heres a version richard eisenburg helped me write
https://gist.github.com/cartazio/9340008
see the linked gist for the full code but heres the meat of it
data Shape (rank :: Nat) a where
Nil :: Shape Z a
(:*) :: !(a) -> !(Shape r a ) -> Shape (S r) a
{-# INLINE reverseShape #-}
reverseShape :: Shape n a -> Shape n a
reverseShape Nil = Nil
reverseShape list = go SZero Nil list
where
go :: SNat n1 -> Shape n1 a-> Shape n2 a -> Shape (n1 + n2) a
go snat acc Nil = gcastWith (plus_id_r snat) acc
go snat acc (h :* (t :: Shape n3 a)) =
gcastWith (plus_succ_r snat (Proxy :: Proxy n3))
(go (SSucc snat) (h :* acc) t)
On Tue, Sep 9, 2014 at 8:55 AM, Cristiano Paris
Hi,
I'm playing around with Type Families so I decided to implement a simple fixed-length Vect of Integers.
Here is my straightforward implementation (ghc 7.8.3):
https://gist.github.com/anonymous/d838e68ce6a02412859f
As you can see, I'm trying to implement a reverse function for my vectors which guarantees that the output vector is the same size as the input one. I tried to express this constraint at the type level.
The problem is that I can't have ghc to type check the reverse function in the non-trivial case:
_________________ Could not deduce (Plus n1 (S m) ~ S (Plus n1 m)) from the context (n ~ S n1) bound by a pattern with constructor CV :: forall n. Int -> Vect n -> Vect (S n), in an equation for ‘vecReverse’ at vect3.hs:30:13-18 Expected type: Vect (Plus n m) Actual type: Vect (Plus n1 (S m)) _________________
Iit has to do with the fact that the type checker can't deduce that:
Plus n1 (S m) ~ S (Plus n1 m) ~ Plus (S n1) m ~Plus n m
I tried to insert the following instance to the family:
Plus n (S m) = S (Plus n m)
but to no avail.
Any clue?
Thanks.
C.
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