CODEWITHSUNDEEP
haskelltypesstatic-typingtype-signature
Andrew Wonnacott
Andrew Wonnacott

Reputation: 902

Haskell type signature with multiple class constraints

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?

didn't do what I wanted.

Upvotes: 35

Views: 17635

Answers (1)

Asherah
Asherah

Reputation: 19347

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.

Upvotes: 67

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