How should I aprroach situations when I know type of needed function but it is not defined yet? I suspect function I'm looking for is easily composable from others, but I don't know where to start looking.
Prelude> :hoogle (b->b->c)->(a->b)->(a->b)->a->a->c
No results found
To be more specific, I have (b->b->c) function at hand but I need to process (lift maybe?) the arguments first. Can hoogle find functions with arguments in different order than specified?
This is close to what I need, but I want to apply different function to each argument.
Prelude Data.Function> :t on
on :: (b -> b -> c) -> (a -> b) -> a -> a -> c
Seeing as your question asked how this function can be composed from other functions, I would like to point out that you can write it as:
import Control.Arrow
on2 :: (b -> b -> c) -> (a -> b) -> (a -> b) -> a -> a -> c
on2 f g h = curry (uncurry f . (g *** h))
However, the straightforward definition suggested by bheklilr is obviously just as concise and much more readable for people who are not used to working with Arrows.
And, yes, as far as I know, Hoogle does also search for functions in which the order of argument types you specified is permuted.