Elm: understanding foldp and mouse-clicks

Question

I'm currently learning Elm. relatively new to functional programming. i'm trying to understand this example from http://elm-lang.org/learn/Using-Signals.elm on counting mouse-clicks. they provide the following code:

clickCount =
    foldp (\click count -> count + 1) 0 Mouse.clicks 

They explain that foldp takes three arguments: a counter-incrementer, which we defined as an anonymous function with two inputs, a starting state 0, and the Mouse.clicks signal.

I do not understanding why we need the variable click in our anonymous function. Why can't we just have \count -> count + 1? Is the extra input getting bound to one of our inputs into foldp?

thanks!

Answer 1

You need it because foldp expects a function with two inputs. In this case, the first input is just ignored by your lambda, but the foldp implementation still puts something in there. Mouse.clicks always puts a sort of do-nothing value called Unit in there.

Some signals have a value associated with them, like Mouse.position, for example. If you wanted to do something like measure how far the mouse has moved, you would need to use that parameter.