ANNOUNCE: GHC 7.0.1 Release Candidate 1

David Fox ddssff at
Wed Sep 29 01:44:52 EDT 2010

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

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