Deriving list scans

I’ve been playing with deriving efficient parallel, imperative implementations of "prefix sum" or more generally "left scan". Following posts will explore the parallel & imperative derivations, but as a warm-up, I’ll tackle the functional & sequential case here.

Folds

You’re probably familiar with the higher-order functions for left and right "fold". The current documentation says:

foldl, applied to a binary operator, a starting value (typically the left-identity of the operator), and a list, reduces the list using the binary operator, from left to right:

foldl f z [x1, x2, ⋯, xn] ≡ (⋯((z `f` x1) `f` x2) `f`⋯) `f` xn

The list must be finite.

foldr, applied to a binary operator, a starting value (typically the right-identity of the operator), and a list, reduces the list using the binary operator, from right to left:

foldr f z [x1, x2, ⋯, xn] ≡ x1 `f` (x2 `f` ⋯ (xn `f` z)⋯)

And here are typical definitions:

foldl  (b → a → b) → b → [a] → b
foldl f z [] = z
foldl f z (x:xs) = foldl f (z `f` x) xs

foldr (a → b → b) → b → [a] → b
foldr f z [] = z
foldr f z (x:xs) = x `f` foldr f z xs

Notice that foldl builds up its result one step at a time and reveals it all at once, in the end. The whole result value is locked up until the entire input list has been traversed. In contrast, foldr starts revealing information right away, and so works well with infinite lists. Like foldl, foldr also yields only a final value.

Sometimes it’s handy to also get to all of the intermediate steps. Doing so takes us beyond the land of folds to the kingdom of scans.

Scans

The scanl and scanr functions correspond to foldl and foldr but produce all intermediate accumulations, not just the final one.

scanl  (b → a → b) → b → [a] → [b]

scanl f z [x1, x2, ⋯ ] ≡ [z, z `f` x1, (z `f` x1) `f` x2, ⋯]

scanr (a → b → b) → b → [a] → [b]

scanr f z [⋯, xn_1, xn] ≡ [⋯, xn_1 `f` (xn `f` z), xn `f` z, z]

As you might expect, the last value is the complete left fold, and the first value in the scan is the complete right fold:

last (scanl f z xs) ≡ foldl f z xs
head (scanr f z xs) ≡ foldr f z xs

which is to say

lastscanl f z ≡ foldl f z
headscanr f z ≡ foldr f z

The standard scan definitions are trickier than the fold definitions:

scanl  (b → a → b) → b → [a] → [b]
scanl f z ls = z : (case ls of
[] → []
x:xs → scanl f (z `f` x) xs)

scanr (a → b → b) → b → [a] → [b]
scanr _ z [] = [z]
scanr f z (x:xs) = (x `f` q) : qs
where qs@(q:_) = scanr f z xs

Every time I encounter these definitions, I have to walk through it again to see what’s going on. I finally sat down to figure out how these tricky definitions might emerge from simpler specifications. In other words, how to derive these definitions systematically from simpler but less efficient definitions.

Most likely, these derivations have been done before, but I learned something from the effort, and I hope you do, too.

Continue reading ‘Deriving list scans’ »

Enhancing a Zip

This post is part four of the zip folding series inspired by Max Rabkin’s Beautiful folding post. I meant to write one little post, but one post turned into four. When I sense that something can be made simpler/clearer, I can’t leave it be.

To review:

  • Part One related Max’s data representation of left folds to type class morphisms, a pattern that’s been steering me lately toward natural (inevitable) design of libraries.
  • Part Two simplified that representation to help get to the essence of zipping, and in doing so lost the expressiveness necessary to define Functorand Applicative instaces.
  • Part Three proved the suitability of the zipping definitions in Part Two.

This post shows how to restore the Functor and Applicative (very useful composition tools) to folds but does so in a way that leaves the zipping functionality untouched. This new layer is independent of folding and can be layered onto any zippable type.

You can get the code described below.

Edits:

  • 2009-02-15: Simplified WithCont, collapsing two type parameters into one. Added functor comment about cfoldlc'.

Continue reading ‘Enhancing a Zip’ »

Proofs for left fold zipping

More beautiful fold zipping

Another lovely example of type class morphisms

I read Max Rabkin’s recent post Beautiful folding with great excitement. He shows how to make combine multiple folds over the same list into a single pass, which can then drastically reduce memory requirements of a lazy functional program. Max’s trick is giving folds a data representation and a way to combine representations that corresponds to combining the folds.

Peeking out from behind Max’s definitions is a lovely pattern I’ve been noticing more and more over the last couple of years, namely type class morphisms.

Continue reading ‘Another lovely example of type class morphisms’ »