I really like the capability that for gives you to keep all the operations controlling the loop - initialisation, incrementing and testing - in one place. In C99 (and Perl) you can declare variables in the for statement, and they will have a scope of precisely the for loop. You'll need an extra block to do that with while. So taking the example above, I assume this is intended to synthesise (updated:) ambrus noted that this is equivalent to:

// UNTESTED
    for(int a = 1; a<10; a++)
    {
        printf("%d\n",a);
    }
[download]

In fact, you'll need an extra block to get the correct scope:

// UNTESTED
// UPDATED: de-obfuscated
    {
        int a = 1;

        while(a<10)
        {
            printf("%d\n",a);
            a++;
        }
    }
[download]

Which is starting to look rather messy.

Context switch back to Perl here!

Synthesising while and foreach from for seems reasonably intuitive to me on the other hand.

# UNTESTED
for (;EXPR;) BLOCK   # leave first and third EXPRs blank to be
                     # equivalent to while (EXPR) BLOCK

# UNTESTED
for (my @list = LIST, VAR = shift @list; # iterate manually through
     @list;                              # LIST in "for" to be
     VAR = shift @list) BLOCK            # somewhat like
                                         # foreach VAR (LIST) BLOCK
                                         # without the usual aliasing
[download]

OK, the foreach is starting to look a bit funny, but a foreach from while will be worse, and spread into the block, rather than be confined to the statement itself.

One issue is that for doesn't have a continue block, but as I come from C I don't tend to use those. continue blocks also split up code that happens every time round the loop, which puts me off. for's third expression is roughly equivalent I suppose, but stuffing a large continue block there could hardly be called good style.

Recursion is another story but I don't think I could live with it as the only looping mechanism. I'd rather have the reverse issue of synthesising recursion, say with a list of hashes, on the rare occasions I really needed it.

updated in response to ambrus's comments.

Also, note:

perl -MO=Deparse -e "for (;<>;){print}"
while (defined($_ = <ARGV>)) {
    print $_;
}
-e syntax OK
[download]

the for deparses as a while!