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mpl/DECISIONS.md
developtheweb f076a1dade Adopt canonical call syntax and wire in orphaned tokens 
- Function calls are f(a, b) via a postfix argument list; juxtaposition
  application removed; nullary calls f() supported
- Wire DEFINITION (x ≜ e), ALLOC/RELEASE postfix (r ⊕ / r ⊖), channel
  operations (⇀_ch e / ↽_ch e), and qualified module access (M‧f)
- Delete tokens with no rule and no example: QUERY (?), EXISTS, IMPORT,
  ARROW, DOT; delete the unary ? prefix operator
- Canonical handler clause: ↯pattern ⟹ expr (pattern is an identifier or
  a string); clauses separated by semicolons
- SEMICOLON has one role: sequence separator with optional trailing use
- Disambiguate braces structurally: record ({a: e}), set ({a, b}), block
- IDENTIFIER no longer allows a leading underscore, so subscripts such as
  ⌉_db_lock and ↽_socket lex as UNDERSCORE + IDENTIFIER
- Add '/' as ASCII alias of ÷; add unary minus; pathLiteral accepts
  🖫identifier as well as 🖫"…"
- Exactly one ASCII escape per glyph (drop \lam, \gets, \ne, \le, \ge,
  \vee, \wedge, \cup alias forms, \N, \Z, \Q, \R, \C, \B)
- Replace deprecated ANTLRInputStream with CharStreams in the test
  harness: it fed UTF-16 units and could never tokenize the
  supplementary-plane glyphs 𝔹, 𝓜 and 🖫
- Update LexerTest/ParserTest to canonical syntax; add coverage for
  subscript lexing, channels, resources, module access, unary minus,
  nullary calls, and negative tests for juxtaposition and the ternary
2026-07-09 02:51:12 -04:00

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Design decisions

One line per decision, with the rejected alternatives named. Governing principles, in order: One Right Answer, the Fatima test, minimal churn against the existing examples.

  • Conditionals are guarded alternatives (condition ⟹ result) | fallback, wired in as a condExpr precedence level; rejected: C-style ternary cond ? a : b (programmer convention, fails the Fatima test), standalone left-recursive conditional rule (ANTLR error 119).
  • Exception handling is a postfix operator expr ↴ { … }; rejected: standalone exceptionHandler rule reachable from atomExpr (mutual left recursion, ANTLR error 119).
  • λ has one token, LAMBDA_VAR, used both as a variable and to open a lambda; rejected: separate LAMBDA token (identical alternatives, fully shadowed, ANTLR warning 184).
  • \implies maps to ⟹ and \Rightarrow maps to ⇒ — one escape, one glyph; rejected: \Rightarrow as an alias of ⟹ (shadowed EXPORT's escape, ANTLR warning 184).
  • The Java package for generated code comes from -package in build.gradle only; rejected: @header package declaration in the grammar (combined with -package it generates a duplicate package statement that does not compile).
  • ANTLR warnings fail the build (-Werror in build.gradle); rejected: warnings as advisory output (they hid the shadowed-token bugs).
  • The choice-type rule is ⟨ expr ⟩ with the interior | consumed by condExpr; rejected: explicit ⟨ expr | expr ⟩ (the interior expr already consumes the bar, making the explicit BAR unreachable).
  • Function calls are f(a, b) — a postfix argument list, nullary f() included; rejected: Haskell juxtaposition f x (programmer convention, fails the Fatima test — children learn f(x) in school).
  • SEMICOLON has one role: sequence separator (trailing ; permitted); the program, blocks, (...), ⌈...⌉, 〔...〕, ⌜...⌝, ⌞...⌟ and ⟳(...) all contain one seqExpr; rejected: a separate statement-terminator rule duplicating the same token (two roles for one symbol).
  • Braces disambiguate structurally: IDENTIFIER : → record, two-plus comma-separated exprs → set, everything else (incl. {} and {x}) → block; a singleton set literal cannot be written (deferred to M1); rejected: parser-order coin flips left undocumented.
  • ≜ defines, ← assigns; both wired into the chain with ≜ binding looser than ←, both right-associative; rejected: ≜ tokenized but unreachable.
  • ⊕/⊖ are postfix resource operators (database ⊕, conn ⊖), matching every example; rejected: prefix form ⊕open(path) that appeared only in the whitepaper.
  • Channel operations are subscripted prefix operators ⇀_ch expr / ↽_ch expr; rejected: leaving SEND/RECEIVE orphaned.
  • ‧ is qualified module access (Mathematics‧sin(angle)), a postfix ‧IDENTIFIER; rejected: leaving MIDDOT orphaned, or . (removed — one access syntax).
  • Handler clauses are ↯pattern ⟹ expr, semicolon-separated, where pattern is an identifier (binds the exception) or a string (matches a message); rejected: ↯e ⇒ expr (⇒ is EXPORT; the clause arrow should mirror the guarded-alternative arrow ⟹).
  • IDENTIFIER may not start with _, so subscripts (⌉_db_lock, ↽_socket) lex as UNDERSCORE + IDENTIFIER; rejected: identifiers with a leading underscore (made every subscript lex as one identifier token).
  • / is an ASCII alias of ÷ (DIV) so π/4 parses; rejected: ÷-only division (unreachable on most keyboards).
  • Unary minus exists (-x), sharing the MINUS token at prefix level; rejected: binary-only minus (cannot write negative numbers); unary plus was NOT added (x ++ y stays invalid).
  • pathLiteral accepts 🖫"…" and 🖫identifier, as required by readFile(🖫path) in example 03.
  • Deleted tokens: ? (QUERY), ∃ (EXISTS), ⇐ (IMPORT), → (ARROW), . (DOT) — defined but used by no parser rule and no example; dead operators are debt; each returns in M1 only with a documented semantic. Rejected: keeping them tokenized-but-unreachable.
  • Exactly one ASCII escape per glyph (\lambda not \lam, \leftarrow not \gets, \neq not \ne, \nat not \N, …); rejected: alias sets (two ways to write the same token).
  • % (modulo), ∑, √, ², |x|, ranges [a..b], indexing/slicing, where, and record field access are NOT in M0 — every document says so instead of using them; deferred to M1 with semantics, not smuggled in via prose.
  • Lambda parameters are a bare comma-separated pattern list (λa, b: body); rejected: parenthesized parameter lists λ(a, b): (two ways to write parameters).