### [Haskell-cafe] Re: appending an element to a list

```
Ronald Guida wrote:

Thank you, apfelmus.  That was a wonderful explanation; the debit
method in [1] finally makes sense.

A diagram says more than a thousand words :)

My explanation is not entirely faithful to Okasaki, let me elaborate.

In his book, Okasaki calls the process of transferring the debits from
the input

xs = x1 : x2 : x3 : ... : xn : []
111 ... 11  0

ys = y1 : y2 : y3 : ... : ym : []
111 ... 11  0

to the output

xs ++ ys = x1 : x2 : x3 : ... : xn : y1 : y2 : y3 : ... : ym : []
222 ... 22111 ... 11  0

debit inheritance. In other words, the debits of xs and ys (here 1 at
each node) are carried over to xs ++ ys (in addition to the debits
created by ++ itself). In the thesis, he doesn't give it an explicit
name, but discusses this phenomenon in the very last paragraphs of
chapter 3.4 .

The act of relocating debits from child to parent nodes as exemplified with

xs ++ reverse ys =
x1 : x2 : x3 : ... : xn : yn : y{n-1} : ... : y1 : []
111 ... 11n0 ... 00  0

xs ++ reverse ys =
x1 : x2 : x3 : ... : xn : yn : y{n-1} : ... : y1 : []
222 ... 2200 ... 00  0

is called debit passing, but Okasaki doesn't use it earlier than in
the chapter Implicit recursive slowdown. But the example I gave here
is useful for understand the scheduled implementation of real time
queues. The trick there is to not create a big suspension with  n
debits but to really physically distribute them across the data structure

x1 : x2 : x3 : ... : xn : yn : y{n-1} : ... : y1 : []
222 ... 2222 ... 22  2

and discharge them by forcing a node with every call to  snoc . I say
physically because this forcing performs actual work, it does not
simply mentally discharge a debit to amortize work that will be done
later. Note that the 2 debits added to each  yi  are an overestimation
here, but the real time queue implementation pays for them nonetheless.

My focus on debit passing in the original explanation might suggest that
debits can only be discharged when actually evaluating the node to which
the debit was assigned. This is not the case, an operation may discharge
any debits, even in parts of the data structure that it doesn't touch.
Of course, it must discharge debits of nodes it does touch.

For instance, in the proof of theorem 3.1 (thesis) for queues, Okasaki
writes We can restore the invariant by discharging the first (two)
debit(s) in  the queue without bothering to analyze which node this
will be. So, the front queue might look like

f1 : f2 : f3 : ... : fn : f{n+1} : f{n+2} : ... : fm : []
001 ... 11n0 ... 00  0

and it's one of the nodes that carries one debit, or it could look like

f2 : f3 : ... : fn : f{n+1} : f{n+2} : ... : fm : []
00 ... 00   n-3   0 ... 00  0

and it's the node with the large amount of debits. In fact, it's not
even immediate that these two are the only possibilities.

However, with the debit passing from my previous post, it's easier to
say which node will be discharged. But even then, only  tail  discharges
exactly the debits of nodes it inspects while the  snoc  operation
discharges debits in the untouched front list. Of course, as soon as
identifying the nodes becomes tractable, chances are that you can turn
it into a real-time data structure.

Another good example are skew heaps from

[2]:
Chris Okasaki. Fun with binary heap trees.
in  J. Gibbons, O. de Moor. The Fun of Programming.
http://www.palgrave.com/PDFs/0333992857.Pdf

Here, the good nodes are annotated with one debit. Every  join
operation discharges O(log n) of them and allocates new ones while
walking down the tree, but the time to actually walk down the tree is
not counted immediately. This is just like (++) walks down the first
list and allocates debits without immediately using O(n) time to do that.

Regards,
apfelmus

PS: In a sense, discharging arbitrary debits can still be explained with
debit passing: first pass those debits to the top and the discharge them
because any operation has to inspect the top.

___

