Re: bin/48843: sh(1): break/continue/return broken inside dot commands

[Aleksej Saushev <asau%inbox.ru@localhost>]

The following reply was made to PR bin/48843; it has been noted by GNATS.

From: Aleksej Saushev <asau%inbox.ru@localhost>
To: gnats-bugs%netbsd.org@localhost
Cc: 
Subject: Re: bin/48843: sh(1): break/continue/return broken inside dot commands
Date: Thu, 05 Jun 2014 02:32:01 +0400

 Richard Hansen <rhansen%bbn.com@localhost> writes:
 
 > The following reply was made to PR bin/48843; it has been noted by GNATS.
 >
 > From: Richard Hansen <rhansen%bbn.com@localhost>
 > To: David Holland <dholland-tech%netbsd.org@localhost>
 > Cc: jarmo.jaakkola%roskakori.fi@localhost, gnats-bugs%NetBSD.org@localhost, 
 >  tech-userlevel%NetBSD.org@localhost
 > Subject: Re: bin/48843: sh(1): break/continue/return broken inside dot 
 > commands
 > Date: Tue, 03 Jun 2014 12:53:56 -0400
 >
 >  On 2014-06-02 13:54, Richard Hansen wrote:
 >  > Note that unlike variable binding, the behavior of dynamic scoping of
 >  > break/continue is a superset of static/lexical scoping (assuming no
 >  > closures):  If the shell does dynamic scoping of break/continue but
 >  > scripts are always written assuming lexical scoping then those scripts
 >  > will still work as expected in all cases.  If POSIX were to specify
 >  > static/lexical scoping then a shell that performs dynamic scoping would
 >  > be conformant but with an extension to the standard.
 >  
 >  Oops -- as pointed out in [1] this is incorrect when the argument to
 >  break/continue is greater than the number of lexically enclosing loops.
 >   An author could write 'break 1000' to make it obvious to other readers
 >  that all loops should be broken.  A script like this might behave
 >  differently on a shell that does dynamic scoping vs. a shell that does
 >  lexical scoping.
 >  
 >  So it looks like POSIX will probably have to choose either lexical
 >  (static) or dynamic for the next major version of the standard (Issue 8).
 >  
 >  David voiced his preference for lexical [2]; does anyone else have an
 >  opinion?
 >  
 >  I think my preference is for dynamic, although I would question the
 >  design decisions of a script author that takes advantage of dynamic
 >  scoping.  Why dynamic over lexical?  Because functionality is lost by
 >  making it lexical:  If a script author wants dynamic but the shell does
 >  lexical, the author is out of luck.  If the shell does dynamic but the
 >  author wants lexical, the author can easily count the number of
 >  lexically enclosing loops and limit the argument of break/continue to
 >  that value.  Dynamic might also be easier to implement.
 
 No, this is wrong. Simulating dynamic scoping with lexical one is clear
 and almost trivial. It also has benefit that you have control over which
 variables have dynamic scope. The opposite is practically impossible.
 
 Besides, the general consensus in programming languages community is
 that dynamic scoping is bad design. It breaks abstractions and thus
 makes writing modular and correct code a lot harder. Choosing dynamic
 scoping over lexical has nearly disastrous consequences on the code
 maintainability.