Re: cond(itionals) with optional execution of statements

[Damien Mattei]

Hi Taylan,

thanks for your ideas ,i  had the same thoughts for some. I will try to
answer below:

On Sun, Sep 12, 2021 at 8:31 PM Taylan Kammer 
wrote:

> On 12.09.2021 19:05, Damien Mattei wrote:
> > Hello Zelphir,
> >
> > condx evaluate all code(s) in the 'exec' block until a conditional is
> true,
> > it then evaluate the consequent code of course.
> > So ,yes your true it saves a lot of nesting parenthesis as in this
> example:
> >
>
> Interesting macro.  I think I've occasionally felt the need for such a
> thing.
>
> Two comments:
>
> 1. Using 'begin' instead of 'exec' might feel more familiar to Schemers,
> and
>since the 'literals' of syntax-rules are matched hygienically starting
>from Guile 2.2 (I think), it might be a good idea to use a common core
>identifier instead of an unbound one like 'exec'.  (Or you could bind
> it.)
>
i had this idea of using begin ,i choose the name condx ,x for execute ,
begin sound less well ;-)
ok i admit it is not a serious argument but i have to keep 'cond' prefix
for naming historically and i do not want to have a long name as it add
some indentation too much.
about 'begin' if found it transparent but it can be confusing with the
'begin' of Scheme,and 'begin' of scheme
is dependant of context:
" begin is special - begin in a body context allows definitions, but begin
in an expression contexts forbids definitions. "
(source :
https://stackoverflow.com/questions/16221336/error-with-define-in-racket/16221820#16221820
)
doc with guile :
https://www.gnu.org/software/guile/manual/html_node/begin.html
so begin is used for sequencing and splicing depending of context, so using
the keyword begin here can cause the user to ask itself about the context
which is not clearly defined in condx , i assume is it a context where
definitions are allowed, but we can use begin as key words , i had this
idea too...
searching about a previous of exec in scheme give no result, it is not used
in guile, racket ,only in others languages : bash, C ? (not sure)

>
> A little elaboration, in case you don't know what I'm talking about: the
> "literals" of a syntax-rules macro are not matched by "name" (the string
> representing the symbol seen in code) but by "variable binding."
>
i understand a bit what you mean but i'm not a easy with that, i've been
facing this problem defining a return keywords too, see later below

>
> For example:
>
>   (let ((else #f))
> (cond
>   ((= 1 0) 'foo)
>   (else 'bar)
>   (#t 'qux)))
>   ; => qux
>
> The code returns qux, because the 'cond' macro doesn't recognize the 'else'
> as the 'else' that it knows of, since it's been rebound via let.
>

i hope no serious programmer will rename 'else' even if i do it in Lisp
cond which have not else clause, so renaming else to #t ( t in lisp) make
lisp compatible with scheme but it is not the case here we are in
Scheme,even if one day hope to port that to Lisp

>
> An example that would work in a conforming R7RS-small implementation; don't
> know if it works with Guile, it's just to explain the principle:
>
>   ;; Import core bindings, but renaming 'else' to 'otherwise'
>   (import (rename (scheme base) (else otherwise)))
>
>   (cond
> ((= 1 0) 'foo)
> ((= 2 3) 'bar)
> (otherwise 'qux))
>
just a comment, otherwise is a long keyword,i'm afraid of identation in can
cause, i assume it was just an example you wrote

>   ; => qux
>
> The (scheme base) library defined by R7RS-small exports the identifier
> 'else'
> which is used by 'cond' for matching.  This allows the programmer to rename
> the 'else' used by 'cond' while importing the base library.  (The 'else' is
> not bound to anything useful, it's just bound at all so it can be renamed.)
>
> Likewise you might want to bind 'exec' to anything and export it along with
> the 'condx' identifier, so if some Schemer uses the identifier 'exec' for
> something different in their code, they can still use your macro by
> renaming
> your 'exec' to something else.  Otherwise there's no way to make it work.
> If 'exec' was unbound during the definition of 'condx' then it must remain
> unbound for 'condx' to recognized it again, meaning it can't be renamed.
>
> Or (IMO better) you could reuse the 'begin' binding (in Guile's case, from
> boot-9, in R7RS-small, from (scheme base)), because it's very unlikely that
> someone will use 'begin' for something else in their code, and it would
> force
> them to rename the core 'begin' to something else and then that would work
> with your code automatically.  E.g. if someone renames 'begin' to 'start'
> it
> will automatically work in your macro if you had defined it with 'begin' in
> the literals list of syntax-rules.
>
> 2. You might be interested in let/ec, which lets you bind a variable to an
>"escape continuation" i.e. a way to "return" from a block of code.
> Here's
>your code using condx rewritten to use let/ec instead:
>

yes very good idea , i already done that but it is in another functio

Re: cond(itionals) with optional execution of statements

[Damien Mattei]

hello Zelphir,
i will answer in mail, note that some answers are perhaps already in my
previous mail in the mailing list

