如何与多个操作员一起使用 Map

Question

is it possible to use map with 2 operators in it like map ((*2)+3) [1,2,3,4] ? If is yes how? i want to make a function that apply (x*2)+3 on a list.

Answer 1

You map a single function. But that function can do whatever you want. Your example can be given as an anonymous function for example:

map (\n -> n * 2 + 3) [1,2,3,4]

But you can also use function composition, which is likely more readable for a case like this where your function is a case of "do one thing, then the other":

map ((+3) . (*2)) [1,2,3,4]

This in turn is equal (by the so-called "functor laws") to

map (+3) . map (*2) $ [1,2,3,4]

Which I personally find a little more readable.

Answer 2

I think it is easy to first introduce a lambda expression here:

map () [1,2,3,4]

This will do exactly what you want. You can perform " function composition " with the (.):: (b -> c) -> (a -> b) -> a -> c function , and thus write this as:

map () [1,2,3,4]

Here you thus can see it as a "chain" of functions, where we pass the input to the rightmost function (here (*2) ), and then the result is passed to the leftmost function (here (+3) ).