[Haskell-cafe] Type error depending on scope of pattern matching

[Richard Eisenberg]

And what's Reverse? I happen to know Reifies. But you'll also get a better (and more likely) answer if you reduce dependencies.

Richard

On Aug 1, 2015, at 8:05 PM, htebalaka <goodingm at gmail.com> wrote:

> Alright. The first two examples show the issue and a fix for it; the last two
> have the same issue but existential quantification prevents the fix.
> 
> grad has type "(Num a, Traversable t) => (forall s. Reifies s Tape => t
> (Reverse s a) -> Reverse s a) -> t a -> t a", and I'm basically trying to
> supply a function to it, either by being polymorphic over all Num (of which
> Reverse s a is an instance), or by providing the specific 'Reifies s Tape =>
> ...' type, neither of which are working.
> 
> 
> 
> 
> amindfv wrote
>> You'll probably get more responses if you paste the code inline.
>> 
>> tom
>> 
>> El Aug 1, 2015, a las 11:25, htebalaka <
> 
>> goodingm@
> 
>> > escribió:
>> 
>>> I'm curious if anyone can tell me why moving the pattern matching in
>>> these
>>> two examples makes it fail to compile, and what can be done to resolve
>>> the
>>> same issue in either of the latter two examples. As far as I can tell
>>> these
>>> should be equivalent. Unfortunately the solution used in the first
>>> examples
>>> won't work in the later ones due to the existential quantification. I've
>>> been asking on #haskell for a few days but haven't been able to find
>>> anyone
>>> who knows what the issue is.
>>> 
>>> Code is available here:  http://lpaste.net/137586
>>> <http://lpaste.net/137586>  
>>> 
>>> 
>>> 
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