#12153: Bug in pattern synonyms with template haskell -------------------------------------+------------------------------------- Reporter: MikeIzbicki | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- #8761 added template haskell support for pattern synonyms, but I believe the implementation has a bug. If I create the following template Haskell syntax trees: {{{ PatSynD AST_sin (RecordPatSyn [a0]) Unidir (ConP HomFree [ViewP (VarE embedSig) (ConP HomSig_sin [VarP a0])]) PatSynD AST_cos (RecordPatSyn [a0]) Unidir (ConP HomFree [ViewP (VarE embedSig) (ConP HomSig_cos [VarP a0])]) }}} the compiler complains that `a0` is being redefined. Of course, this should be allowed because `a0` is local to each pattern synonym. A temporary workaround is to rename each variable to be unique to the pattern. The following template haskell compiles just fine. {{{ PatSynD AST_sin (RecordPatSyn [a0_sin]) Unidir (ConP HomFree [ViewP (VarE embedSig) (ConP HomSig_sin [VarP a0_sin])]) PatSynD AST_cos (RecordPatSyn [a0_cos]) Unidir (ConP HomFree [ViewP (VarE embedSig) (ConP HomSig_cos [VarP a0_cos])]) }}} This is with GHC 8.1.20160518 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12153 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler