Hello everyone,

I am just starting with Haskell so please bear with me.

Here's my question:

Consider the below definition / output:

Prelude> :t (+)

(+) :: (Num a) => a -> a -> a

What I understand from the above is that "+" is a function that takes two args

which are types of anything that IS-AN instance of "Num" (Int, Integer, Float, Double)

and returns an instance of "Num".

Hence this works fine:

Prelude> 4.3 + 2

6.3

But I can't understand why this doesn't work:

Prelude> 4.3 + 4 :: Int

<interactive>:1:0:

    No instance for (Fractional Int)

      arising from the literal `4.3' at <interactive>:1:0-2

    Possible fix: add an instance declaration for (Fractional Int)

    In the first argument of `(+)', namely `4.3'

    In the expression: 4.3 + 4 :: Int

    In the definition of `it': it = 4.3 + 4 :: Int

I expected that the second addition would work as both "Float" and "Int" are

instances of "Num". Is it that since both the formal args are defined as "a" they

have to be exactly the same instances? Had "+" been defined something like: 

(+) :: (Num a, Num b) => a -> b -> a

my second addition would have worked?

Please let me know what I am missing.

Regards,

Venu Chakravorty.

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