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

[htebalaka]

Yeah, that seems to be the issue, but pattern matching with let doesn't seem
to work well with existentials, (GHC's brain explodes, and informs me to use
a case statement), so the fix there doesn't seem to work either :(

David Kraeutmann wrote
> I guess that pattern matching on (P f) removes the universal
> quantification you placed on it and it just defaults to Integer. Seems to
> be related to https://ghc.haskell.org/trac/ghc/ticket/10450.
> 
> On 8/3/2015 6:43 PM, htebalaka wrote:
>> I managed to remove the Data.Reflection and Numeric.AD dependency
>> entirely.
>> In this case, Reverse is just a newtyped Num with a phantom variable
>> added.
>> Still, the same issue occurs. In one case the type gets defaulted to
>> Integer
>> if I try to pass a "forall a. Num a => t a -> a" to grad (which has type
>> "Num a, Traversable t => (forall s. t (Reverse s a) -> Reverse s a) -> t
>> a
>> -> t a"). Then it complains that Integer isn't Reverse s a, and won't
>> compile.
>>
>> In the other where I try to specifically pass the a value with type
>> "forall
>> s. t (Reverse s a) -> Reverse s a" it think the type variable would
>> escape
>> its scope, but I don't see why it should, or how to prevent that in the
>> "doesnt3" example.
>>
>> I have no idea how to apply the fix used in the first two examples to the
>> later two, or why it should even be necessary.
>>
>>
>>
>>
>>
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