# [Haskell-cafe] Would you mind explain such a code ?

Roman Cheplyaka roma at ro-che.info
Thu Sep 10 05:47:05 EDT 2009

```* zaxis <z_axis at 163.com> [2009-09-10 00:51:21-0700]
>
>
> As I understand  foldr (\x g -> g . (`f`x)) id xs  will return a function
> such as (`f` 3).(`f` 2).(`f` 1) . You have already made it clear !  However,
> why  does the "step" function below has three parameters ? I think foldr
> will call step using two parameters, the 1st is list element and the 2nd is
> a funtion whose initial value is id).

That's right.

step x g a = g (f a x)

is, thanks to currying, another way to write

step x g = \a -> g (f a x)

This is what we want -- function of two arguments, which returns a new
value of accumulator (which in this case is a function itself).

And

g (f a x)

can be rewritten as

g ((f a) x) = g (a `f` x) = g ((`f` x) a) = (g . (`f` x)) a,

thus

step x g = \a -> g (f a x) = \a -> (g . (`f` x)) a = g . (`f` x)

(the last step is called 'eta-reduction'). Remember that g is a previous
value of accumulator and step x g is its new value, so on each step we
compose accumulator with (`f` x) to get the new value. So in the end it will
look like you wrote above.

> myFoldl f z xs = foldr step id xs z
>     where step x g a = g (f a x)
>
>
> staafmeister wrote:
> >
> >
> >
> > zaxis wrote:
> >>
> >> myFoldl :: (a -> b -> a) -> a -> [b] -> a
> >>
> >> myFoldl f z xs = foldr step id xs z
> >>     where step x g a = g (f a x)
> >>
> >> I know myFoldl implements foldl using foldr. However i really donot know
> >> how it can do it ?
> >>
> >> Please shed a light one me, thanks!
> >>
> >
> > Hi,
> >
> > Nice example! Well this is indeed an abstract piece of code. But basically
> > foldl f z xs starts with z and keeps applying (`f`x) to it
> > so for example foldl f z [1,2,3] = ((`f`3).(`f`2).(`f`1)) z
> >
> > Because functions are first-class in haskell, we can also perform a foldl
> > where instead of calculating the intermediate values we calculate the
> > total function, i.e. ((`f`3).(`f`2).(`f`1)) and apply it to z.
> >
> > When the list is empty z goes to z, so the start function must be id.
> > So we can write
> > (`f`3).(`f`2).(`f`1) = foldr (\x g -> g . (`f`x)) id xs
> >
> > This is almost in your form.
> >
> > Hope this helps,
> > Gerben
> >
>
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