[Issue 3889] New: Forbid null as representation of empty dynamic array

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=3889

           Summary: Forbid null as representation of empty dynamic array
           Product: D
           Version: 2.040
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at eml.cc 2010-03-07 07:08:13 PST ---
I prefer a language that uses literals that are as much as possible clear,
readable and unambiguous. In D "null" can represent empty pointers, empty class
references, and empty dynamic arrays:


int[] foo(int[] a) {
    return null;
}
int[] foo2(int[] a) {
    return [];
}
void main() {
    foo(cast(int[])null);
    foo(null);
    foo([]);
    foo2(cast(int[])null);
    foo2(null);
    foo2([]);
}


But "null" is not a clear way to represent an empty array, that is a struct of
two items (this reminds me of the untidy usage of 0 to represent a null pointer
in C). So I propose to remove the usage of "null" to represent an empty dynamic
array (the Delight D-derived language already allows only [] for such purpose).


Note: [] is not fully unambiguous, it doesn't specify a type. Specifying a type
can be useful, because you can use it to give as argument to a template
function an array of the correct type:


import std.stdio: writeln;
void foo(T)(T[] a) {
    writeln(typeid(T));
}
void main() {
    // OK, int[]
    foo(cast(int[])null);

    // OK, int[]
    foo(cast(int[])[]);

    // Partially OK, inside foo 'a' is of type void[]
    foo([]);

    // Bad, Error: template test.foo(T) does not match any function template
declaration
    foo(null);
}


In such situations cast(int[])[] is enough. I think this idiom isn't common in
D code.

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