[Haskell-cafe] foldr (.) id

[Greg Fitzgerald]

sorry for the buggy code

> let parseOrIgnore p s = either (const s) id $ parse p s
> let parseAllOrIgnore = compose . map parseOrIgnore [p1, p2, p3]
> parseAllOrIgnore "abbbcccbbba"

On Fri, Oct 26, 2012 at 2:11 PM, Greg Fitzgerald <garious at gmail.com> wrote:

> 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 at gmail.com> wrote:
>
>> >>>>> Greg Fitzgerald <garious at 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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