Re: LOGO-L> visual versus text programming

[Jacques Duthen <inpact10@clr48pl.der.edf.fr>]

Brian Harvey wrote:
> 
> KERRB@Magill.UniSA.edu.au writes:
> >What is the distinction b/w creating procedures and composing functions??
> 
> By "composing" I didn't mean "writing," as in composing music.  I meant
> the mathematical sense, in which composing functions f and g means
> creating the new function h(x) = f(g(x)).
> 
> In Logo terms, procedures are divided into commands, which do things,
> and operations, which compute values; the operations correspond to
> mathematical functions.  (Strictly speaking, an operation is a function
> only if it always gives the same output for the same input(s), so for
> example RANDOM is an operation but not a function.)  The question of
> composition doesn't come up for commands, because they don't produce
> a value that can be used as input to another procedure.  With commands,
> what you have is a *sequence* of events, rather than a *composition*
> of functions.

Note that, in Lisp, to make a sequence with,say, f1 f2 f3, you have to
"compose" them inside the "sequencer" progn or prog1 or whatever:
  (progn (f1) (f2) (f3))
So, you really apply "progn" to the results returned by f1 f2 and f3,
which consists in keeping only the last one (a lisp fun always returns
at least nil).
In Logo, the way to combine operations is usualy in an implicit "progn",
but it could be with a "protect":
  protect f1 f2 f3
which means: do f1, and then f2, even if f1 fails.

In a multithreaded Logo, the combinator could be a "parallel progn":
  start-all f1 f2 f3
which means: start f1 f2 and f3 simultaneously.

So operations might be combined, but the combinator would act
on the control flow, not the data flow, since there is none for
operations.

-- 
| # # | # # # | # # | # # # | # # | # # # | Jacques Duthen          |
| # # | # # # | # # | # # # | # # | # # # | duthen@club-internet.fr |
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