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

Another use is to implement class member functions that have a dummy argument which should not be evaluated (used to resolve ambiguous types). Example that could be in Data.bits:

instance Bits Int where
  isSigned = const True
  bitSize  = const wordSize
  ...

By using const we explicitly say that we are defining constant values.

Personally I dislike the use of dummy parameters, but if they are used in a class then this is a rather nice way of writing instances.

Answer 2

Say you want to create a list of Nothings equal to the length of a string. As const returns its first argument, no matter the second, you can do:

listOfNothings :: String -> [Maybe Char]
listOfNothings = (map . const) Nothing

or, more explicitly:

listOfNothing st = map (const Nothing) st

Answer 3

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.