On the Austin Group mailing list, David Korn (of ksh93 fame) complained[1] that bash's 'local' uses dynamic scoping, but that ksh's 'typeset' uses static scoping, and argued that static scoping is saner since it matches the behavior of declarative languages like C and Java (dynamic scoping mainly matters in functional languages like lisp): [1] https://www.opengroup.org/sophocles/show_mail.tpl?CALLER=show_archive.tpl&source=L&listname=austin-group-l&id=14951 I'm trying to standardize the notion of local variables for the next revision of POSIX, but before I can do so, I need some feedback on two general aspects: 1. Implementation aspect: How hard would it be to add static scoping to dash? Is it something that should be added in addition to dynamic scoping, via the use of an option to select the non-default mode (for example, 'local -d' to force dynamic, 'local -s' to force static, and 'local' to go with default scoping)? Or should dash switch entirely to static scoping (my gut feel is that static scoping may be more efficient to implement, which fits in line with dash's desire to be as lean as possible)? 2. User aspect: Is anyone aware of a script that intentionally uses the full power of dynamic scoping available through 'local' which would break if scoping switched to static? Here's a sample shell script that illustrates the difference between the two scoping methods (note that ksh only provides nested scoping via its typeset builtin, and only when using the function reserved word). $ ksh -c 'function f1 { typeset a=local; f2; echo $a; }; function f2 { echo $a; a=changed; }; a=global; f1; echo $a' global local changed $ dash -c 'f1 () { typeset a=local; f2; echo $a; }; f2 () { echo $a; a=changed; }; a=global; f1; echo $a' local changed global In static scoping, function f2 does not shadow a declaration of a, so references to $a within f2 refer to the global variable. The local variable a of f1 can only be accessed within f1; the behavior of f2 is the same no matter how it was reached. In dynamic scoping, function f2 looks up its call stack for the closest enclosing scope of a variable named a, and finds the local one declared in f1. Therefore, the behavior of f2 depends on how f2 is called. -- Eric Blake eblake@xxxxxxxxxx +1-801-349-2682 Libvirt virtualization library http://libvirt.org
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