[Haskell-cafe] Lazy series [was : Preventing sharing]
jerzy.karczmarczuk at unicaen.fr
Fri Jan 8 23:24:23 UTC 2016
Le 08/01/2016 21:33, Tom Ellis a écrit :
> On Fri, Jan 08, 2016 at 03:18:38PM -0500, Doug McIlroy wrote:
> exps = 1 + integral exps
> Can you explain why the first line won't work for lazy lists in a strict
> language? It seems perfectly fine to me.
If I understand well the issue, simply because at the RHS exps is not a
thunk, it is evaluated, which breaks down the co-recursivity of the
definition, even if the operators are delayed.
In a strict language you would have to use a kind of macros to make it work.
Good, old days...
I believe I wrote a paper on lazy power series already in 1996 (but
Douglas McI. hasn't read it).
(Theoretical Computer Science 187, pp. 203–219, (1997).)
I still have a copy:
and I tried hard to do some Computer Algebra with that, trying to
implement by force some lazy algorithms in a strict CA language MuPAD
(similar to Maple, with strong OO structure). It was clumsy and almost
nobody was convinced, when I presented this in Paderborn, the home of MuPAD.
Then MuPad quit the free software world, got attached to MathWorks, and
I simply forgot this experiment. I tried to convince some students to
continue it using Mathematica (Hold, HoldRest, etc. permit to construct
delayed lists; in general, a rewriting system seems better adapted to
such kind of implementation, than purely procedural languages), but the
results were ugly, and my dear students asked me to give them some other
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