Re: [Haskell-beginners] Could not deduce (Matrix m (Maybe a)) from the context (Matrix m a)

Am Freitag 29 Januar 2010 10:45:32 schrieb Lyndon Maydwell:

Thanks Daniel.

It works, but I'm a bit confused as to why the extra type information is needed.

Well, you call fromRows on the result of vicinityRows. vicinityRows m x y :: [[Maybe a]] fromRows :: Matrix mat b => [[b]] -> mat b So for fromRows (vicinityRows m x y) to be well typed, you need an instance Matrix matr (Maybe a) where ... for some matrix type matr. That has to come from somewhere. It might come from an instance Matrix m a => Matrix m (Maybe a) where ... or you can supply it as a constraint on the function vicinityMatrix. However, having a look, none of the methods in the class look like they depend on the actual type a, so it might be better to have class Matrix m where fromRows :: [[a]] -> m a toList :: m a -> [a] rows :: m a -> Integer columns :: m a -> Integer row :: m a -> Integer -> [a] column :: m a -> Integer -> [a] at :: m a -> Integer -> Integer -> a (!!!) :: m a -> Integer -> Integer -> a vicinityRows :: m a -> Integer -> Integer -> [[Maybe a]] vicinityMatrix :: m a -> Integer -> Integer -> m (Maybe a) -- No constraint needed!! neighbours :: m a -> Integer -> Integer -> [a]

On Fri, Jan 29, 2010 at 5:17 PM, Daniel Fischer

wrote:

...

Well, the error says the compiler (the type checker) can't deduce the context (Matrix m (Maybe a)) from the givens. If you supply that information,

vicinityMatrix :: Matrix m (Maybe a) =>               m a -> Integer -> Integer -> m (Maybe a)

it'll work.

...

Thanks guys.

---

My Matrix class definition follows below:

module Matrix (Matrix) where

import Data.Array import Data.Maybe (catMaybes) import Control.Monad (guard)

class Matrix m a   where     fromRows       :: [[a]] -> m a

<snip>

...

...

    vicinityRows   :: m a   -> Integer -> Integer -> [[Maybe a]]     vicinityMatrix :: m a   -> Integer -> Integer -> m (Maybe a)