# [Haskell-cafe] Bifold: a simultaneous foldr and foldl

Thu Dec 2 04:35:21 CET 2010

```On 11/30/10 13:43, Noah Easterly wrote:
> On Tue, Nov 30, 2010 at 9:37 AM, Larry Evans <cppljevans at suddenlink.net
>
>     suggested to me that bifold might be similar to the function, Q, of
>     section 12.5 equation 1) on p. 15 of:
>
>      http://www.thocp.net/biographies/papers/backus_turingaward_lecture.pdf
>
[snip]
>
> [snip]
>
> Thanks, Larry, this is some interesting stuff.
>
> I'm not sure yet whether Q is equivalent - it may be, but I haven't been
> able to thoroughly grok it yet.
>
> For uniformity, I shifted the notation you gave to Haskell:
>
>   (.^) :: (a -> a) -> Int -> a -> a
>   f .^ 0 = id
>   f .^ n = f . (f .^ (n - 1))
>
>   (./) :: (b -> c -> c) -> [a -> b] -> (a->c) -> a -> c
>   (./) = flip . foldr . \h f g -> h <\$> f <*> g
>
>   _Q_ :: (b -> c -> c) -> (a -> b) -> (a -> a) -> (a -> c) -> a -> c
>   _Q_ h i j k = h <\$> i <*> (k . j)
>
> So the shorthand just states the equivalence of (_Q_ h i j) .^ n and
> (./) h [ i . (j .^ m) | m <- [0 .. n-1] ] . ( . (j .^ n))
>
> Looking at it that way, we can see that (_Q_ h i j) .^ n takes some
> initial value, unpacks it into a list of size n+1 (using i as the
> iterate function),
> derives a base case value from the final value (and some function k)
> maps the initial values into a new list, then foldrs over them.
>
> The _f_ function seems to exist to repeat _Q_ until we reach some
> stopping condition (rather than n times)
>
>   _f_ :: (b -> c -> c) -> (a -> b) -> (a -> a) -> (a -> Bool) -> (a ->
> c) -> a -> c
>  _f_ h i j p q a = if p a then q a else _Q_ h i j (_f_ h i j p q) a
>
> No simple way to pass values from left to right pops out at me, but I
> don't doubt that bifold could be implemented in foldr, and therefore
> there should be *some* way.
>

Hi Noah,

The attached is my attempt at reproducing your code and also
contains an alternative attempt at emulating the code in
section 12.5 of:

http://www.thocp.net/biographies/papers/backus_turingaward_lecture.pdf

However, ghci compilation of bifold produces an error message:

BifoldIfRecur.hs:20:19: parse error on input `='

OTOH, when this code is commented out and the test variable
is printed, the output is:

[1,2,3,999,3,2,1]
[3,2,1,999]
[1,2,3,999]
[(),(),()]

The first line is for a call to if_recur.  The other two are for
foldl and foldr where the binary operator is (flip (:)) and
(:), respectively.  The suffix after 999 of the 1st line suggests to
me that if_recur does something like foldr with the else_ function
is called, after which something like foldr is done, as indicated
by the [1,2,3] prefix before 999 of the 1st line.  So it seems
that both foldr and foldl are being done during if_recur, IOW,
it's a kinda bifold also.

Hopefully this sheds some light on how section 12.5 is related to
bifold; however, I'm still not completely sure what that relation
is :(

-regards,
Larry

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