ANNOUNCE: GHC 7.0.1 Release Candidate 1

[David Fox]

On Tue, Sep 28, 2010 at 8:36 PM, Antoine Latter <aslatter at gmail.com> wrote:
> On Tue, Sep 28, 2010 at 10:10 PM, David Fox <ddssff at gmail.com> 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.