digitalmars.D.bugs - [Issue 5399] New: Return the result of a nonvoid function in a void function
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http://d.puremagic.com/issues/show_bug.cgi?id=5399
Summary: Return the result of a nonvoid function in a void
function
Product: D
Version: D2
Platform: x86
OS/Version: Windows
Status: NEW
Keywords: accepts-invalid
Severity: normal
Priority: P2
Component: DMD
AssignedTo: nobody puremagic.com
ReportedBy: bearophile_hugs eml.cc
--- Comment #0 from bearophile_hugs eml.cc 2011-01-01 13:49:57 PST ---
In D (rightly) you can't return a value different from void inside a void
function. But this code compiles and runs with no errors in DMD 2.051 (bug
found by Daren Scot Wilson):
int foo() { return 1; }
void main() {
return foo();
}
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bearophile_hugs eml.cc changed:
What |Removed |Added
----------------------------------------------------------------------------
Severity|normal |enhancement
--- Comment #1 from bearophile_hugs eml.cc 2011-01-01 23:00:02 PST ---
This isn't a bug, I was wrong. D2 specs clearly show this is working as
expected:
http://www.digitalmars.com/d/2.0/statement.html
ReturnStatement:
return;
return Expression ;
Expression is allowed even if the function specifies a void return type. The
Expression will be evaluated, but nothing will be returned. If the Expression
has no side effects, and the return type is void, then it is illegal.
Indeed, this too compiles:
void main() {
int x;
return x++;
}
But this is a potentially dangerous corner case in the return rules.
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Walter Bright <bugzilla digitalmars.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
CC| |bugzilla digitalmars.com
Resolution| |INVALID
--- Comment #2 from Walter Bright <bugzilla digitalmars.com> 2011-01-01
23:33:12 PST ---
It is not a dangerous corner case, it is a deliberate design choice. It is
meant to facilitate writing generic code so that the same code can be generated
for void and non-void return values.
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nfxjfg gmail.com changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |nfxjfg gmail.com
--- Comment #3 from nfxjfg gmail.com 2011-01-02 05:45:57 PST ---
(In reply to comment #2)
It is not a dangerous corner case, it is a deliberate design choice. It is
meant to facilitate writing generic code so that the same code can be generated
for void and non-void return values.
Wow, D is really full of idiocy, isn't it?
If you wanted to facilitate that, you'd just allow declaring void variables and
all that. I've written generic code where _that_ would have been quite useful.
But returning non-void values in void functions? Garbage language feature.
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Iain Buclaw <ibuclaw ubuntu.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |ibuclaw ubuntu.com
--- Comment #4 from Iain Buclaw <ibuclaw ubuntu.com> 2011-01-02 06:24:38 PST ---
(In reply to comment #2)
It is not a dangerous corner case, it is a deliberate design choice. It is
meant to facilitate writing generic code so that the same code can be generated
for void and non-void return values.
Just for clarification, so you allow this, but the return value is always
ignored? (ie: 0)
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Andrei Alexandrescu <andrei metalanguage.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Priority|P2 |P5
Status|RESOLVED |REOPENED
CC| |andrei metalanguage.com
Resolution|INVALID |
Severity|enhancement |major
--- Comment #5 from Andrei Alexandrescu <andrei metalanguage.com> 2011-01-02
06:48:17 PST ---
This is a very problematic misfeature that takes no effort to remove. In
particular I confirm it is of no or negative use to generic programming.
Walter, please let's remove it in the next release. Thank you.
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--- Comment #6 from nfxjfg gmail.com 2011-01-02 07:46:29 PST ---
I think the issue is with allowing stuff like this:
Result wrapCall(alias call, Result, Args...)(Args args) {
return call(args);
}
And then making it work even if the result of the call is void:
wrapCall(&something, void, int, int)(1, 2);
That requires that you can return a void value. Returning a void normally
wouldn't make sense, but as you can see it simplifies generic programming.
Somehow it made sense in Walter's head to allow returning _anything_ from a
void function. (It would make sense if void would work like Scala's Unit, but
void doesn't.)
Walter, please explain.
By the way if D were really orthogonal and would follow any logic, you wouldn't
have any problem with this code:
Result wrapCall(alias call, Result, Args...)(Args args) {
try {
return call(args);
} catch {
writefln("call failed!");
return Result.init;
}
}
This works, except when Result is void. Then you have to use static if,
duplicate the core code around the actual call if that is more complicated than
in the given example, and so on. (I had this in real world code.)
Sure makes a lot of sense.
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--- Comment #7 from Andrei Alexandrescu <andrei metalanguage.com> 2011-01-02
09:05:29 PST ---
(In reply to comment #6)
I think the issue is with allowing stuff like this:
Result wrapCall(alias call, Result, Args...)(Args args) {
return call(args);
}
And then making it work even if the result of the call is void:
wrapCall(&something, void, int, int)(1, 2);
That requires that you can return a void value. Returning a void normally
wouldn't make sense, but as you can see it simplifies generic programming.
Yes, that's a classic in C++ too. My assessment refers not to forwarding across
void function, but to void functions returning non-void expressions.
To clarify: forwarding from one void function to another void function is
useful. Having a void function return a non-void value should be removed.
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yebblies <yebblies gmail.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |yebblies gmail.com
--- Comment #8 from yebblies <yebblies gmail.com> 2011-06-29 18:58:29 EST ---
I'll copy what I said in issue 3746:
Without this feature, what should happen with lazy void?
void lazyFunc(lazy void a) { a; }
void main()
{
int i;
lazyFunc(i++);
}
lazyFunc(i++) is currently re-written to something like
lazyFunc({return i++;})
This of course is broken if returning a non-void from a void function is
disallowed.
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Don <clugdbug yahoo.com.au> changed:
What |Removed |Added
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CC| |clugdbug yahoo.com.au
--- Comment #9 from Don <clugdbug yahoo.com.au> 2011-06-29 02:29:05 PDT ---
(In reply to comment #8)
I'll copy what I said in issue 3746:
Without this feature, what should happen with lazy void?
void lazyFunc(lazy void a) { a; }
void main()
{
int i;
lazyFunc(i++);
}
lazyFunc(i++) is currently re-written to something like
lazyFunc({return i++;})
This of course is broken if returning a non-void from a void function is
disallowed.
What's wrong with
lazyFunc( { i++; return;})
?
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yebblies <yebblies gmail.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |patch
--- Comment #10 from yebblies <yebblies gmail.com> 2011-06-30 03:02:12 EST ---
(In reply to comment #9)
What's wrong with
lazyFunc( { i++; return;})
?
For some reason I was thinking this would skip the 'expression has no effect'
errors.
https://github.com/D-Programming-Language/dmd/pull/174
Fixing this found a bug in phobos.
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Walter Bright <bugzilla digitalmars.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|REOPENED |RESOLVED
Resolution| |FIXED
Severity|major |enhancement
--- Comment #11 from Walter Bright <bugzilla digitalmars.com> 2011-10-09
14:54:12 PDT ---
https://github.com/D-Programming-Language/dmd/commit/c942d51c8b1103d5ce4c3dfc03ae77c07c687cd6
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