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Pattern matching on data constructors with Rank N functions #42

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fxdpntthm opened this issue May 24, 2022 · 0 comments
Open

Pattern matching on data constructors with Rank N functions #42

fxdpntthm opened this issue May 24, 2022 · 0 comments
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Low Priority Things that are good to have but not needed

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@fxdpntthm
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fxdpntthm commented May 24, 2022 •
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newtype NT m a = MkNT {unNT :: forall b. a -> m b}

foobar1 :: (b -> a) -> NT m a -> NT m b
foobar1 f nt = MkNT $ \ a -> unNT nt (f a)

foobar2 :: (b -> a) -> NT m a -> NT m b
foobar2 f (MkNT nt) = MkNT $ \ a -> nt (f a)

Although they both should work, foobar1 doesn't. The use of unNT gives rise to a wanted m @ b constraint.
one could type foobar1 :: Total m => (b -> a) -> NT m a -> NT m b. The total constraint is needed here as m as it can be applied to any b
@fxdpntthm fxdpntthm added the Low Priority Things that are good to have but not needed label May 24, 2022
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