[Haskell-cafe] foldr (.) id

[Tony Morris]

It's the Endo monoid.

?> :t ala Endo foldMap -- see newtype package
ala Endo foldMap :: Foldable t => t (a -> a) -> a -> a
?> ala Endo foldMap [(+1), (*2)] 8
17
?> :i ala
ala ::
  (Newtype n o, Newtype n' o') =>
  (o -> n) -> ((o -> n) -> b -> n') -> b -> o'
      -- Defined in Control.Newtype


On 27/10/12 04:41, Greg Fitzgerald wrote:
> Hi Haskellers,
>
> I've recently found myself using the expression: "foldr (.) id" to compose
> a list (or Foldable) of functions.  It's especially useful when I need to
> map a function over the list before composing.  Does this function, or the
> more general "foldr fmap id", defined in a library anywhere?  I googled and
> hoogled, but no luck so far.
>
> Thanks,
> Greg
>
>
>
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-- 
Tony Morris
http://tmorris.net/


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