Question

This question is about type in Haskell programming language.

When I type

:t []

I get the return as:

[] :: [a]

What is [a]?

Answer 1

Rewriting it like so might make things clearer:

[] :: forall a. [a]

So, since it doesn’t contain any value, haskell can’t decide on a type and leaves it open. If you use it in a typed expression, however, like

x = []

f :: [Int] -> [Int]
f l = l

y = f x

it automatically gets resolved to [Int], since it is used as this type.