[Issue 3971] Syntax & semantics for array assigns

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http://d.puremagic.com/issues/show_bug.cgi?id=3971



--- Comment #19 from bearophile_hugs at eml.cc 2011-11-15 23:56:31 PST ---
(In reply to comment #18)
> See:
> 
> http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D&article_id=149289

This is from that post:

This also means this is currently accepted:

void main() {
    int[3] a;
    a = 1;
    assert(a == [1, 1, 1]);
}


While this is not accepted:

void main() {
    int[] b = new int[3];
    b = 1;
    assert(b == [1, 1, 1]); //Error: cannot implicitly convert expression (1)
of type int to int[]
}



I'd like D to require  a[]=1  in that first case too.

I'd like the [] to be required every time an O(n) vector operation is done,
for:
- constancy with all other vector operations among two arrays, that require [];
- and to avoid unwanted (and not easy to spot in the code) O(n) operations;
- bugs and confusion in D newbies that don't have memorized all current special
cases.

On the other hand Don says that [] is only required for lvalues.

I think this boils to a new table like this:


Rhs is an array, is it compilable?
a       / b             a=b     a[]=b   a=b[]   a[]=b[]
int[3u] / int[3u]       FALSE   true    FALSE   true
int[3u] / int[]         FALSE   true    FALSE   true
int[]   / int[3u]       FALSE   true    FALSE   true
int[]   / int[]         true    true    true    true

Rhs is a element, is it compilable?
a                       a=N     a[]=N   a[0..2]=N
int[3u]                 FALSE   true    true
int[]                   false   true    true


Now if there's a [] on the left, then it's an O(n) vector operation (like a
copy), otherwise it's O(1).

That also means:

void main() {
    int[] a = new int[3];
    int[] b = new int[3];    
    a = b; // OK, copies just array fat reference
}

void main() {
    int[3] a, b;
    a = b; // Not OK, hidden vector op
}


I am not sure this new table is fully correct, but it's a start. Fixes of
mistakes are welcomes.

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

This is an alternative proposal.

On the other hand this vector op syntax doesn't currently compile:

void main() {
    int[3] a, b;
    a[] += b;
}


So if array assign is seen as a normal vector op, then the [] is needed on the
right too:


Rhs is an array, is it compilable?
a       / b             a=b     a[]=b   a=b[]   a[]=b[]
int[3u] / int[3u]       FALSE   FALSE   FALSE   true
int[3u] / int[]         FALSE   FALSE   FALSE   true
int[]   / int[3u]       FALSE   FALSE   FALSE   true
int[]   / int[]         true    FALSE   FALSE   true

Rhs is a element, is it compilable?
a                       a=N     a[]=N   a[0..2]=N
int[3u]                 FALSE   true    true
int[]                   false   true    true


Where the two cases with dynamic arrays are syntax errors to keep more
symmetry:

void main() {
    int[] a = new int[3];
    int[] b = new int[3];
    a[] = b; // error
    a = b[]; // error
}

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