[m-dev.] do {} while(0) pattern in macros

[Paul Bone]

Hi Mercury team.

I remember learning the pattern:

	#define PZ_WRITE_INSTR_1(code, w1, tok)           \
		do {                                          \
			if (opcode == (code) && width1 == (w1)) { \
				token = (tok);                        \
				goto write_opcode;                    \
			}                                         \
		} while (0)

For macros that expand to statements while working on Mercury.  While I
think that I once knew why we did this, I have now forgotten and would like
to re-learn.

Can someone remind me why we do this?  My guess it's to make sure that the
macro is always a single statement and cannot be used as an expression or
expand to multiple statements in a misleading way such as in:

    if (cond)
        CALL_MY_MACRO(a, b);

If that's the case, why don't we just put them inside some curly brackets.

	#define MAYBE_A_SAFE_MACRO(a, b, c)                 \
		{                                               \
            a_statement(a, b);                          \
            another_statement(c);                       \
		}

Thanks.


-- 
Paul Bone
http://paul.bone.id.au