On Tuesday 28 September 2010 11:10:58 pm David Fox wrote:
I'm seeing errors like this in various places, which I guess are coming from the new type checker:
Data/Array/Vector/Prim/BUArr.hs:663:3: Couldn't match type `s' with `s3' because this skolem type variable would escape: `s3' This skolem is bound by the polymorphic type `forall s. ST s a' The following variables have types that mention s fill0 :: MBUArr s e -> ST s Int (bound at Data/Array/Vector/Prim/BUArr.hs:669:5) In the first argument of `runST', namely `(do { marr <- newMBU n; n' <- fill0 marr; unsafeFreezeMBU marr n' })'
I would love to hear an explanation about what they mean and what to do about them. (This one is from uvector.)
This appears to be due to MonoLocalBinds. The issue boils down to: foo = runST (bar 5) where bar x = return x The necessary type for bar is forall s. n -> ST s n n being bound in the type of foo (or, feel free to make it concrete). However, GHC will not infer this when the non-generalizing let is turned on. But, no rigid s will do, because it's expected to be a quantified variable by runST. As for why this happens in uvector: -fglasgow-exts is one of the ghc-options in the cabal file. Alternately, a type signature could be given. -- Dan