CODEWITHSUNDEEP
haskell
thor
thor

Reputation: 22460

Changing the return type of a function in Haskell?

Is there is a succinct way to change the return type of a function?

Consider for example comparing in Data.Ord. It returns an Ordering. Yet I am only interested in whether the two items are equal or not. So I need to convert the resulting Ordering to a Bool.

A straight forward way I can think of is:

isEqualOn f x y = if comparing f x y==EQ then True else False

(or isEqualOn f x y = comparing f x y==EQ as here as pointed out in the comments).

Is there a more compositional way to do this (sort of adapting comparing or writing it in a pointless manner) without having to write out everything? Ideally, I am looking for something that works on n-ary functions as well.

-- Update --

As suggested by the answers/comments, the specific example above (isEqualOn) can be implemented using the standard on function as on (==). However, my question is about the general technique to change function return types in a compositional/pointless manner as exemplified by the SEC approach in the comments.

Upvotes: 3

Views: 407

Answers (2)

HaskellElephant
HaskellElephant

Reputation: 9891

As mentioned you can use the function on from Data.Function, but perhaps you are looking for the operator == which returns a Bool. In which case your function can be written as:

isEqualOn f x y = fx == fy

and personally I would never write this particular function but "inline" it where it is used.

On the more general matter of "changing the return type". What you are looking for is composition:

isEq Eq = True
isEq _  = False

equal x y = isEq (comparing x y)

or more haskell flavored:

equal x = isEq . comparing x

Upvotes: 0

behzad.nouri
behzad.nouri

Reputation: 77951

Your definition of isEqualOn would be the same as on (==):

\> import Data.Function (on)
\> :t (on (==))
(on (==)) :: Eq a1 => (a -> a1) -> a -> a -> Bool

for example to compare on absolute value:

\> let isEqualOn = on (==)
\> isEqualOn abs 2 (-2)
True

Upvotes: 5

Related Questions