What's the point of 'const' in the Haskell Prelude?

Question

Looking through the Haskell Prelude, I see a function const:

const x _ = x 

I can't seem to find anything relevant regarding this function.

What's the point? Can anyone give an example of where this function might be used?

Answer 1

It's useful for passing to higher-order functions when you don't need all their flexibility. For example, the monadic sequence operator >> can be defined in terms of the monadic bind operator as

x >> y = x >>= const y 

It's somewhat neater than using a lambda

x >> y = x >>= \_ -> y 

and you can even use it point-free

(>>) = (. const) . (>>=) 

although I don't particularly recommend that in this case.

Answer 2

To add to hammar's excellent direct answer: humble functions like const and id are really useful as a higher order function for the same reason that they are fundamental in the SKI combinator calculus.

Not that I think haskell's prelude functions were modeled consciously after that formal system or anything. It's just that creating rich abstractions in haskell is very easy, so you often see these types of theoretical things emerge as practically useful.

Shameless plug, but I blogged about how the Applicative instance for (->) are actually the S and K combinators here, if that's the kind of thing you're into.