you want the following (which doesnt require undediable instances)
data Nat = Z | S Nat
type family U (x :: Nat) where
U 0 = Z
U n = S (U (n-1))
this lets you convert type lits into your own peanos or whatever
hat tip to richard eisenburg for showing me this trick on the mailing list
a while ago
On Sat, Oct 25, 2014 at 9:53 AM, Barney Hilken
If you define your own type level naturals by promoting
data Nat = Z | S Nat
you can define data families recursively, for example
data family Power :: Nat -> * -> * data instance Power Z a = PowerZ data instance Power (S n) a = PowerS a (Power n a)
But if you use the built-in type level Nat, I can find no way to do the same thing. You can define a closed type family
type family Power (n :: Nat) a where Power 0 a = () Power n a = (a, Power (n-1) a)
but this isn't the same thing (and requires UndecidableInstances).
Have I missed something? The user guide page is pretty sparse, and not up to date anyway.
If not, are there plans to add a "Successor" constructor to Nat? I would have thought this was the main point of using Nat rather than Int.
Barney.
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