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digitalmars.D.bugs - [Issue 3514] New: opApply should be the first-choice foreach iteration method.

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=3514

           Summary: opApply should be the first-choice foreach iteration
                    method.
           Product: D
           Version: 2.036
          Platform: Other
        OS/Version: Windows
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: dsimcha yahoo.com


--- Comment #0 from David Simcha <dsimcha yahoo.com> 2009-11-16 06:18:55 PST ---
struct Foo {
    uint front() {
        return 1;
    }

    int opApply(int delegate(ref uint) dg) {
        return 1;
    }
}

void main() {
    Foo foo;
    foreach(elem; foo) {}
}

test8.d(68): Error: no property 'empty' for type 'Foo'
test8.d(68): Error: no property 'popFront' for type 'Foo'

Clearly, DMD saw that front() was present and tried to use range foreach.  This
is incorrect because:

1.  Only part of the range interface existed.  The opApply interface was
complete and should have worked.

2.  If someone defines both a range interface and an opApply interface with the
same types, they probably have a good reason, since ranges serve other
purposes, but opApply exists **only** for foreach.

3.  Some things, like iterating over trees, can be done more efficiently with
control of the stack than without.

Also, once opSlice becomes able to define implicit conversions to ranges for
foreach loops, any opApply's defined should take precedence over this for the
reasons mentioned above.

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--- Comment #1 from David Simcha <dsimcha yahoo.com> 2009-11-16 06:21:38 PST ---
Forgot to mention:  Also, when using virtual functions, it may sometimes be
reasonable to define an opApply as an optimization for foreach loops, even if
the range-based foreach has the exact same semantics.  For example:

class Foo {
    SomeType front() {  return _front;}
    void popFront() {  
        // do stuff.
    }

    bool empty() {
        return _empty;
    }
}

In this case each iteration will require three virtual function calls, whereas
if opApply were used, the overhead would be reduced to a single delegate call.

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Don <clugdbug yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |clugdbug yahoo.com.au


--- Comment #2 from Don <clugdbug yahoo.com.au> 2009-11-16 06:40:54 PST ---
Doing this will also fix bug 2984 (which is marked as a regression, but isn't
really).

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Leandro Lucarella <llucax gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |llucax gmail.com
         Resolution|                            |DUPLICATE


--- Comment #3 from Leandro Lucarella <llucax gmail.com> 2009-11-16 12:44:28
PST ---
*** This issue has been marked as a duplicate of issue 2984 ***

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Steven Schveighoffer <schveiguy yahoo.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|DUPLICATE                   |


--- Comment #4 from Steven Schveighoffer <schveiguy yahoo.com> 2009-11-16
13:07:44 PST ---
This bug supersedes bug 2984, because solving this bug will not only address
the problem in 2984, but address a very important problem that is not
identified in 2984.  That is:  if opApply is present *at all* it should
override any range functionality when sending to foreach.  This includes the
case not identified in 2984 in which valid range operations are present
*alongside* opApply.  e.g.:

import std.stdio;

struct S
{
    int front()
    {
        return 0;
    }

    bool empty()
    {
        return true;
    }

    void popFront()
    {
    }

    int opApply(int delegate(ref int x) dg)
    {
        writeln("inside opapply");
        int x = 0;
        return dg(x);
    }
}

void main()
{
    S s;
    foreach(i; s)
    {
        writeln("inside loop %d", i);
    }
}

Currently outputs nothing, it should output:
inside opapply
inside loop 0

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--- Comment #5 from Leandro Lucarella <llucax gmail.com> 2009-11-16 13:46:50
PST ---
Then bug 2984 should be marked as a duplicate for this bug, right? There is
definitely duplication in this 2 bugs, and there is no point to keep both open.

I won't do it just in case I'm messing it up again ;)

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--- Comment #6 from Steven Schveighoffer <schveiguy yahoo.com> 2009-11-16
14:01:40 PST ---
Yeah, probably :)  I'll do it.

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Steven Schveighoffer <schveiguy yahoo.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |2korden gmail.com


--- Comment #7 from Steven Schveighoffer <schveiguy yahoo.com> 2009-11-16
14:04:03 PST ---
*** Issue 2984 has been marked as a duplicate of this issue. ***

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--- Comment #8 from Leandro Lucarella <llucax gmail.com> 2009-12-15 16:52:04
PST ---
http://www.dsource.org/projects/dmd/changeset/295

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Walter Bright <bugzilla digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bugzilla digitalmars.com


--- Comment #9 from Walter Bright <bugzilla digitalmars.com> 2009-12-18
21:02:43 PST ---
More discussion:

http://www.digitalmars.com/d/archives/digitalmars/D/opApply_Vs._Ranges_What_should_take_precedence_101124.html

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Walter Bright <bugzilla digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|REOPENED                    |RESOLVED
         Resolution|                            |FIXED


--- Comment #10 from Walter Bright <bugzilla digitalmars.com> 2009-12-31
11:20:21 PST ---
Fixed dmd 2.038

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Dec 31 2009