Haskell functions left-to-right

Question

I have a function that I use quite frequently, which allows me to write my code in a way which seems more natural to me.

infixl 6 $:
($:) :: a -> (a -> b) -> b
a $: f = f a

This lets me do something like

let x = getData
        $: sort
        $: group
        $: aggregate

instead of

let x = aggregate 
        $ group 
        $ sort 
        $ getData

I recently learned that Clojure has something like this built in (I don't know much Clojure, but I think it would be written (-> getData sort group aggregate)?) which makes me wonder if Haskell has it built in as well. Hoogle doesn't have any results though.

Are there any standard libs with something similar included? It probably makes my code hard for others to read if I have such a common part is idiosyncratic.

Answer 1

The reverse application operator your are describing is now part of the standard package base (since 4.8.0) as the & operator.

Note that this operator is defined with a lower precedence than the one you proposed (infixl 1).

Answer 2

There's nothing like this built in, but Control.Category.(>>>) is close: it's flip (.), so you can write

f x = x $: sort $: group $: aggregate

as

f = sort >>> group >>> aggregate

There's no shortage of definitions and names for your ($:) combinator. I think functions tend to suit the pipeline style more often than simple applications, so I don't feel any great need for it; (>>>) is a bit ugly, though.

(Besides, Haskell's non-strict semantics mean that the flow of data isn't necessarily in the direction the arrows are pointing here; after all, aggregate could provide the first constructor before sort even gets a chance to look at the argument. So I tend to just use (.) and ($); I'm used to the order.)