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.