[Haskell-cafe] Really need some help understanding a solution

Luke Palmer lrpalmer at gmail.com
Thu Mar 26 17:05:23 EDT 2009

On Thu, Mar 26, 2009 at 12:21 PM, GüŸnther Schmidt <gue.schmidt at web.de>wrote:

> Hi guys,
> I tried for days now to figure out a solution that Luke Palmer has
> presented me with, by myself, I'm getting nowhere.

Sorry, I meant to respond earlier.

They say you don't really understand something until you can explain it to a
six year old.  So trying to explain this to a colleague made me realize how
little I must understand it :-).  But I'll try by saying whatever come to

*Lazy* list processing is all about *right* associativity.  We need to be
able to output some information knowing that our input looks like a:b:c:...,
where we don't know the ...  I see IntTrie [a] as an infinite collection of
lists (well, it is [[a]], after all :-), one for each integer.  So I want to
take a structure like this:


And turn it into a structure like this:

0 -> ...
1 -> 2:...
2 -> ...
3 -> 4:5:...

(This is just a list in my implementation, but I intended it to be a trie,
ideally, which is why I wrote the keys explicitly)

So the yet-unknown information at the tail of the list turns into
yet-unknown information about the tails of the keys.  In fact, if you
replace ... with _|_, you get exactly the same thing (this is no

The spine of this trie is maximally lazy: this is key.  If the structure of
the spine depended on the input data (as it does for Data.Map), then we
wouldn't be able to process infinite data, because we can never get it all.
So even making a trie out of the list _|_ gives us:

{ 0 -> _|_, 1 -> _|_, 2 -> _|_, ... }

I.e. the keys are still there.  Then we can combine two tries just by
combining them pointwise (which is what infZipWith does).  It is essential
that the pattern matches on infZipWith are lazy. We're zipping together an
infinite sequence of lists, and normally the result would be the length of
the shortest one, which is unknowable.  So the lazy pattern match forces the
result ('s spine) to be infinite.

Umm... yeah, that's a braindump.   Sorry I couldn't be more helpful.  I'm
happy to answer any specific questions.


> He has kindly provided me with this code:
> import Data.Monoid
> newtype IntTrie a = IntTrie [a]
>    deriving Show
> singleton :: (Monoid a) => Int -> a -> IntTrie a
> singleton ch x = IntTrie $ replicate ch mempty ++ [x] ++ repeat mempty
> lookupTrie :: IntTrie a -> Int -> a
> lookupTrie (IntTrie xs) n = xs !! n
> instance (Monoid a) => Monoid (IntTrie a) where
>    mempty                            = IntTrie (repeat mempty)
>    mappend (IntTrie xs) (IntTrie ys) = IntTrie (infZipWith mappend xs ys)
> infZipWith f ~(x:xs) ~(y:ys) = f x y : infZipWith f xs ys
> test =  mconcat [singleton (n `mod` 42) [n] | n <- [0..]] `lookupTrie` 10
> It's supposed to eventually help me group a list of key value pairs and
> then further process them in a linear (streaming like) way.
> The original list being something like [('a', 23), ('b', 18), ('a', 34)
> ...].
> There are couple of techniques employed in this solution, but I'm just
> guessing here.
> The keywords I've been looking up so far:
> Memmoization, Deforestation, Single Pass, Linear Map and some others.
> Can someone please fill me in?
> Günther
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