[Top][All Lists]
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: Dotted pair call argument
From: |
David Kastrup |
Subject: |
Re: Dotted pair call argument |
Date: |
Wed, 22 Feb 2012 10:06:28 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.0.92 (gnu/linux) |
Mark H Weaver <address@hidden> writes:
> David Kastrup <address@hidden> writes:
>
>>> Scheme has a very useful property which your proposed syntax would
>>> destroy: any valid expression can be substituted for any other valid
>>> expression, and the result has the same meaning except for the
>>> substitution.
>>
>> guile> (display . (close (current-output-port)))
>> #<primitive-procedure close>guile>
>>
>> Now try
>>
>> (define x (close (current-output-port)))
>> (display . x)
>
> Admittedly I could have been more clear, but I certainly didn't mean to
> imply that anything that _looks_ like a valid expression can be
> replaced. That would be absurd.
Exactly.
> What I meant is that any _subexpression_ can be replaced with any other
> valid expression, without changing the meaning of the program in any
> other way.
So the solution would be to not call dotted pair endings of argument
lists a "subexpression", and everybody will be happy.
> Whether something is a subexpression depends on its _position_ within
> the larger expression.
Yes. That's the point. The dotted list end is a specific position.
Not "subexpression" position. If a list is there, it is evaluated
element by element. If a non-list is there, we get an error. Instead,
I prefer evaluating it and using the evaluated result, whatever it may
be, as the argument list tail. Personally, I would not even demand
actual argument lists to be proper lists as long as the declared
argument list is a dotted list as well: you can still match arguments
then.
> The only advantage I see to this proposed syntax is that in some
> restricted cases it is more aesthetically pleasing.
apply can't handle improper lists either.
> I suspect that most experienced Schemers have at some point wondered
> why dotted-tail notation is not allowed in procedure calls. I
> certainly have, but upon further consideration I became convinced that
> the pitfalls of adopting such an ambiguous and potentially confusing
> syntax far outweigh the advantages.
Nothing that is currently valid would change its meaning.
--
David Kastrup