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 t

Answer 1

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

Suppose you would like to generate all subsequences of a given list.

For each list element, at a given point you have a choice of True (include it in the current subsequence) or False (do not include it). This can be done using the filterM function.

Like this:

 λ> import Control.Monad
 λ> :t filterM
 filterM :: Applicative m => (a -> m Bool) -> [a] -> m [a]
 λ> 

For example, we want all the subsequences of [1..4].

 λ> filterM  (const [True, False])  [1..4]
 [[1,2,3,4],[1,2,3],[1,2,4],[1,2],[1,3,4],[1,3],[1,4],[1],[2,3,4],[2,3],[2,4],[2],[3,4],[3],[4],[]]
 λ>