[Haskell-cafe] Reversing a fixed-length vector

[Carter Schonwald]

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 <frodo at theshire.org> wrote:

> 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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