[sesuximo]

Imo foo[x][y] is better for many reasons:

- compiler can break it apart so foo[x] is evaluated once for many ys just like any other function

- you can make types that are unaware of their rank eg vector doesn’t know if it’s a 1D vector of int or a 2D vector of vectors

- “seamlessly” handles jagged arrays

[jiggawatts]

Which reminds me of an insightful observation that the array indexing operator [] and the function call operator () are basically doing the same thing: evaluating a function and returning a value.

So what you've just described is basically Currying, or partial function evaluation. It's a bit like defining "al(i)" as array lookup for an index 'i' instead of "[i]". Then the following are equivalent:

    something[i][j]
    something.al(i).al(j)

What people also want are:

    something[i,j]
    something.al(i,j)

In languages like Haskell, every function can always be evaluated argument-by-argument, with no special effort by the programmer. The compiler takes care of generating the code for the various intermediate call forms. In languages like C++, it would be a significant hassle to create all of the various partially-evaluated wrapper types.

[gpderetta]

In C++ you can create a simple generic reusable currying wrapper. Boost as a few. The reality is that the vast majority of C++ programmers don't see the need.

[Animats]

Trying to do this this through Currying turns a simple 2D array access into a quite complicated operation. One that a compiler has to be really smart to turn into a gheap 2D array indexing operation again. This really matters in number-crunching code, where it seems that almost everything is about multiplying arrays.

[gpderetta]

Of course it wouldn't make much sense to do it for indexing. I was discussing currying as a generally useful tool.

[Athas]

It's not the same when you support slicing. foo[x:y][z] is not the same as foo[x:y,z]. I'm not sure anyone is proposing slicing to be supported with the indexing operator in C++, though, so it might not matter.

[Bancakes]

It's semantically the same and the compiler should know that. But I do agree [][] makes more sense. The jth index of the ith array.

[mhh__]

They'd probably be lowered to exactly the same code, so the compiler argument isn't very good.

[sesuximo]

If the user has access to both indices, they can make partially hosting harder/impossible