How do we know if a typeclass is a sub

2018-06-15 07:36:40

If we do let add ab = a+b then add :: Num a => a -> a -> a .

We also know that 1.5 :: Fractional a => a . And add 1 1.5 works flawlessly.

If I understand correctly every type which has an instance of Fractional has also an instance of Num , but how is this fact made explicit?

It's in the definition of the Fractional class:

class Num a => Fractional a where
   ...

You can see it typing :i Fractional in GHCi, or on the Haddock docs.

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