• d-bugmail puremagic.com (104/104) Jan 09 2012 http://d.puremagic.com/issues/show_bug.cgi?id=7255

d-bugmail puremagic.com writes:

http://d.puremagic.com/issues/show_bug.cgi?id=7255

           Summary: Struct array assign?
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: bearophile_hugs eml.cc



Currently (DMD 2.058 Head) D allows code like this:


struct MyTypedefInt {
    int x;
    this(int xx) { this.x = xx; }
    // void opAssign(int xx) { this.x = xx; }
}
void main() {
    MyTypedefInt x = 1; // OK
}



But if you put the elements into an array they aren't accepted (the same
problem happens with BigInts, etc):


struct MyTypedefInt {
    int x;
    this(int xx) { this.x = xx; }
}
void main() {
    MyTypedefInt[2] a3 = [1, 2]; // Error: cannot implicitly convert expression
}



This too is refused:

struct MyTypedefInt {
    int x;
    this(int xx) { this.x = xx; }
    // void opAssign(int xx) { this.x = xx; }
}
void main() {
    MyTypedefInt[2] a3 = [{ 1 }, { 2 }];
}


test.d(7): Error: struct MyTypedefInt has constructors, cannot use {
initializers }, use MyTypedefInt( initializers ) instead
test.d(7): Error: struct MyTypedefInt has constructors, cannot use {
initializers }, use MyTypedefInt( initializers ) instead


This is accepted:

struct MyTypedefInt {
    int x;
    this(int xx) { this.x = xx; }
}
void main() {
    MyTypedefInt[2] a3 = [MyTypedefInt(1), MyTypedefInt(2)];
}


but it's not so nice to write, even if you shorten the name of MyTypedefInt to
a single char:

struct MyTypedefInt {
    int x;
    this(int xx) { this.x = xx; }
}
void main() {
    alias MyTypedefInt M;
    MyTypedefInt[2] a3 = [M(1), M(2)];
}



So in this issue I ask if it's possible and wise to support code like:
MyTypedefInt[2] a3 = [1, 2];
MyTypedefInt[] a4 = [1, 2];
MyTypedefInt[][] a5 = [[1, 2], [3, 4]];

---------------

Here is an usage example of code like that. This is Ada code:


subtype Number is Natural range 0 .. 9;
subtype Valid_Number is Number range 1 .. 9;
type Board is array (Valid_Number, Valid_Number) of Number;
Sample_Board : Board := (1 => (3, 9, 4, 0, 0, 2, 6, 7, 0),
                         2 => (0, 0, 0, 3, 0, 0, 4, 0, 0),
                         3 => (5, 0, 0, 6, 9, 0, 0, 2, 0),
                         4 => (0, 4, 5, 0, 0, 0, 9, 0, 0),
                         5 => (6, 0, 0, 0, 0, 0, 0, 0, 7),
                         6 => (0, 0, 7, 0, 0, 0, 5, 8, 0),
                         7 => (0, 1, 0, 0, 6, 7, 0, 0, 8),
                         8 => (0, 0, 9, 0, 0, 8, 0, 0, 0),
                         9 => (0, 2, 6, 4, 0, 0, 7, 3, 5));



With the change I have suggested (plus Phobos-defined TypeDef and SubRange) you
are able to write something similar in D2 too (here I have ignored
Valid_Number, the type of the array index), despite it looks a bit worse:


alias TypeDef!(SubRange!(size_t, 0, 10)) Number;
alias TypeDef!(Number[9][9]) Board;
Board sampleBoard = [[3, 9, 4, 0, 0, 2, 6, 7, 0],
                     [0, 0, 0, 3, 0, 0, 4, 0, 0],
                     [5, 0, 0, 6, 9, 0, 0, 2, 0],
                     [0, 4, 5, 0, 0, 0, 9, 0, 0],
                     [6, 0, 0, 0, 0, 0, 0, 0, 7],
                     [0, 0, 7, 0, 0, 0, 5, 8, 0],
                     [0, 1, 0, 0, 6, 7, 0, 0, 8],
                     [0, 0, 9, 0, 0, 8, 0, 0, 0],
                     [0, 2, 6, 4, 0, 0, 7, 3, 5]];


This *encourages* to write D code with more static safety, that for me is one
of the main purposes of using static typing in the first place. D is designed
to write more than just game engines :-)

Are implicit conversions dangerous? The purpose of this idea is to be handy and
to favor the writing of more statically safe code (making something more handy
pushes people to do it more). But if implicit conversions are not safe, then it
fails being at being more statically safe.

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