[Haskell-cafe] Reversing a fixed-length vector

[Erik Hesselink]

The problem with a class based approach is that the class context will
show up everywhere, even though all instances have already been
defined.

Erik

On Tue, Sep 9, 2014 at 4:12 PM, adam vogt <vogt.adam at gmail.com> wrote:
> Hi Cristiano,
>
> You can also convince ghc about (Plus n1 (S m) ~ S (Plus n1 m)) by
> writing VecReverse as a class:
> https://gist.github.com/aavogt/15399cbdd5e74d0e9cd8
>
> Regards,
> Adam
>
> On Tue, Sep 9, 2014 at 10:07 AM, Cristiano Paris <frodo at theshire.org> wrote:
>> Gosh! I suspected I hit a non-trivial problem... cool!
>>
>> C.
>>
>> On Tue, Sep 9, 2014 at 3:56 PM, Carter Schonwald
>> <carter.schonwald at gmail.com> wrote:
>>>
>>> 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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>>>> Fingerprint = 4575 4FB5 DC8E 7641 D3D8  8EBE DF59 B4E9 C17E 53C6
>>>>
>>>>
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>>>
>>
>>
>>
>> --
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>>
>>
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