Haskell type signature with multiple class constraints

haskell types static-typing type-signature
Andrew Wonnacott picture Andrew Wonnacott · Jun 19, 2012 · Viewed 13.5k times · Source

How can I have multiple class constraints, so if A is an Eq and B is a Num, I could say either 

f :: Eq a => a -> b`

or 

f :: Num b => a -> b

So, how can I have Eq a => and Num b => at the same time?

  • f :: Eq a => Num b => a -> b,
  • f :: Eq a -> Num b => a -> b, and
  • f :: Eq a, Num b => a -> b

didn't do what I wanted.

Answer

Asherah picture Asherah · Jun 19, 2012

They're usually called class constraints, as Eq and Num are called type-classes.

How about this?

f :: (Eq a, Num b) => a -> b

The parentheses are significant.

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