[Haskell-cafe] Problems on installing sizes-types package

[Mateusz Kowalczyk]

On 21/02/14 18:52, Rodrigo Ribeiro wrote:
> Hello to all!
> 
> I'm trying to install the sized-types package, a dependency for kansas lava:
> 
> http://hackage.haskell.org/package/sized-types
> 
> using cabal, and the following error message is returned:
> 
> Data/Sized/Sparse/Matrix.hs:61:26:
>     Could not deduce (Eq a) arising from a use of `prune'
>     from the context (Size m, Size n, Size m', Size n', n ~ m', Num a)
>       bound by the type signature for
>                  mm :: (Size m, Size n, Size m', Size n', n ~ m', Num a) =>
>                        Matrix (m, n) a -> Matrix (m', n') a -> Matrix (m,
> n') a
>       at Data/Sized/Sparse/Matrix.hs:50:7-112
> 
> Is it a known issue? Is there some way to solve it? I'm using GHC 7.6.3 and
> plataform 2013.2.0.0.
> 
> Best,
> 
> Rodrigo
> 
> 
> 
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> 

You can just add the ‘Eq a’ constraint to the relevant line and it all
compiles fine with 7.6.3. It might be worth-while to e-mail the authors
and ask for a new release with the fix in place (and removing all the
hard tabs from the code…). You can use ‘cabal unpack sized-types’ to
fetch the package source, make the change and then ‘cabal install’ it
from there.

The relevant line becomes:
mm :: (Eq a, Size m, Size n, Size m', Size n', n ~ m', Num a) => Matrix
(m,n) a -> Matrix (m',n') a -> Matrix (m,n') a

-- 
Mateusz K.