conformance/corpus/020_guarded_alternatives/meta.json

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+{"status": "unratified", "source": "coverage", "decision": "", "notes": "the canonical conditional: guarded alternatives with fallback"}

conformance/corpus/020_guarded_alternatives/program.mpl

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+grade ≜ λs: (s ≥ 90 ⟹ "A") | ((s ≥ 80 ⟹ "B") | "C");
+✎ grade(95);
+✎ grade(85);
+✎ grade(70);

conformance/corpus/021_no_guard_match/expected.out

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+⊥
+⊥

conformance/corpus/021_no_guard_match/meta.json

@@ -0,0 +1 @@
+{"status": "unratified", "source": "coverage", "decision": "", "notes": "a guard that never fires, with no | fallback, surfaces as ⊥"}

conformance/corpus/021_no_guard_match/program.mpl

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+✎((false ⟹ 1));
+x ← (false ⟹ 1);
+✎ x;

conformance/corpus/022_guard_truthiness/expected.out

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+one
+no
+num
+no
+no

conformance/corpus/022_guard_truthiness/meta.json

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+{"status": "unratified", "source": "coverage", "decision": "", "notes": "guard conditions: true or non-zero number fire; strings/lists never do"}

conformance/corpus/022_guard_truthiness/program.mpl

@@ -0,0 +1,5 @@
+✎((1 ⟹ "one") | "no");
+✎((0 ⟹ "zero") | "no");
+✎((2.5 ⟹ "num") | "no");
+✎(("s" ⟹ "str") | "no");
+✎(([1] ⟹ "list") | "no");

conformance/corpus/023_alt_chain/expected.out

@@ -0,0 +1,2 @@
+3
+2

conformance/corpus/023_alt_chain/meta.json

@@ -0,0 +1 @@
+{"status": "unratified", "source": "coverage", "decision": "", "notes": "fall-through chains take the first firing arm"}

conformance/corpus/023_alt_chain/program.mpl

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+✎((false ⟹ 1) | (false ⟹ 2) | 3);
+✎((false ⟹ 1) | (true ⟹ 2) | 3);

conformance/corpus/024_def_assign_rebind/expected.out

@@ -0,0 +1,4 @@
+1
+2
+3
+5

conformance/corpus/024_def_assign_rebind/meta.json

@@ -0,0 +1 @@
+{"status": "unratified", "source": "coverage", "decision": "", "notes": "≜ and ← both rebind freely; ← works without a prior ≜"}

conformance/corpus/024_def_assign_rebind/program.mpl

@@ -0,0 +1,8 @@
+x ≜ 1;
+✎ x;
+x ← 2;
+✎ x;
+x ≜ 3;
+✎ x;
+y ← 5;
+✎ y;

conformance/corpus/025_def_assign_value/expected.out

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+7
+8
+2
+2

conformance/corpus/025_def_assign_value/meta.json

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+{"status": "unratified", "source": "coverage", "decision": "", "notes": "both binding forms are expressions returning the bound value; ≜ chains right"}

conformance/corpus/025_def_assign_value/program.mpl

@@ -0,0 +1,5 @@
+✎(a ≜ 7);
+✎(a ← 8);
+b ≜ c ≜ 2;
+✎ b;
+✎ c;

conformance/corpus/026_def_vs_assign_scope/expected.out

@@ -0,0 +1,2 @@
+99
+99

conformance/corpus/026_def_vs_assign_scope/meta.json

@@ -0,0 +1 @@
+{"status": "unratified", "source": "coverage", "decision": "", "notes": "inside a λ, ← mutates the captured outer binding; ≜ creates a local one"}

conformance/corpus/026_def_vs_assign_scope/program.mpl

@@ -0,0 +1,7 @@
+x ← 1;
+f ≜ λy: {x ← 99; x};
+f(0);
+✎ x;
+g ≜ λy: {x ≜ 55; x};
+g(0);
+✎ x;

conformance/corpus/027_block_sequencing/expected.out

@@ -0,0 +1,3 @@
+3
+⊥
+4

conformance/corpus/027_block_sequencing/meta.json

@@ -0,0 +1 @@
+{"status": "unratified", "source": "coverage", "decision": "", "notes": "a block yields its last expression; {} yields ⊥; trailing ; permitted"}

conformance/corpus/027_block_sequencing/program.mpl

@@ -0,0 +1,3 @@
+✎({1; 2; 3});
+✎({});
+✎({4;});

