Question

The function const is defined in Prelude as:

const x _ = x

In GHCi, when I tried

Prelude> const 6 5  -> Gives 6

But when I tried

Prelude> const id 6 5 -> Gives 5

Even after making changes like

Prelude> (const id 6) 5 -> Gives 5

Shouldn’t this function give 6 as the output of function id has type id :: a -> a which should bind like

Prelude> (const 6) 5 -> Gives 6

Why does function const behave differently?

Answer 1

You seem to be thinking that this is equivalent to const (id 6) 5, where id 6 evaluates to 6, but it isn’t. Without those parentheses, you’re passing the id function as the first argument to const. So look at the definition of const again:

const x _ = x

This means that const id 6 = id. Therefore, const id 6 5 is equivalent to id 5, which is indeed 5.