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 <jwieg...@gmail.com> wrote: > >>>>> Greg Fitzgerald <gari...@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 >
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