Re: Why not support (begin), (cond), (case-lambda), etc?

[Mark H Weaver <mhw@netris.org>]

David Kastrup <dak@gnu.org> writes:

> Mark H Weaver <mhw@netris.org> writes:
>
>> I'd like to argue in favor of supporting (begin), (cond), (case-lambda)
>> and other such degenerate forms, for the same reason that we support
>> (*), (+), and (let () ...).
>
> Actually, I'd like to see (let ()) and (lambda ()) work too for similar
> reasons (basically equivalent to (if #f #f)).

I agree.

>> Imagine if we didn't support (*) and (+).  Then you couldn't simply
>> write (apply + xs) to add a list of numbers; instead you'd have to write
>> (if (null? xs) 0 (apply + xs)).
>
> (apply + 0 xs) but it still is a distraction.

True, and indeed still an ugliness, even if one with fewer extra
characters.  I myself work hard to make my programs as simple as elegant
as possible.

>> The same argument applies to (begin), (cond), and (case-lambda).  They
>> simplify writing robust syntax transformers without having to handle
>> degenerate cases specially.
>
> Correct me if I am wrong (I actually have not really understood syntax
> transformers), but the usual patterns of xxx ... can't be empty (that
> is,

Actually, you are wrong here.  "e ..." can be empty.  If you want to
prohibit the empty list, you need to write something like "e0 e ..."
instead.

However, "e ..." _does_ require a proper list, i.e. without a dotted
tail at the end.  For example "(define (proc arg ...) e0 e ...)" will
_not_ match (define (map f . xs) <blah>).  To match cases like that, you
need to instead make your pattern "(define (proc . args) e0 e ...)".

>> Apart from this general argument, I can think of one particularly
>> compelling reason to support (begin).  Suppose you have a macro that
>> generates a sequence of local definitions.  How do you return an empty
>> sequence of definitions without terminating definition context?
>
> (begin (values))

No, that doesn't work.  (values) is an expression, not a definition, and
thus (values) terminates definition context.  (Within a local block,
local definitions cannot follow an expression).

  scheme@(guile-user)> (let () (begin (values)) (define x 3) x)
  While compiling expression:
  ERROR: Syntax error:
  unknown location: definition in expression context in subform x of 3

     Mark