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mpl/DECISIONS.md
developtheweb 5230273ab1 Fix grammar compilation errors and latent build breakers 
- Restructure conditionals into a condExpr precedence level and exception
  handling into a postfix rule, removing the mutual left recursion through
  expr/atomExpr (ANTLR error 119)
- Rewrite pattern as patternAtom (COMMA patternAtom)*, removing left
  recursion with an empty-matchable tail (ANTLR error 148)
- Drop the LAMBDA token fully shadowed by LAMBDA_VAR (warning 184)
- Give \Rightarrow to EXPORT only; IMPLIES keeps \implies (warning 184)
- Remove @header package declaration that duplicated the -package argument
  and made the generated parser uncompilable
- Point the ANTLR source set at src/main/antlr4 so Gradle actually finds
  the grammar
- Treat ANTLR warnings as errors (-Werror)
- Record decisions in DECISIONS.md
2026-07-09 02:44:14 -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).