[Python-ideas] Loop labels

[Matt Joiner]

Yeah it's definitely non trivial. Reading this is what got me thinking
about it in Python: mortoray.com/2011/10/23/the-ideal-language-has-goto/
On Mar 9, 2012 1:07 PM, "David Townshend" <aquavitae69 at gmail.com> wrote:

> The biggest problem is not the new syntax.  It's the new type of object
> that would be needed, Label, which lies in some magical place half way
> between a name and a keyword. What would be the result of the following
> code?
>
> loops = []
> for i in range(4) as label:
>     print(type(label), dir(label))
>     loops.append(label)
> for label in loops as newlabel:
>     break label
>
> David
> On Mar 9, 2012 1:08 AM, "Westley Martínez" <anikom15 at gmail.com> wrote:
>
>> On Fri, Mar 09, 2012 at 02:52:08AM +0800, Matt Joiner wrote:
>> > Several languages allow break and continue statements to take a count
>> > or loop label for greater flexibility.
>> >
>> > Dealing with nested loops and control flow around them is definitely
>> > something I and probably most programmers deal with everyday.
>> > Generally for very complex control flow one might employ functions,
>> > and use return statements to work around any shortcomings. This is not
>> > always ideal in CPython because of the performance cost of function
>> > calls, and lack of anonymous functions. The other work around is
>> > usually goto statements, which clearly aren't available or appropriate
>> > in Python.
>> >
>> > So what about the following extensions?
>> >
>> > Allow for and while statements to be labelled using "as".
>> >
>> > Allow break and continue to take the name of a containing loop, or an
>> > integer. The corresponding named loop, or the nth containing loop are
>> > treated as though they are the nearest enclosing loop.
>> >
>> > loop_label ::=  identifier
>> >
>> > break_stmt ::=  "break" [decimalinteger | loop_label]
>> >
>> > continue_stmt ::=  "continue" [decimalinteger | loop_label]
>> >
>> > while_stmt ::=  "while" expression ["as" loop_label] ":" suite
>> >                 ["else" ":" suite]
>> >
>> > for_stmt ::=  "for" target_list "in" expression_list ["as" loop_label]
>> ":" suite
>> >               ["else" ":" suite]
>> >
>> > Here's a really naive example:
>> > for a in b as c:
>> >     for d in e:
>> >         break c # or break 2
>> >     for f in g:
>> >         continue c # or continue 2
>>
>> -1; if we do this we might as well add goto labels.
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