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Proposal: Remove ad-hoc handling of case formatting in the parser #4012

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#4241
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Proposal: Remove ad-hoc handling of case formatting in the parser#4012
#4241
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type: breaking changetype: enhancement
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v0.15.0
@natefaubion

Description

@natefaubion
natefaubion
opened on Feb 9, 2021

Summary

The parser has a couple of ad-hoc cases in the parser that should not be necessary.

purescript/lib/purescript-cst/src/Language/PureScript/CST/Parser.y

Lines 404 to 411 in e56d28b

-- These special cases handle some idiosynchratic syntax that the current
-- parser allows. Technically the parser allows the rhs of a case branch to be
-- at any level, but this is ambiguous. We allow it in the case of a singleton
-- case, since this is used in the wild.
| 'case' sep(expr, ',') 'of' '\{' sep(binder1, ',') '->' '\}' exprWhere
{ ExprCase () (CaseOf $1 $2 $3 (pure ($5, Unconditional $6 $8))) }
| 'case' sep(expr, ',') 'of' '\{' sep(binder1, ',') '\}' guardedCase
{ ExprCase () (CaseOf $1 $2 $3 (pure ($5, $7))) }

Motivation

I originally added these cases to keep some core modules compiling that use extremely odd formatting for single-branch cases. Normally these would run afoul the offside rule, but the old parser accepted them due to various idiosyncrasies.

https://github.com/purescript/purescript-transformers/blob/42175954755a0fa0fe688579309032a948d919fb/src/Control/Monad/Writer/Trans.purs#L82-L86

Proposal

We should remove these cases in 0.15 and update the core libraries. We can warn in a followup 0.14 release.

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