#9671: Allow expressions in patterns -------------------------------------+------------------------------------- Reporter: | Owner: Iceland_jack | Status: new Type: feature | Milestone: 7.10.1 request | Version: Priority: normal | Keywords: patterns, pattern Component: Compiler | synonyms Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: | Related Tickets: None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by simonpj): Your examples all use constants, but I assume you are going to allow this: {{{ f :: Person -> Map Person Age -> Age f p (Lookup p a) = a f p _ = error ("No info on " ++ show p) }}} Now this is jolly confusing. In the pattern `(Lookup p a)`, `p` is a bound occurence while `a` is a binding occurrence. This distinction is made syntactically by view patterns: {{{ f :: Person -> Map Person Age -> Age f p ((\m -> lookup m p) -> a) = a }}} Here the stuff on the left of the view-pattern arrow `->` are bound occurrences, while the stuff on the right is binding. You could even have {{{ f p (Lookup p p) = p }}} meaning {{{ f p1 (Lookup p1 p2) = p2 }}} To me, the lack of syntactic cues for what is bound and what is binding seems quite serious. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9671#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler