Does the Zig compiler consider arrays with comptime variable lengths as possible zero length arrays?

Question

I'm experimenting with n-dimensional arrays in Zig.

const expectEqual = std.testing.expectEqual;

fn NdArray(comptime n: comptime_int, comptime shape: [n]comptime_int) type {
    if (shape.len == 0) {
        // zero dimensional array, return the scalar type
        return u8;
    } else {
        return struct {
            // positive dimensional array, return an array of arrays one dimension lower
            data: [shape[0]]NdArray(n - 1, shape[1..].*)
        };
    }
}

test "NdArray test" {
    const expected = struct {
        data: [2]struct {
            data: [6]struct {
                data: [9]struct {
                    data: u8
                }
            }
        }
    };
    expectEqual(NdArray(3, [3]comptime_int{ 2, 6, 9 }), expected);
}

But I get a compile error:

11:25: error: accessing a zero length array is not allowed
            data: [shape[0]]NdArray(n - 1, shape[1..].*)
                        ^

I don't see any way for the compiler to reach line 11, when shape has zero length. Does the compiler just forbid indexing of shape , because it doesn't have length expressed by an integer literal?

Answer 1

More of an extended comment than an answer, I think as tuket says, this seems to be compiler related. I look forward to a better explanation than the one I'm about give =D

It looks like like the struct child scopes (if such a thing applies here) are evaluated before the outer scope. It seems to work if you shift the shape[0] reference to the parent scope:

fn NdArray(comptime n: comptime_int, comptime shape: [n]comptime_int) type {
    if (shape.len == 0) {
        // zero dimensional array, return the scalar type
        return u8;
    } else {
        var foo = shape[0];
        return struct {
        // positive dimensional array, return an array of arrays one dimension lower
            data: [foo]NdArray(n - 1, shape[1..].*)
        };
    }
}

Since your error will come from the final pass of this recursion, another option would be to rewrite it in a non-recursive way.