[Haskell-cafe] working my way through Data.Type.Equality...my head hurts...
nicholls.mark at vimn.com
Tue Jan 13 12:39:23 UTC 2015
Full code here…
Lets start at the beginning…
Ø data a :~: b where
Ø Refl :: a :~: a
Looks reasonable….” a :~: b” is inhabited by 1 value…of type “a :~: a”
Ø sym :: (a :~: b) -> (b :~: a)
Ø sym Refl = Refl
hmmm… (a :~: b) implies that a is b
so (b :~: a)
Ø -- | Transitivity of equality
Ø trans :: (a :~: b) -> (b :~: c) -> (a :~: c)
Ø trans Refl Refl = Refl
Ø -- | Type-safe cast, using propositional equality
Ø castWith :: (a :~: b) -> a -> b
Ø castWith Refl x = x
Ø -- | Generalized form of type-safe cast using propositional equality
Ø gcastWith :: (a :~: b) -> ((a ~ b) => r) -> r
Ø gcastWith Refl x = x
I don’t even understand the signature
What does “~” mean…(it’s something that comes out in error messages when my types are all messed up)….and then there’s a “=>” going on…in the middle of a signature….I know “=>” in the context of “Num a => a -> a -> a”
What would a value of type “((a ~ b) => r)” look like?
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