Re: ANNOUNCE: GHC 7.0.1 Release Candidate 1

On Tue, Sep 28, 2010 at 8:36 PM, Antoine Latter wrote:

On Tue, Sep 28, 2010 at 10:10 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' })'

GHC no longer generalizes local bindings - which means that while previously the 'where' defined function would have had a type:

...

fill0 :: forall s .  ... -> ST s Int

Where the elipsis indicates an unspecified portion of the local type signature.

I would recommend writing a type signature, but that requires writing a type signature for it's helper function:

...

fill s i = ...

Except I don't know how to write a type signature for this.

The value 's' passed in is bound by pattern matching on this guy's constructor:

data Stream a = forall s. Stream (s -> Step s a) !s Int

in the top-level function, so I don't even know if it has a type I can name.

I'll try to boil this down to something I can put on hpaste.

Antoine

I have to say, as an everyday joe programmer, that the obscurity of this message disturbs me. I hope I don't see it very often.