Haskell type signature with multiple class constraints

问题

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.

回答1:

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.

来源：https://stackoverflow.com/questions/11093847/haskell-type-signature-with-multiple-class-constraints