On Sun, Sep 12, 2021 at 8:36 PM Zelphir Kaltstahl <
zelphirkaltst...@posteo.de> wrote:

> Hi Damien!
>
> I see! Indeed, some indentation happening there. I get the point now,
> thank you. Could be well worth it, but it depends on how much you get out
> of it, compared to how much additional mental load you get for introducing
> a new control flow concept.
>
yes i agree. i have not developped a lot with this control flow condx , we
can do the same in a python style with a few 'if' and 'return' and 'when'
and that give already enought lisibility, but as cond exist in Scheme
historically but well used todays i wanted to improve it a bit.
'condx' overpass the cond limitation, cond limitation are the causes that
force me to introduce 'return' in 'def'inition of function 'de base' i
think we must include 'return' in every function definition and my keyword
'def' (see previous mail) is derived from python ,it is shorter ans save a
few char in indentation too...

> Sometimes it is worth taking a step back and refactoring part of the logic
> out into a separate procedure, which then starts at a lower level of
> indentation again, avoiding the problem. I don't know your scenario though,
> so this is only a general idea.
>
> I would probably write the `cond` version a bit different, to avoid
> `begin` and break a few lines at other places to avoid the indentation
> taking a lot of horizontal space. I am not sure it makes sense in your
> context, but here it is:
>
> 
> (define (ssigma-proto L t)
>
>   (set! cpt {cpt + 1})
>
>   (define ls (length L))
>   (define dyn (array-ref dyna ls t))
>
>   ;; dyna[ls][t] means 0: unknown solution, 1: solution found, 2: no solution
>
>   (cond
>[(not (zero? dyn)) (one? dyn)]
>[(null? L) (array-set! dyna 2 ls t) #f] ;; return #f
>[else
> (let [(c (first L))]
>   (cond
>[{c = t} ;; c is the solution
> (array-set! dyna 1 ls t)
> #t] ;; return #t
>[else
> (let [(R (rest L))]
>   (cond
>[{c > t}   ;; continue searching a solution in the rest
> (let [(s (ssigma-proto R t))]
>   (array-set! dyna (one-two s) ls t)
>   s)] ;; return s
>[else
> ;; c < t at this point
> ;; c is part of the solution or his approximation
> ;; or c is not part of solution
> (let [(s {(ssigma-proto R {t - c}) or (ssigma-proto R t)})]
>   (array-set! dyna (one-two s) ls t)
>   s)]))]))]))
> 
>
> (Removed some empty lines for brevity in this e-mail. Sometimes I like
> good empty lines too! Could the `set!` be replaced using a `define`?)
>

if you look at my previous mail the code i inserted at end ,it goes in the
same sense

> I guess the expressions in braces are using infix operations like the `or`
> or `=` in the condition of `{c = t}`.
>
yes it is not developped by me but it is in SRFI 105 named 'curly infix':
https://srfi.schemers.org/srfi-105/srfi-105.html
a great SRFI ! i think, that scheme needs a bit of infix for some
mathematical expression or testing ,it allow to write
expression as in natural language {x > 3} is easiest to understand than (>
x 3),all is explained in the part RATIONALE of the SRFI
if you want to use it with guile,as far as i remember you just have to add
this in your .guile file:
(read-enable 'curly-infix)
to have it activated everywhere in code, it is fully compatible with other
features of Scheme

SRFI 105 also allow to use notation like {dyna[ls t]} for array and
notation for functions such as {sin(x)}

i'm developping an extension to  scheme that will allow more things (see
example in previous mail), i just have to package it in a module and write
some documentation about it, i hope to release it first for Guile  in this
month and for other scheme implementation later...

Damien


Best regards,
> Zelphir
> On 9/12/21 7:05 PM, Damien Mattei wrote:
>
> Hello Zelphir,
>
> condx evaluate all code(s) in the 'exec' block until a conditional is
> true, it then evaluate the consequent code of course.
> So ,yes your true it saves a lot of nesting parenthesis as in this example:
>
> here a code with condx and without it:
>
>
> (define (ssigma-proto-condx L t)
>
>   (set! cpt {cpt + 1})
>
>   (define ls (length L))
>   (define dyn (array-ref dyna ls t))
>
>   ;; dyna[ls][t] means 0: unknown solution, 1: solution found, 2: no solution
>
>   (condx [(not (zero? dyn)) (one? dyn)]
>[(null? L) (array-set! dyna 2 ls t) #f] ;; return #f
>   
>[exec (define c (first L))]
>   
>;; c is the solution
>[{c = t} (array-set! dyna 1 ls t) #t]  ;; return #t
>   
>[exec (define R (rest L))]
>   
>;; continue searching a solution in the rest
>[{c > t} (define s (ssigma-proto R t))
> (array-set! dyna
>