Hmm, neato.  but didn't make life any easier!

Data.Monoid> (appEndo . mconcat . map Endo) [(+10), (+20)] 3

33

Data.Monoid> (foldr (.) id) [(+10), (+20)] 3

33

I had hoped for something like:

> mconcat [(+10), (+20)] 3

But I suppose that's nonsense, considering this works:

> mconcat [(++"10"), (++"20")] "3"

"310320"

I think this is the most general solution?

import Control.Category

import Data.Foldable

import Prelude hiding (foldr, (.), id)

compose :: (Foldable t, Category cat) => t (cat a a) -> cat a a

compose = foldr (.) id

Usage:

> compose [(+10), (+20)] 3

Real-world use case:

> let parseOrIgnore p = either (const s) id . parse p s

> parseAllOrIgnore = compose . map parseOrIgnore [p1, p2, p3]

Naming:

"(.)/compose" is consistent with "(+)/sum", "(*)/product", "(&&)/and", etc.

Thoughts?

-Greg

On Fri, Oct 26, 2012 at 12:31 PM, John Wiegley <jwiegley@gmail.com> wrote:

>>>>> Greg Fitzgerald <garious@gmail.com> writes:

> I've recently found myself using the expression: "foldr (.) id" to compose a
> list (or Foldable) of functions.

You want the Endo monoid:

    ghci> appEndo (Endo (+ 10) <> Endo (+ 20)) $ 3
      33

John