```

### [Haskell-cafe] Re: appending an element to a list

```
Abhay Parvate wrote:

I somehow thought it would be easy to talk about complexity of calculating
individual elements in an infinite list should be sufficient, but that seems
to be involved, and my over-generalization doesn't seem to work. Thanks for
the link; particularly it has reference to Wadler's papers exactly on this
problem.

Note however that Wadler's and similar formalisms are still a
unsatisfactory in that they are quite clumsy to work with, it's quite
tedious/impossible to analyze examples with a lot of lazy evaluation.
But they are a good guideline.

In his book about purely functional data structures [1], Okasaki takes a
different approach; each node of a data structure is given a debit, a
cost to evaluate it. For instance, consider

xs = x1 : x2 : x3 : ... : xn : []
111 ... 11  0

ys = y1 : y2 : y3 : ... : ym : []
111 ... 11  0

The numbers below indicate the time it takes to evaluate the node to
weak head normal form. For demonstration purposes, I arbitrarily chose 1
for each (:) here.

The combined list will then have debits like

xs ++ ys = x1 : x2 : x3 : ... : xn : y1 : y2 : y3 : ... : ym : []
222 ... 22111 ... 11  0

In other words, the ys list is copied verbatim but each element of xs
incurs an additional cost of 1, corresponding to one step in the
evaluation of the concatenation with (++).

In order to force/inspect a constructor/node, you have to pay off its
debits first. In the above example,  head (xs ++ ys)  would have to pay
2 units of time (one unit for  head xs  and one for the (++)). Now, the
thing about debits is that we can relocate them to the top and only
overestimate the total running time if we do that. For instance, we
could push all debits to the top

xs ++ ys = x1   : x2 : x3 : ... : xn : y1 : y2 : y3 : ... : ym : []
2n+m   00 ... 00000 ... 00  0

so that evaluating  head (xs ++ ys)  is now estimated to cost (2n+m)
units of time while the rest is free/fully evaluated.

The above example is rather useless, but consider the case  n == m  and

xs = x1 : x2 : x3 : ... : xn : []
000 ... 00  0

ys = y1 : y2 : y3 : ... : yn : []
000 ... 00  0

i.e. two fully evaluated lists of the same length. Then, we have

xs ++ reverse ys =
x1 : x2 : x3 : ... : xn : yn : y{n-1} : ... : y1 : []
111 ... 11n0 ... 00  0

because reversing the list ys is monolithic, i.e. looking at its  head
already forces the tail of the list. But now, we can distribute the
debits upwards

xs ++ reverse ys =
x1 : x2 : x3 : ... : xn : yn : y{n-1} : ... : y1 : []
222 ... 2200 ... 00  0

and thereby amortize the cost of reversing the second lists over the n
elements of the first list. This is used in the implementation of purely

[1]: Chris Okasaki. Purely Function Data Structures.
http://www.cs.cmu.edu/~rwh/theses/okasaki.pdf
(This is the thesis on which the book is based.)

Regards,
apfelmus

___

```

### Re: [Haskell-cafe] Re: appending an element to a list

```Thank you, apfelmus.  That was a wonderful explanation; the debit
method in [1] finally makes sense.

[1]: Chris Okasaki. Purely Function Data Structures.
http://www.cs.cmu.edu/~rwh/theses/okasaki.pdf
___

```

### [Haskell-cafe] Re: appending an element to a list

```
Tillmann Rendel wrote:

Abhay Parvate wrote:

I think I would like to make another note: when we talk about the complexity
of a function, we are talking about the time taken to completely evaluate
the result. Otherwise any expression in haskell will be O(1), since it
just creates a thunk.

I don't like this notion of complexity, since it seems not very suited
for the analysis of composite expression in Haskell.

Is this intuitive view generalizable to arbitrary datatypes (instead of
lists) and formalized somewhere?

Regards,
apfelmus

___

```

### Re: [Haskell-cafe] Re: appending an element to a list

```I somehow thought it would be easy to talk about complexity of calculating
individual elements in an infinite list should be sufficient, but that seems
to be involved, and my over-generalization doesn't seem to work. Thanks for
the link; particularly it has reference to Wadler's papers exactly on this
problem.

Abhay

On Sun, Jun 1, 2008 at 1:07 PM, apfelmus [EMAIL PROTECTED] wrote:

Tillmann Rendel wrote:

Abhay Parvate wrote:

I think I would like to make another note: when we talk about the
complexity
of a function, we are talking about the time taken to completely evaluate
the result. Otherwise any expression in haskell will be O(1), since it
just creates a thunk.

I don't like this notion of complexity, since it seems not very suited for
the analysis of composite expression in Haskell.

Is this intuitive view generalizable to arbitrary datatypes (instead of
lists) and formalized somewhere?

Regards,
apfelmus

___

___

