Could not deduce (Show t)

Question

I have this piece of code in Haskell that refuses to compile:

data (Eq a, Num a, Show a) => Mat a = Mat {nexp :: Int, mat :: QT a}
 deriving (Eq, Show)

data (Eq a , Show a ) => QT a = C a | Q (QT a ) (QT a ) (QT a ) (QT a )
 deriving (Eq, Show)


cs:: (Num t) => Mat t -> [t]

cs(Mat nexp (Q a b c d)) =(css (nexp-1) a c)++(css (nexp-1) b d)
    where
        css 0 (C a) (C b) = (a-b):[]
        css nexp (Q a b c d) (Q e f g h) = (zipWith (+) (css (nexp-1) a c) (css (nexp-1) e g))++(zipWith (+)(css (nexp-1) b d) (css (nexp-1) f h))

I have this error:

Could not deduce (Show t) arising from a use of `Mat'
from the context (Num t)
bound by the type signature for cs:: Num t => Mat t -> [t]

I searched online and found many similar questions, but nothing seems to get close to my problem. How can I make this code work?

You shouldn't use constraints on data types: http://stackoverflow.com/questions/12770278/typeclass-constraints-on-data-declarations — Tikhon Jelvis, Jul 31 '13 at 13:36

score 10 · Accepted Answer · edited May 23 '17 at 12:28

10

Class constraints on data types don't mean anything.

See this related question

Remove the constraint on the data type.

data Mat a = Mat {nexp :: Int, mat :: QT a}
    deriving (Eq, Show)

edited May 23 '17 at 12:28

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answered Jul 31 '13 at 13:35

Don Stewart

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Could not deduce (Show t)

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