Re: lcc first experience

In article
<cf0c2e4c-e25f-4ceb-a45d-5e056817552f@[EMAIL PROTECTED]
>,
Paul Hsieh  <websnarf@[EMAIL PROTECTED]
> wrote:

>C is an uncommon language in that it does not require a comparison
>operator in its if statements, and that all statements have side-
>effects. 

ANSI X3.159-1989  (aka C89)
Appendix A Language Syntax Summary

A.2.3 Statements

 (3.6) statement:
    labeled-statement
    compound-statement
    expression-statement
    selection-statement
    iteration-statement
    jump-statement

 (3.6.1) labeled-statement:
   identifier : statement
   case constant-expression : statement
   default : statement

In the first of those choices of 3.6.1, the identifier is a label.
Does a label have a side-effect? If not, then whether the first form
of labeled statement has a side effect depends upon whether the
statement on the right hand side -always- has a side effect, a point
which remains to be proved at this point in the analysis.

In the second of those choices of 3.6.1, does a constant-expression
have a side effect? If not, then whether the second form of labeled
statement has a side effect depends upon whether the statement
on the right hand side -always- has a side effect, a point which
remains to be proved at this point in the analysis.

In the third of these choices of 3.6.1, does the presence of 'default'
have a side effect? If not then etc..

  (3.6.2) compound-statement:
        {declaration-list$opt$ statement-list$opt$}

Please excuse my typographic mangling of the subscript 'opt' to $opt$.

Does the -absence- of a declaration-list always have a side-effect
in a compound-statement? If not then determining whether
compound-statements
always have side-effects, reduces down to determining whether
{statement-list$opt$} always has a side-effect. In such a statement,
does the -absence- of the statement-list always have a side-effect?
That is, is there a side-effect associated with the compound-statement
  {}
?? If not then we have identified a compound-statement with no side-
effect, and have thus identified a statement with no side-effect,
which in turn would allow us to substitute in {} for the 'statement'
****tion in all three choices of labeled-statement, so unless there
is a side-effect associated with the presence of a label or
constant-expression or 'default' label, labeled-statements need not
have any side-effects.

Similarily, expression-statement (3.6.3) is  expression$opt$
and by simply exercising our option to not put in an expression,
we appear to have found another form of statement that has no
side-effects.

If we examine section 3.6.3 of C89 properr, we find that it is entitled

  3.6.3 Expression and Null Statements
and contains a paragraph:

  A null statement (consisting of just a semicolon) performs no
operations.

Personally, I find it difficult to see how there could be a "side-effect"
associated with a statement that the C89 standard *specifically*
says "performs no operations", but perhaps I'm just funny that way.


We can continue this analysis through selection-statement and
interation-statement and find several forms of each of those which have 
no side-effects.


This brings us to (3.6.6) jump-statement
  goto identifier ;
  continue ;
  break ;
  return expression$opt$ ;

At this point there is room for disagreement over what constitutes a
'side-effect'. If a 'continue;' appears as the terminal statement
inside a loop, then has it had a side-effect or not, considering the
action would be the same if it was not there.  If a 'break' appears as
the terminal statement of a loop which does not have post-statements
with side-effects (e.g., the third part of the for() is empty or has no
side effects) and which would not have executed any more interations
anyhow, then is it a side-effect? If a goto statement branches to the
next line, has it had a side-effect? If a return statement with no
expression appears as the last line of a void function, then
does that return have a side-effect?

I would be prepared to accept for the purposes of discussion
a definition that indicated that a jump-statement always has a
side-effect. But jump-statement is only one of the seven forms
of C89 statements and the other six all have valid (and sometimes useful)
forms that have no side-effects.

I must therefore disagree with the claim that,

>all statements have side-
>effects. 

as there are numerous C statements possible that have no side-
effects.
-- 
   "I was very young in those days, but I was also rather dim."
   -- Christopher Priest