What does the `<<` operator mean in elm?

Question

In the following code taken from Elm Form Example, line 122, what does the << operator mean?

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Answer 1

My second attempt :D

<< vs <|

The difference between << and <| is that << is used to compose functions and <| is used to omit parentheses.

Why it works like that

Let's look at type annotation found here:

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

This definition tells us, that when you pass two functions to function <<, you will get function a -> c.

Example with demo

hi a =
    a + 2
hello a =
    a * 2
bye =
    hello << hi
c =
    bye 3

c returns value 10.

Read more about:

• infix operators - first argument on the left of function,
• partial application - when you pass one argument to function expecting two arguments, you get function expecting one argument.