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Fix grammar compilation errors and latent build breakers

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- 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
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developtheweb 2026-07-09 02:44:14 -04:00
parent 590762cad3
commit 5230273ab1
3 changed files with 59 additions and 20 deletions
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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).