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digitalmars.D.bugs - [Issue 827] New: Trying to break out of a labelled BlockStatement breaks out of a for loop at its beginning

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=827

           Summary: Trying to break out of a labelled BlockStatement breaks
                    out of a for loop at its beginning
           Product: D
           Version: 1.00
          Platform: PC
        OS/Version: Windows
            Status: NEW
          Keywords: accepts-invalid, wrong-code
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla digitalmars.com
        ReportedBy: smjg iname.com


----------
import std.stdio;

void main() {
    block: {
        for (int i = 0; i < 10; i++) {
            if (i == 5) break block;
        }
        writefln("Within block");
    }
    writefln("Outside block");
}
----------
Within block
Outside block
----------

I was expecting it to print only "Outside block".  However, according to the
spec, the code shouldn't compile:

http://www.digitalmars.com/d/statement.html
"If break is followed by Identifier, the Identifier  must be the label of an
enclosing while, for, do or switch statement, and that statement is exited. It
is an error if there is no such statement."

The restriction to while, for, do or switch seems arbitrary, but still....


--
Jan 10 2007
next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=827


thomas-dloop kuehne.cn changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  BugsThisDependsOn|                            |199
         OS/Version|Windows                     |All

Bug 827 depends on bug 199, which changed state.

Bug 199 Summary: Label causes scope to collapse into parent
http://d.puremagic.com/issues/show_bug.cgi?id=199

           What    |Old Value                   |New Value
----------------------------------------------------------------------------
             Status|REOPENED                    |RESOLVED
         Resolution|                            |WORKSFORME
             Status|RESOLVED                    |REOPENED
         Resolution|WORKSFORME                  |



------- Comment #1 from thomas-dloop kuehne.cn  2007-04-05 05:26 -------
I'm not sure this is the same as issue 199 but 199's "collapsing scope" looks 
like the root cause.

Added to DStress as
http://dstress.kuehne.cn/nocompile/b/break_13_A.d
http://dstress.kuehne.cn/nocompile/b/break_13_B.d


--
Apr 05 2007
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=827


bugzilla digitalmars.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |WONTFIX




------- Comment #2 from bugzilla digitalmars.com  2008-06-23 17:10 -------
It works as spec'd, and you're right it is the same issue as 199. Won't change
behavior for the same reason. Note that if you write it as (inserting an if
statement):

import std.stdio;

void main() {
    block: if (1) {
        for (int i = 0; i < 10; i++) {
            if (i == 5) break block;
        }
        writefln("Within block");
    }
    writefln("Outside block");
}

it won't compile per the spec.


--
Jun 23 2008
prev sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=827


smjg iname.com changed:

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




------- Comment #3 from smjg iname.com  2008-06-23 18:04 -------
(In reply to comment #2)

 It works as spec'd,


No it doesn't.  The statement from the spec that I already quoted is still
there, word for word.


 and you're right it is the same issue as 199.


Maybe within the compiler, but not insofar as according to the language, the
label is of the BlockStatement not of the ForStatement therein.  What the label
labels and whether the BlockStatement opens a new scope or not are essentially
distinct concepts.


 Won't change behavior for the same reason.


If you're referring to bug 199 comment 6, neither point seems to me to apply
here:


 I don't want to change this because it could break existing code,


Somebody who wants the existing behaviour of my code example can just use

void main() {
    block: for (int i = 0; i < 10; i++) {
        if (i == 5) break block;
    }
    writefln("Within block");
    writefln("Outside block");
}

so what's there to break?


 and there doesn't seem to be a compelling reason to do so.


I certainly think there is: the nasty shock a programmer gets on trying this
code, expecting it to (a) be legal (b) if so, behave in a way that makes
intuitive sense.

(In reply to comment #2)

 Note that if you write it as (inserting an if statement):


<snip>

 it won't compile per the spec.


Maybe.  But is this really relevant?  Nobody's going to do this just to make
the compiler catch the error, since doing so would imply that the coder's own
eyes have already caught the error.


--
Jun 23 2008