conformance/corpus/028_block_no_scope/expected.out

@@ -0,0 +1 @@
+9

conformance/corpus/028_block_no_scope/meta.json

@@ -0,0 +1 @@
+{"status": "unratified", "source": "coverage", "decision": "", "notes": "braces do NOT create a scope: bindings made inside leak out"}

conformance/corpus/028_block_no_scope/program.mpl

@@ -0,0 +1,2 @@
+{y ← 9; 0};
+✎ y;

conformance/corpus/029_comments/expected.out

@@ -0,0 +1,3 @@
+1
+2
+3

conformance/corpus/029_comments/meta.json

@@ -0,0 +1 @@
+{"status": "unratified", "source": "coverage", "decision": "", "notes": "line comments and nested block comments"}

conformance/corpus/029_comments/program.mpl

@@ -0,0 +1,4 @@
+-- line comment
+✎ 1; {- block -} ✎ 2;
+{- outer {- nested -} outer -}
+✎ 3;

conformance/corpus/030_bot/expected.out

@@ -0,0 +1,3 @@
+⊥
+x⊥
+[⊥, 1]

conformance/corpus/030_bot/meta.json

@@ -0,0 +1 @@
+{"status": "unratified", "source": "coverage", "decision": "", "notes": "⊥ displays as ⊥ and concatenates/nests as a value"}

conformance/corpus/030_bot/program.mpl

@@ -0,0 +1,3 @@
+✎ ⊥;
+✎("x" + ⊥);
+✎ [⊥, 1];

conformance/corpus/031_show_all_types/expected.out

@@ -0,0 +1,8 @@
+42
+1.5
+s
+true
+false
+[1]
+⊥
+λ

conformance/corpus/031_show_all_types/meta.json

@@ -0,0 +1 @@
+{"status": "unratified", "source": "coverage", "decision": "", "notes": "one ✎ per value type"}

conformance/corpus/031_show_all_types/program.mpl

@@ -0,0 +1,9 @@
+f ≜ λx: x;
+✎ 42;
+✎ 1.5;
+✎ "s";
+✎ true;
+✎ false;
+✎ [1];
+✎ ⊥;
+✎ f;

conformance/corpus/032_comparisons/expected.out

@@ -0,0 +1,6 @@
+true
+true
+false
+false
+true
+true

conformance/corpus/032_comparisons/meta.json

@@ -0,0 +1 @@
+{"status": "unratified", "source": "coverage", "decision": "", "notes": "all six comparison operators on numbers"}

conformance/corpus/032_comparisons/program.mpl

@@ -0,0 +1,6 @@
+✎(1 < 2);
+✎(2 ≤ 2);
+✎(3 > 4);
+✎(4 ≥ 5);
+✎(1 = 1);
+✎(1 ≠ 2);

conformance/corpus/033_string_compare/expected.out

@@ -0,0 +1,4 @@
+true
+true
+true
+true

conformance/corpus/033_string_compare/meta.json

@@ -0,0 +1 @@
+{"status": "unratified", "source": "coverage", "decision": "", "notes": "lexicographic order and structural equality on strings"}

conformance/corpus/033_string_compare/program.mpl

@@ -0,0 +1,4 @@
+✎("a" < "b");
+✎("abc" = "abc");
+✎("abc" ≠ "abd");
+✎("b" ≥ "a");

conformance/corpus/034_cross_type_equality/expected.out

@@ -0,0 +1,6 @@
+false
+false
+true
+true
+true
+false

conformance/corpus/034_cross_type_equality/meta.json

@@ -0,0 +1 @@
+{"status": "unratified", "source": "coverage", "decision": "", "notes": "equality is structural per type and false across types; ⊥ equals ⊥"}

conformance/corpus/034_cross_type_equality/program.mpl

@@ -0,0 +1,6 @@
+✎(1 = "1");
+✎(true = 1);
+✎([1] = [1]);
+✎([] = []);
+✎(⊥ = ⊥);
+✎("" = ⊥);

conformance/corpus/035_mixed_compare/expected.out

@@ -0,0 +1,3 @@
+true
+false
+true

conformance/corpus/035_mixed_compare/meta.json

@@ -0,0 +1 @@
+{"status": "unratified", "source": "coverage", "decision": "", "notes": "ordering across types follows host coercion — observed, flagged as a judgment call"}

conformance/corpus/035_mixed_compare/program.mpl

@@ -0,0 +1,3 @@
+✎(1 < "2");
+✎("10" < 9);
+✎(true < 2);