Mail Archives: geda-user/2015/10/27/14:26:46
Karl,
sorry for delay,
On Tue, Oct 20, 2015 at 11:54:38PM +0200, karl AT aspodata DOT se wrote:
...
> As for nr 4, well yes interpreted lang. takes you faster to runtime,
> but I don't see what the author see is so special about the while do,
There is usually no need for 'while' and 'do' since most things are done
just using tail calls (improved recursion), and those things are just
wrappers for them (and there are procedures to see how they wrap them).
> or about closures, I read this:
>
> https://www.gnu.org/software/guile/manual/html_node/Closure.html#Closure
>
> but I fail to see any definition of what a closure really is, is it a
> (let ...) thing containing a lambda expression ?
> (And lambda, isn't that just a function without name ?)
Yes, lambda is just a function without name, and it is sometimes very
useful because it is not always convenient to name a block of code in
order just to use it several times. It also allows using of currying
[1]. All this is about 'lambda calculus' [2] that is a scientific
phenomenon in programming being a foundation of several modern
programming languages. In two words, it is sometimes convenient to split
functionality if you know that F (Y, X) = F ( Y(X) ). Say, in the
following example I separate the map function and another function 'a'
which I cannot name yet:
(define f
(lambda (a ls)
(map
(lambda (x) (a x)) ; here x is any member of the list ls
ls)))
Then I can do (f + ls) or (f - ls) to add or subtract members of a list.
If the unnamed function gets complicated enough, or when I want to use
it in other places, I can split it off at any time and write
(define (myfunc .....))
(define f
(lambda (a ls)
(map myfunc ls)))
>
> I read
>
> https://www.gnu.org/software/guile/manual/html_node/Serial-Number.html#Serial-Number
>
> but isn't that the same as
>
> static some_struct x;
> void some_init() { x = ...; };
> some_run() { do something; }
>
> It feels like I'm blind and don't see what's so special about this.
> And reading thoose two links I get a feeling that the author says the
> same thing about something I'm used to, but he/she uses for me
> unfamiliar words, which make it a really hard to understand it.
The major thing about closures is that they save not only code, but also
their toplevel environment in them (the underlying technique, AFAIK, is
catching current program stack), thereby allowing encapsulation
without using any OOP. You can create opaque structures with getters
and setters allowing users use only them to access data, where you can
remember toplevel variables you want. Then there would be far less
possibilities to break anything (compared with plain C constants and
functions, IIUC). Besides, as it is shown in the manual, you can easily
copy the functionality of closure you made by creating the closure
inside another procedure (this is used as an example of making several
independent serial numbers generators). Yes, you can always duplicate
things in C (since at least Guile core is written in C), however you'll
have to do more work then. This is the same thing as if I would compare
C and asm. (Almost) all things you do in C you could do in asm (I'm
wording 'almost' and 'could' because some things would require too much
work, especially ones ensuring portability). However, C is much better
than asm for most cases, you know :)
Returning to closures, please try searching e.g. 'closure advantages'
in google for more info on them. You'll also see that closures are used
in many modern hi-level languages.
>
> So, is there a scheme book for c programmers ?
Not that I know of. AIUI, Scheme must not be C dependent. OTOH, as long
as Guile is GNU Scheme aimed at extending programs in C, and if you want
to be aware of functions which could be used for that, you can look
through the fifth chapter of the guile info manual (it is available
on-line, search for 'guile Programming and C' and you'll find [3] :)).
> And, you know, there are dictionaries available when travelling abroad,
> which one should I use when going to scheme-land ?
>
I don't think I understand you here. If you mean any unknown terms,
you may try to look up them in wikipedia.
Cheers,
Vladimir
[1] https://en.wikipedia.org/wiki/Currying
[2] https://en.wikipedia.org/wiki/Lambda_calculus
[3] https://www.gnu.org/software/guile/manual/html_node/Programming-in-C.html
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