#12188: Pattern variables bound by PatternGuards are not accessible in where clause -------------------------------------+------------------------------------- Reporter: heisenbug | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 Resolution: invalid | Keywords: 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 heisenbug): Replying to [comment:4 simonpj]:
Richard is right. In Haskell
* The patterns of the function definition scope over guards, where clause, and body * The where clause scopes over the guards, and body. * The guards scope over the body only
Once could define the language differently, but that's the way it is in Haskell.
Oh, pity. Surely those rules were set in stone when pattern guards weren't a thing yet. Thanks for the `let` tip, I'll look into it and see how it helps in my case.
You can include let-binding in the guards, if you want: {{{ foo :: Int -> Bool foo a | Just b <- return a , let c = a == b = c }}}
-- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12188#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler