#9755: Unhelpful error message when -XScopedTypeVariables is omitted -------------------------------------+------------------------------------- Reporter: bitemyapp | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 7.8.3 Resolution: | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: | Blocked By: None/Unknown | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by simonpj): I believe that the real problem is that in the definition of {{{ vecIndexIx :: (Ix ix, Bounded ix) => Vector a -> ix -> a vecIndexIx vec ix = vec ! Ix.index (minBound :: ix, maxBound :: ix) ix }}} you probably think that the uses of `ix` in the RHS mean the same `ix` as in the type signature. To achieve that you need `-XScopedTypeVariables` and an explicit `forall`: {{{ vecIndexIx :: forall a ix. (Ix ix, Bounded ix) => Vector a -> ix -> a vecIndexIx vec ix = vec ! Ix.index (minBound :: ix, maxBound :: ix) ix }}} Now it works fine. What you wrote is equivalent to {{{ vecIndexIx vec ix = vec ! Ix.index (minBound :: forall ix. ix, maxBound :: forall ix. ix) ix }}} where the type signature `blah :: ix` is universally quantified to `blah :: forall ix. ix`. It's an easy mistake to make. It might be a good idea if GHC spotted type variables that have the same name as one belonging to an enclosing, but un-scoped, type signature, and suggested this change. If someone wanted to try that, I could advise. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9755#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler