#11367: [Regression] Only one clause allowed in (explicitly bidirectional) pattern synonyms -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: | PatternSynonyms Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Well in normal code you can't write {{{ f :: Int -> Int }}} and omit the declaration of `f`. You have to write {{{ f :: Int -> Int f = undefined }}} So it's consistent to require the same for pattern synonyms. Usually `where` clauses contain zero or more bindings, which is why an empty `where` is usually ok. But here it must contain exactly one. (Two would not make sense either.) For my money, I think it's maybe a mistake for the unidirectional/bidirectional split to be so quietly signaled. One could imagine {{{ pattern unidirectional Q a = pat pattern bidirectional P a b = pat pattern bidirectional R x = pat where R = ... }}} But opinions vary. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11367#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler