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digitalmars.D.bugs - [Issue 4661] New: Array of lazy sequence

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=4661

           Summary: Array of lazy sequence
           Product: D
           Version: D2
          Platform: x86
        OS/Version: Windows
            Status: NEW
          Keywords: rejects-valid
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody puremagic.com
        ReportedBy: bearophile_hugs eml.cc


--- Comment #0 from bearophile_hugs eml.cc 2010-08-17 06:01:25 PDT ---
Problem in building an array of lazy sequences. This D2 code:


import std.algorithm: map;
void main() {
    auto r1 = map!q{a+1}([1, 2, 3]);
    auto r2 = map!q{a+2}([1, 2, 3]);
    auto a = [r1, r2];
}


Fails, and DMD 2.048 shows:
Error: incompatible types for ((r1) ? (r2)): 'Map!(result,int[])' and
'Map!(result,int[])'


Additionally, the error message lacks a line number.

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next sibling parent d-bugmail puremagic.com writes:
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David Simcha <dsimcha yahoo.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |dsimcha yahoo.com
         Resolution|                            |INVALID


--- Comment #1 from David Simcha <dsimcha yahoo.com> 2010-08-17 15:51:49 PDT ---
This bug is invalid because of the way template instantiation from string
lambdas works.  Instantiating map with q{a + 1} produces a completely different
type than map instantiated with q{a + 2}, and therefore storing them in an
array should not be possible.  Take a look at how Map and
std.functional.unaryFun() work.  Also, to illustrate my point, note that the
following code works:

import std.algorithm: map;
void main() {
    auto r1 = map!q{a+1}([1, 2, 3]);
    auto r2 = map!q{a+1}([1, 2, 3]);
    auto a = [r1, r2];
}

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Brad Roberts <braddr puremagic.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
                 CC|                            |braddr puremagic.com
         Resolution|INVALID                     |


--- Comment #2 from Brad Roberts <braddr puremagic.com> 2010-08-17 16:25:19 PDT
---
Take a step back and rethink your analysis.  This example is a straight forward
case of int[] to int[] mapping.  They _should_ be compatible types.  The map
function is irrelevant to the types involved.  If the result of the map
functions differed, then you'd be right.

Also, the lack of a line number is a bug regardless of the validity of the
operation.

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David Simcha <dsimcha yahoo.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|rejects-valid               |diagnostic
            Summary|Array of lazy sequence      |Array Literal Incompatible
                   |                            |Type Error Msg Should
                   |                            |Include Line Number


--- Comment #3 from David Simcha <dsimcha yahoo.com> 2010-08-17 17:39:01 PDT ---
Ok, so this is a legit diagnostic bug.  I'll grant you that.  However, thinking
that Map!("a + 1", Range) and Map!("a + 2", Range) are the same type represents
a serious misunderstanding of template alias parameters.  The binding of the
lambda function to the Map instance happens at compile time, not runtime. 
Let's try a thought experiment, thinking at the C level.  What would happen if
Map!"a + 2" were cast to Map!"a + 1" and front() were called?

Map!"a + 1".front() would be called.  Inlined (probably) in Map!"a + 1".front()
is something like return _input.front + 1.

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--- Comment #4 from Brad Roberts <braddr puremagic.com> 2010-08-17 17:46:40 PDT
---
You're miring your logic in the implementation rather than the concept of what
map is.  Pretend it was implemented as:

T[] map(T)(mapfunc, T[]);

would we even be having this discussion?

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--- Comment #5 from David Simcha <dsimcha yahoo.com> 2010-08-17 18:40:32 PDT ---
No, I'm saying that since the lambda function is bound at compile time, this
isn't a bug.  It's an intentional design tradeoff.  It **can't** be fixed
unless the lambda function is bound at runtime instead, which would prevent it
from being inlined, prevent implicit instantiation of the lambda function if
it's a template, etc.

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Andrei Alexandrescu <andrei metalanguage.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |andrei metalanguage.com


--- Comment #6 from Andrei Alexandrescu <andrei metalanguage.com> 2010-08-17
19:58:25 PDT ---
I agree that the initial code shouldn't work. This code should:

import std.algorithm: map;
void main() {
    auto r1 = map!q{a+1}([1, 2, 3]);
    auto r2 = map!q{a+2}([1, 2, 3]);
    auto a = array(chain(r1, r2));
}

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--- Comment #7 from bearophile_hugs eml.cc 2010-08-18 03:23:38 PDT ---
Given the way templates work in D, the original code can't work, but I'd like
to receive the error line number here.

This problem is a good example to show the difference between a structural type
system and a nominative one.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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yebblies <yebblies gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch
                 CC|                            |yebblies gmail.com


--- Comment #8 from yebblies <yebblies gmail.com> 2011-06-09 00:54:18 PDT ---
Test case for the error message, and a similar case for associative array
initializers:

void main() {

    auto a = [1, null];
    auto b = [1 : 1, null : null];    
}

https://github.com/D-Programming-Language/dmd/pull/99

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--- Comment #9 from yebblies <yebblies gmail.com> 2011-06-09 08:40:44 PDT ---
*** Issue 5518 has been marked as a duplicate of this issue. ***

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prev sibling parent d-bugmail puremagic.com writes:
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Walter Bright <bugzilla digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|REOPENED                    |RESOLVED
                 CC|                            |bugzilla digitalmars.com
         Resolution|                            |FIXED


--- Comment #10 from Walter Bright <bugzilla digitalmars.com> 2011-06-11
12:50:14 PDT ---
https://github.com/D-Programming-Language/dmd/commit/86ea84f72c3b82ae034776e58a96054ab97f2138

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