Why i get this error in my haskell code

Question

Why i get this error in my code: code:

module Task5 where

import Prelude

data Stream a = a :& Stream a

infixl 4 :&

add :: Num a => a -> a -> a
add a b = a + b

instance Num (Stream a ) where
    (+) (ia:&a) (ib :& b) = (ia + ib) :& ((+) a b)

error:

Task5.hs:14:33:
    No instance for (Num a) arising from a use of `+'
    Possible fix:
      add (Num a) to the context of the instance declaration
    In the first argument of `(:&)', namely `(ia + ib)'
    In the expression: (ia + ib) :& ((+) a b)
    In an equation for `+':
        + (ia :& a) (ib :& b) = (ia + ib) :& ((+) a b)

I don't understand how to correct this , i tried it for all week , but not find any solution. So , can you say me what i should correct?

Ganesh Sittampalam · Accepted Answer · 2014-07-01 05:50:52Z

4

As the error suggests, you need to add Num a to the "context" of your instance declaration:

instance Num a => Num (Stream a) where

Otherwise the ia + ib operation doesn't have any (+) operation available, as that is acting on the individual stream members.

answered Jul 1, 2014 at 5:50

Ganesh Sittampalam

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