Question

For this piece of haskell code:

isIn :: (Eq a) => [a] -> [a] -> Bool
needle `isIn` haystack = any (needle `isPrefixOf`) (tails haystack)

This is a function definition I believe. How do I understand what the input arguments are and what the return type is?

For example: what is (Eq a)?

Answer 1

Eq is what’s called a typeclass. It declares a few functions, in this case == and friends, and we can make instances of that typeclass which provide definitions for == and others.

This means that when we have something that’s an instance of the Eq typeclass, we know we can use == on it.

The trick here is that in our function, we need to have types which make it possible to check that they’re equal. If we just had [a] -> [a] -> Bool then we’d be in trouble because we’d have just promised that our implementation works on things without an == operator which it doesn’t.

Because of this we use the => which adds context to our function definition. It says something like “This will work for any a as long as a is an instance of the Eq typeclass”. That way, we can use == safely and know that all our argument types will implement it appropriately.

Quick Illustration

This is an error:

doIfEqual :: a -> a -> (a -> a -> [a])
doIfEqual a b f = if a==b then f a b else []

but this works because we specify a is an instance of Eq

doIfEqual (Eq a) => a -> a -> (a -> a -> [a])
doIfEqual a b f = if a==b then f a b else []