#8779: Exhaustiveness checks for pattern synonyms -------------------------------------+------------------------------------- Reporter: nomeata | Owner: mpickering Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.8.1 checker) | Keywords: Resolution: | PatternSynonyms Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D2669 Wiki Page: | -------------------------------------+------------------------------------- Comment (by rwbarton): I feel like my suggestion, whatever it was, must have been misinterpreted... I'm concerned about situations in which there are no view patterns, at all. Take this scenario. I have a data type with a lot of `Either` fields {{{#!hs data T = T String (Either Int T) (Either Int T) (Either Int T) (Either Int T) }}} and I need to write a bunch of functions like {{{#!hs f (T s (Left _) (Left _) (Left n) (Right _)) = replicate n s f (T s (Left _) (Left n) (Right _) (Right _)) = replicate (n-1) s -- ... }}} It sure would be handy if I could define {{{#!hs pattern L <- Left _ pattern R <- Right _ }}} to condense the long left-hand sides. It just seems so obvious that I should be able to use a ''pattern synonym'' as a shorthand for a pattern, without having to go through extra contortions to appease the exhaustiveness checker. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8779#comment:47 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler