Question

In Haskell, why does this compile:

splice :: String -> String -> String
splice a b = a ++ b
main = print (splice "hi" "ya")

but this does not:

splice :: (String a) => a -> a -> a
splice a b = a ++ b
main = print (splice "hi" "ya")

>> Type constructor `String' used as a class

I would have thought these were the same thing. Is there a way to use the second style, which avoids repeating the type name 3 times?

Answer 1

The => syntax in types is for typeclasses.

When you say f :: (Something a) => a, you aren’t saying that a is a Something, you’re saying that it is a type “in the group of” Something types.

For example, Num is a typeclass, which includes such types as Int and Float.
Still, there is no type Num, so I can’t say

f :: Num -> Num
f x = x + 5

However, I could either say

f :: Int -> Int
f x = x + 5

or

f :: (Num a) => a -> a
f x = x + 5