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From: Bart <bc@freeuk.com>
Newsgroups: comp.lang.c
Subject: Re: question about linker
Date: Sat, 7 Dec 2024 14:57:17 +0000
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On 07/12/2024 13:04, Bart wrote:
> On 07/12/2024 11:34, Janis Papanagnou wrote:
>> On 06.12.2024 02:20, Bart wrote:
> 
>>> I used to be puzzled by this too: 'while' can both start a while
>>> statement, and it can delimit a do-while statement. How is that 
>>> possible?
>>
>> A keyword (like 'while') is just a lexical token that can be used in
>> different syntactical contexts.
> 
> Is it common to both start and end a statememt with the same keyword?
> 
> 
>   (Even with different semantics, if a
>> language designer thinks this is a good idea for his case.)
>>
>> You may be less confused with using Pascal;
> 
> Now you are demonstrating my point about being treated like a beginner. 
> And it is exasperating.
> 
> This is a point of language design. C's approach works - just. But it 
> relies on some subtlety. 'while (cond)' both starts a statement, and can 
> end a statement:
> 
>     do while(cond) do ; while (cond);
> 
> The second 'while' here starts a nested while-loop inside a do-while 
> loop. Not confusing? It is to me! I couldn't finish my example as I got 
> lost (but as it happens, this is valid code, partly by luck).
> 
> Of course, if you put aside all other matters and concentrate on the 
> minutiae of the syntax details, then it is all 'obvious'. I think it 
> needs to be a lot more obvious.
> 
>>    while positive-case do ...
>>    until negative-case do ...
>>    do ... while positive-case
>>    do ... until negative-case
>> (depending on language with this of similar syntax).
>>
>> There's nothing wrong using the same keyword for the same type of
>> condition, rather it's good to have it defined that way.
> 
> The problem here is exactly the same: how do you tell whether the 
> 'while' in 'do ... while' is ending this 'do'-loop, or starting a nested 
> while-loop?
> 
> You don't think there is an element of ambiguity here?
> 
> Let's take my example again:
> 
>     do while(cond) do ; while (cond);
> 
> And make a small tweak:
> 
>     do ; while(cond) do ; while (cond);
> 
> It still compiles, it is still valid code. But now it does something 
> utterly different. (One is two nested loops, the other is two 
> consecutive loops - I think!)

Correction: the versions I tested don't have that 'do' in the middle. 
The two examples need to be written like this to be valid:

     do while (cond) ; while (cond);
     do ; while (cond) ; while (cond);

So this /is/ two nested loops followed by two consecutive ones. The 
difference is that semicolon.

In a syntax where newlines may be injecting semicolons, such a subtle 
distinction is not strong enough. As I originally said, it's fragile.

Those suggesting that semicolons are unimportant details of syntax in C 
are wrong. They can make a significant difference and contribute to errors:

    for (i=0; i<n; ++i);
         printf("%d\n", i);