Proposal – let’s backport Go := to C

The Go language was designed with the intention of replacing C and C++ over much of their ranges. While the large additions to Go – notably automatic memory allocation with garbage collection – attract attention, there is one small addition that does an impressive job of helping code be more concise while not being tied to any of the large ones.

I refer to the := variant of assignment, which doesn’t seem to have a name of its own in the Go documentation but I will pronounce “definement”. It must have an unbound name on its left (receiving) side and an expression on the right (sending) side. The semantics are to declare the name as a new variable with the type of the right-hand expression, then assign it the value.

Here’s the simplest possible example. This

void foo(int i)
{
int x;
x = bar(i);
 
/* More code that operates on i and x */
}

becomes this:

void foo(int i)
{
x := bar(i)
 
/* More code that operates on i and x */
}

A way to think about definement is that it generates a variable declaration with an initialization. In modern C these can occur anywhere a conventional assignment can.

Definement is a simple idea, but a remarkably productive one. It declutters code – scalar and struct local-variable declarations just vanish. This has two benefits; (1) it improves readability, and thus maintainability; and (2) it eliminates a class of silly errors due to multiple declarations falling out of sync – for example, when changing the return type of a function (such as bar() in the above example), you no longer gave to go back and tweak the declaration of every variable that receives a result from its callsites.

Definement syntax also has the property that, if we were to implement it in C, it would break cleanly and obviously on any compiler that doesn’t support it. The sequence “:=” is not a legal token in current C. In gcc you get a nice clean error message from trying to compile the definement:

foo.c: In function ‘foo’:
foo.c:3:5: error: expected expression before ‘=’ token
x := i
^

This makes it a low-risk extension to implement – there’s no possibility of

it breaking any existing code.

It is worth noting that this will actually be slightly simpler to implement in C than it is in Go, because there are no untyped constants in C.

I think there’s a relatively easy way to get this into C.

First, write patches to implement it in both gcc and clang. This shouldn’t be difficult, as it can be implemented as a simple parser change and a minor transformation of the type-annotated AST – there are no implications for code generation at all. I’d be surprised if it took a person familiar with those front ends more than three hours to do.

Second, submit those patches simultanously, with the notes attached to each referencing the other one.

Third, wait for minor compilers to catch up. Which they will pretty quickly, judging by the history of other pure-syntax enhancements such as dot syntax for structure initialization.

Fourth, take it to the standards committees.

OK, am I missing anything here? Can any of my readers spot a difficulty I haven’t noticed?

Will anyone who already knows these front ends volunteer to step up and do it? I certainly could, but it would be more efficient for someone who’s already climbed the learning curve on those internals to do so. If it helps, I will cheerfully write tests and documentation.

EDIT: No, we can’t backport C++ “auto” instead – it has a different and obscure meaning in C as a legacy from B (just declares a storage class, doesn’t do type propagation). Mind you I’ve never seen it actually used, bu there’s still a nonzero risk of collision with old code.