```

### Re: [Haskell-cafe] Re: appending an element to a list

```
Abhay Parvate wrote:

I think I would like to make another note: when we talk about the complexity
of a function, we are talking about the time taken to completely evaluate
the result. Otherwise any expression in haskell will be O(1), since it just
creates a thunk.

I don't like this notion of complexity, since it seems not very suited
for the analysis of composite expression in Haskell. For example,

repeat 42

has infinite complexity according to your definition (it doesn't even
terminate if completely evaluated), but

take 5 \$ repeat 42

has only constant complexity even if fully evaluated. It is not clear
how to reuse the finding about the complexity of (repeat 42) to
determine the complexity of (take 5).

Instead, my view of complexity in lazy languages includes the
interesting behaviour of the rest of the program as variables. For
example, (repeat 42) needs O(n) steps to produce the first n elements of
its output. Now, (take 5 x) restricts x to the first 5 elements, so
(take 5 \$ repeat 42) needs O(min(n, 5)) = O(1) steps to produce the
first n elements of its output.

Is this intuitive view generalizable to arbitrary datatypes (instead of
lists) and formalized somewhere?

Tillmann
___

```

### [Haskell-cafe] Re: appending an element to a list

```Tillmann Rendel [EMAIL PROTECTED] writes:

Hi! (Cool, another guy from DAIMI on haskell-cafe!)

Another (n - 1) reduction steps for the second ++ to go away.

last (o ++ l)
A)  ~  last ('o' :  ++ l))
L)  ~  last ( ++ l)
A)  ~  last (l)
L)  ~  'l'

And the third and fourth ++ go away with (n - 2) and (n - 3) reduction
steps. Counting together, we had to use

n + (n - 1) + (n - 2) + ... = n!

reduction steps to get rid of the n calls to ++, which lies in O(n^2).
Thats what we expected of course, since we know that each of the ++
would need O(n) steps.

I really liked the excellent and very clear analysis! But the last
formula should be:

n + (n - 1) + (n - 2) + ... = n * (n+1) / 2

which is still of order n^2.

--
Martin Geisler

VIFF (Virtual Ideal Functionality Framework) brings easy and efficient
SMPC (Secure Multi-Party Computation) to Python. See: http://viff.dk/.

pgpxgbfpNjJtq.pgp
Description: PGP signature
___

```

### Re: [Haskell-cafe] Re: appending an element to a list

```I think I would like to make another note: when we talk about the complexity
of a function, we are talking about the time taken to completely evaluate
the result. Otherwise any expression in haskell will be O(1), since it just
creates a thunk.
And then the user of the program is to be blamed for running the program,
since that is what caused evaluation of those thunks :)

Abhay

2008/5/31 Martin Geisler [EMAIL PROTECTED]:

Tillmann Rendel [EMAIL PROTECTED] writes:

Hi! (Cool, another guy from DAIMI on haskell-cafe!)

Another (n - 1) reduction steps for the second ++ to go away.

last (o ++ l)
A)  ~  last ('o' :  ++ l))
L)  ~  last ( ++ l)
A)  ~  last (l)
L)  ~  'l'

And the third and fourth ++ go away with (n - 2) and (n - 3) reduction
steps. Counting together, we had to use

n + (n - 1) + (n - 2) + ... = n!

reduction steps to get rid of the n calls to ++, which lies in O(n^2).
Thats what we expected of course, since we know that each of the ++
would need O(n) steps.

I really liked the excellent and very clear analysis! But the last
formula should be:

n + (n - 1) + (n - 2) + ... = n * (n+1) / 2

which is still of order n^2.

--
Martin Geisler

VIFF (Virtual Ideal Functionality Framework) brings easy and efficient
SMPC (Secure Multi-Party Computation) to Python. See: http://viff.dk/.

___