Trying to apply map to "inner" arguments of functions in Haskell

Question

Let's define an arbitrary function

someFunc a b = ...

if I ever need it, I know I can do something like

map (someFunc a) [b0, b1, b2, ..., bn]

and I'll get as result

[(someFunc a b0), (someFunc a b1), ..., (someFunc a bn)]

There is nothing new here. But what if instead of using the map's 2nd argument to vary b, I wanted to vary a (a "inner" argument)?

map (someFunc ? b) [?0, ?1, ?2, ..., ?n]

Is there any way to accomplish this in Haskell? If not, what would be the work around for this?

I know I probably wasn't very clear about what I'm posting about. If needed, I can try to reformulate my question :(

Trust me (or anyone who did a bit of Haskell) - you'll need implicit currying. Propably because it becomes second nature, but at the very least so you can read the code point-free fanatics sometimes produce. — user395760
– user395760, Commented Dec 1, 2010 at 22:36

sepp2k · Accepted Answer · 2010-12-01 22:25:05Z

11

You can either use a lambda

map (\a -> someFunc a b) ...

or the higher order functionflip, which returns the given function with its arguments flipped around:

map (flip someFunc b) ...

answered Dec 1, 2010 at 22:25

sepp2k

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C. A. McCann Over a year ago

Don't forget (ab)using backticks and infix sections, e.g. (`someFunc` b). Usually not recommended, but can be acceptable for two-argument functions and/or functions intended to be used infix, like on.

Paul Johnson · Accepted Answer · 2010-12-01 22:26:36Z

4

You could use

flip :: (a -> b -> c) -> b -> a -> c

So you would say

map (flip someFunc b) [a1...]

For more complicated cases with more arguments you would have to use a lambda. In theory you can always do it with the right combination of flips and arguments, but the lambda version will probably be more readable.

answered Dec 1, 2010 at 22:26

Paul Johnson

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Antal Spector-Zabusky Over a year ago

You could also use lots and lots of flips: \x -> f x a1 a2 a3 a4 a5, for instance, is equivalent to flip (flip (flip (flip (flip f a1) a2) a3) a4) a5 (code obtained via the pointfree program). But, uh, don't do this :-)

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