[Haskell-cafe] Graphical graph reduction

Mike Thyer mike at thyer.name
Mon Feb 25 19:04:32 EST 2008

Kai wrote:
> Thank you all for showing interest and responding.
>> Check out http://thyer.name/lambda-animator/. Requires Java.
> Wow, this is SUCH a cool tool. Best discovery in a long time! I think
> I need to brush up on my lambda-calculus, because it took me some time
> to figure out what settings to use to get something similar to
> Haskell. Function strategy "substitute by name", Argument strategy
> "call by need" and Reduce to "weak head normal form" seem to work OK,
> though.

Yes those are the correct settings for a Haskell-like reduction order.

> One issue, though. When trying out my infinite-list-of-fibs example, I
> have an auxiliary function
> (define nth (lambda (n xs) (if (= n 0)(head xs)(nth (- n 1) (tail xs)))))
> and this is not behaving the way I want, i.e. like the Haskell function
> nth n (x:xs) = if n == 0 then x else nth (n-1) xs
> because it doesn't do pattern matching, i.e. it doesn't force the
> argument to be evaluated until it fits the x:xs pattern. I can't
> figure out to simulate this eager pattern in the lisp that the
> lambda-animator uses. This means that if I e.g. do a (nth 8 fibs) in
> the animator, I get a long string of tail's before it actually starts
> expanding the fibs list...

You can use the seq function to force arguments to be evaluated.  For example:

(define nth (lambda (n xs) (seq xs if (= n 0)(head xs)(nth (- n 1) (tail xs)))))

The input language is a curried dialect of Scheme.  The seq function only takes two arguments.  It reduces its first argument and then returns its second.  This ensures the pair at the root of xs is
evaluated before the evaluation of the function body continues.

You can similarly use seq to prevent an unbounded build up of suspended computations in the fibs list, for example:

(define head-strict-cons (lambda (h t) (seq h cons h t)))
(define zipWith (lambda (f xs ys) (head-strict-cons (f (head xs) (head ys)) (zipWith f (tail xs) (tail ys)))))
(define fibs (letrec ((fibs (cons 1 (cons 1 (zipWith + fibs (tail fibs)))))) fibs))
(define nth (lambda (n xs) (seq xs if (= n 0)(head xs)(nth (- n 1) (tail xs)))))
(define fib (lambda (n) (nth n fibs)))

You'll see that the size of the graph is bounded and that no more than three elements of the infinite fibs list exist at any one time.

Lambda Animator doesn't automatically do any of the deforestation that a Haskell compiler will, so there are more reduction steps and larger graphs in the reduction of the above program than you would
expect with a compiled Haskell program.


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