The comma operator

The comma operator

I’ll prefix this with the fact that I have never done any sort of language development, am only vaguely familiar with the terminology and am making wild speculation around a shower thought; is , an operator?

I automatically lumped it in with as a syntactical token (see above if this is not a thing), things like []{}();; and in most languages it probably is. But an interesting way of looking at it is like the : (cons) operator in Haskell

You have, lets say, a function call:

printf("Hello %s, today is %d kelvin!", "Dylan", 21+274);

I think most people parse the comma here as a delimiter, delimiting where one expression ends and the next begins. But we can also look at it as constructing the list of arguments to pass to the function:

    List args = "Hello %s, today is %d kelvin!", "Dylan", 21+274;
    // Haskell cons operator:
    // "Hello %s, today is %d kelvin!" : "Dylan" : 21 + 274 : []

Here we approach C’s array initilization (sub out the curlies from squares and you’ve got popular list initialisation a la python and js).

parameter args[] = {"Hello %s, today is %d kelvin!", "Dylan", 21+274};

Note: The included Haskell example won’t compile because it can’t handle lists with multiple types – just like the last C example, but it’s more for illustration than anything.

So, what if we just say functions accept lists of args (I think this is close to how LISP works – where an expression is a list with the first value being the function – but I’m not a LISP programmer and haven’t written anything in it…). For type-checking reasons we could generate anonymous types for the parameters (sort of how you might define a Props type for a JSX component), however the potential power of implementing the language in this way is defeated unless you can do compile-time type-checking on lists constructed at runtime…

    // Prototype in some made-up language
    printweather: (name: string, temp: int) -> int
    // Function call
    printweather "Dylan", 21+22

Now visually, we’re approaching functional languages and LISPs, but with commas…

Similarities to Python

In Python you can create tuples in this manner:

    >>> 1,

What’s neat about the Python example is that it illustrates something that perhaps the example in the previous example didn’t: the end of the statement is inferring the cons to the empty list (see the Haskell example where we needed to explicitly :[] to construct a list).

However when you approach it from this angle, the Python analogy breaks pretty quickly:

    >>> [1,]
    >>> x = 1,
    >>> [x]
    >>> list(x)

If Python was consistant with this approach both expressions would’ve evaluated to what the second did – a list containing a single tuple. Obviously this wouldn’t be very useful for a list construction, so the [] acts like a call to the list constructor.

Python’s array deconstruction is also interesting to consider here, in how it relates to the argument list being expanded in the function called:

    >>> one, = [1]
    >>> one
    >>> name, temp = "Dylan", 21+22


So this wasn’t very satisfying, nothing transformative came about considering a comma-operator in a traditional imperitive statically-typed language context, but I mostly wanted to write down this thought so I didn’t forget it :_). Maybe I should’ve mentioned that at the start of the article…