Bdtjtk

I do not understand why i Show typeclass complains on using a custom type since i already provided an instance for it :

Custom Type

data Numeric=I Int | D Double 



instance Show Numeric where

        show (I x)=show x

        show (D y)=show y



 instance Num Numeric where

        (+) (I a) (I b) =I (a+b)

        (+) (D a) (I b) =D (a+ fromIntegral b)

        (+) (I a) (D b)=D (fromIntegral a+b)

        (-) (D a) (I b)= D (a- fromIntegral b)

        (-) (I a) (D b)=D(fromIntegral a -b)

Method that complains

arrayToString::Num a=>[a]->String

arrayToString arr =intercalate "," $ map show arr

So given my type that implements Num and Show typeclasses i do not understand why it has renders this error when i am feeding arrayToString a [Numeric] value

Error

* Could not deduce (Show a) arising from a use of `show'

      from the context: Num a

        bound by the type signature for:

                   arrayToString :: forall a. Num a => [a] -> String

        at Types.hs:40:5-37

      Possible fix:

        add (Show a) to the context of

          the type signature for:

            arrayToString :: forall a. Num a => [a] -> Strin

If you want to show something then use the Show constraint:

arrayToString :: (Show a) => [a] -> String

i do not understand why it has renders this error when i am feeding arrayToString a [Numeric] value

Notice the function arrayToString doesn't have anything specific to Num or Numeric. All it does is render a string for anything Showable (via show).

The problem is that you are not using your custom type.

The given type signature is arrayToString :: forall a. Num a => [a] -> String. You have universally quantified over all possible types a that are members of the Num typeclass. This includes your type but also every other numeric type that exists and ever will exist. This is too bold a statement.

You might want to consider changing the signature to arrayToList :: [Numeric] -> String, using your type directly, instead of relying on parametric polymorphism.

Also, as a side-note, is a singly linked list (or a cons-list), not an array.

EDIT: To translate this issue into logic, what you have done is to say

• There is a set of things called Numeric
  


• Elements of Numeric are members of the classes Num and Show
  


• Therefore, for every possible type ever that is in the Num class, I can turn a list of that type into a string

You have drastically increased the scope of the statement so that it includes a whole bunch of types we know nothing about and have no proof that they can be turned into strings.