#16213: Unnecessary error -------------------------------------+------------------------------------- Reporter: pjljvdlaar | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Replying to [ticket:16213 pjljvdlaar]:
Yet, both patterns are matched, i.e. '[] and (x ': xs), so I was surprised the compiler could not figure this out!
This isn't how type class instances work. Having an `Show (TermList '[])` instance and a `Show (TermList (x ': xs))` instance doesn't imply an instance for `forall (v :: [Bool]). Show (TermList v)`. In the instance you're trying to write, GHC needs a `Show` instance for an arbitrary list `v`, and GHC can't know //a priori// whether that list is `'[]` or `(_ ': _)`. That being said, there's not much sense in writing two separate `Show` instances like this. Why not just combine them into a single instance, like so? {{{#!hs instance Show (TermList v) where show TNil = "Nil" show (TCons a b) = "(Cons " ++ show a ++ " " ++ show b ++ ")" }}} This actually gives you an instance for `forall (v :: [Bool]). Show (TermList v)`, so you avoid your earlier issue entirely. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16213#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler