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From: minforth <minforth@gmx.net>
Newsgroups: comp.lang.forth
Subject: Re: Parsing timestamps?
Date: Wed, 2 Jul 2025 01:26:18 +0200
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In-Reply-To: <20250701234705.000044ce@tin.it>

Am 01.07.2025 um 23:47 schrieb peter:
> On Tue, 01 Jul 2025 11:40:38 -0700
> Paul Rubin <no.email@nospam.invalid> wrote:
> 
>> Hans Bezemer <the.beez.speaks@gmail.com> writes:
>>>>> But such would indicate a deficiency in Forth.  Do C programmers
>>>>> reach a point at which they can't go forward? ...
>>> Another great argument to leave Forth and embrace C! Why painfully
>>> create kludge to cram into a language that was clearly not created
>>> for that when you have a language available that was actually
>>> DESIGNED with those requirements in mind?!
>>
>> I'm not sure what you're getting at here, though I see the sarcasm.
>>
>> Is the kludge locals?  They don't seem that kludgy to me.
>> Implementing them in Forth is straightforward and lots of people have
>> done it.
>>
>> Finally, a fancy enough Forth compiler can do the same things that a C
>> compiler does.  Those compilers are difficult to write, but they exist
>> (VFX, lxf, etc.).  I don't know if locals make writing the compiler
>> more difficult.  But the user shouldn't have to care.
> 
> The code generator in lxf has no knowledge of what a local is.
> locals are conceptually placed on the return stack. lxf is as smart
> about the return stack as the data stack. that is why it can produce
> very efficient code for simple examples like 3DUP. The actual
> implementation of local in the interpreter is just a few lines of code.
> The difference with locals will be seen when you have a boundary block,
> IF statement, a call etc that require a known state of the stacks.
> The real problem for me with locals is that their scope is to the end
> of the definition. With the stack you end the scope of an item with a
> drop and extend it with a dup, very elegant!
> A multipass compiler can of course find the scope of each local but at
> the cost of more complexity.
> 
> In lxf64 I have introduced a local stack with the same capabilities as
> the data and return stack. I am not sure yet if this is better.
> 
> The nice thing is that I now have >ls ls> and ls@. Compared with the
> return stack this also works across words. One word can put stuff on
> the localstack and another retrieve it. This is sometimes very useful.

In a sense, such locals become global. I am not sure if this opens the 
way inadvertently for hard-to-detect bugs. One rarely discussed property 
of locals is that they offer data encapsulation (or have scope in C 
terminology).

Only one useful application comes to my mind: sharing locals between
quotation and its parent function, i.e. for creating closures. But who 
needs thema anyway?