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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