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Move case statement style discussion to m4 quoting section.
* doc/autoconf.texi (Limitations of Builtins): Move comparison of quoting styles... (Balancing Parentheses): ...to this new node. Suggested by Ralf Wildenhues. Signed-off-by: Eric Blake <ebb9@byu.net>
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Eric Blake
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@ -1,3 +1,11 @@
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2008-11-21 Eric Blake <ebb9@byu.net>
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Move case statement style discussion to m4 quoting section.
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* doc/autoconf.texi (Limitations of Builtins): Move comparison of
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quoting styles...
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(Balancing Parentheses): ...to this new node.
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Suggested by Ralf Wildenhues.
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2008-11-20 Eric Blake <ebb9@byu.net>
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Factor more common code out of AT_CHECK into shell function.
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@ -445,6 +445,7 @@ M4 Quotation
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* Quotation and Nested Macros:: Macros calling macros
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* Changequote is Evil:: Worse than INTERCAL: M4 + changequote
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* Quadrigraphs:: Another way to escape special characters
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* Balancing Parentheses:: Dealing with unbalanced parentheses
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* Quotation Rule Of Thumb:: One parenthesis, one quote
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Using @command{autom4te}
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@ -9292,6 +9293,7 @@ former helps one to follow the latter.
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* Quotation and Nested Macros:: Macros calling macros
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* Changequote is Evil:: Worse than INTERCAL: M4 + changequote
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* Quadrigraphs:: Another way to escape special characters
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* Balancing Parentheses:: Dealing with unbalanced parentheses
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* Quotation Rule Of Thumb:: One parenthesis, one quote
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@end menu
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@ -9778,6 +9780,126 @@ invention, and I suppose it could have been a common pun around the
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Cambridge University computer lab at the time.
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@end quotation
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@node Balancing Parentheses
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@subsection Dealing with unbalanced parentheses
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@cindex balancing parentheses
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@cindex parentheses, balancing
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@cindex unbalanced parentheses, managing
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One of the pitfalls of portable shell programming is that @command{case}
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statements require unbalanced parentheses (@pxref{Limitations of
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Builtins, , Limitations of Shell Builtins}). With syntax highlighting
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editors, the presence of unbalanced @samp{)} can interfere with editors
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that perform syntax highlighting of macro contents based on finding the
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matching @samp{(}. Another concern is how much editing must be done
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when transferring code snippets between shell scripts and macro
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definitions. But most importantly, the presence of unbalanced
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parentheses can introduce expansion bugs.
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For an example, here is an underquoted attempt to use the macro
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@code{my_case}, which happens to expand to a portable @command{case}
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statement:
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@example
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AC_DEFUN([my_case],
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[case $file_name in
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*.c) echo "C source code";;
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esac])
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AS_IF(:, my_case)
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@end example
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@noindent
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In the above example, the @code{AS_IF} call underquotes its arguments.
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As a result, the unbalanced @samp{)} generated by the premature
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expansion of @code{my_case} results in expanding @code{AS_IF} with a
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truncated parameter, and the expansion is syntactically invalid:
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@example
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if :; then
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case $file_name in
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*.c
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fi echo "C source code";;
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esac)
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@end example
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If nothing else, this should emphasize the importance of the quoting
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arguments to macro calls. On the other hand, there are several
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variations for defining @code{my_case} to be more robust, even when used
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without proper quoting, each with some benefits and some drawbacks.
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@itemize @asis
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@item Creative literal shell comment
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@example
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AC_DEFUN([my_case],
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[case $file_name in #(
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*.c) echo "C source code";;
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esac])
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@end example
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@noindent
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This version provides balanced parentheses to several editors, and can
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be copied and pasted into a terminal as is. Unfortunately, it is still
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unbalanced as an Autoconf argument, since @samp{#(} is an M4 comment
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that masks the normal properties of @samp{(}.
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@item Quadrigraph shell comment
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@example
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AC_DEFUN([my_case],
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[case $file_name in @@%:@@(
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*.c) echo "C source code";;
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esac])
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@end example
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@noindent
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This version provides balanced parentheses to even more editors, and can
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be used as a balanced Autoconf argument. Unfortunately, it requires
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some editing before it can be copied and pasted into a terminal, and the
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use of the quadrigraph @samp{@@%:@@} for @samp{#} reduces readability.
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@item Quoting just the parenthesis
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@example
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AC_DEFUN([my_case],
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[case $file_name in
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*.c[)] echo "C source code";;
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esac])
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@end example
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@noindent
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This version quotes the @samp{)}, so that it can be used as a balanced
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Autoconf argument. As written, this is not balanced to an editor, but
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it can be coupled with @samp{[#(]} to meet that need, too. However, it
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still requires some edits before it can be copied and pasted into a
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terminal.
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@item Double-quoting the entire statement
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@example
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AC_DEFUN([my_case],
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[[case $file_name in #(
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*.c) echo "C source code";;
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esac]])
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@end example
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@noindent
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Since the entire macro is double-quoted, there is no problem with using
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this as an Autoconf argument; and since the double-quoting is over the
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entire statement, this code can be easily copied and pasted into a
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terminal. However, the double quoting prevents the expansion of any
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macros inside the case statement, which may cause its own set of
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problems.
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@item Using @code{AS_CASE}
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@example
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AC_DEFUN([my_case],
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[AS_CASE([$file_name],
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[*.c], [echo "C source code"])])
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@end example
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@noindent
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This version avoids the balancing issue altogether, by relying on
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@code{AS_CASE} (@pxref{Common Shell Constructs}); it also allows for the
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expansion of @code{AC_REQUIRE} to occur prior to the entire case
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statement, rather than within a branch of the case statement that might
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not be taken. However, the abstraction comes with a penalty that it is
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no longer a quick copy, paste, and edit to get back to shell code.
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@end itemize
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@node Quotation Rule Of Thumb
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@subsection Quotation Rule Of Thumb
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@ -14901,113 +15023,13 @@ $ @kbd{case foo in (foo) echo foo;; esac}
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@end example
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@noindent
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@cindex balancing parentheses
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@cindex parentheses, balancing
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The leading @samp{(} can be omitted safely. Unfortunately, there are
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contexts where unbalanced parentheses cause other problems, such as when
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using a syntax-highlighting editor that searches for the balancing
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counterpart, or more importantly, when using a case statement as an
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underquoted argument to an Autoconf macro:
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@example
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AC_DEFUN([my_case],
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[case $file_name in
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*.c) echo "C source code";;
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esac])
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AS_IF(:, my_case)
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@end example
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@noindent
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In the above example, the unbalanced @samp{)} in the premature expansion
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of @code{my_case} results in expanding @code{AS_IF} with a truncated
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parameter, and the expansion is syntactically invalid:
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@example
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if :; then
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case $file_name in
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*.c
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fi echo "C source code";;
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esac)
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@end example
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@noindent
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If nothing else, this should emphasize the importance of the quoting
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rule of thumb (@pxref{Quotation Rule Of Thumb}), that you should single
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quote all macro arguments that might be re-expanded, and double-quote
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macro arguments that are literal text. On the other hand, there are
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several variations for defining @code{my_case} to be more robust, each
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with some benefits and some drawbacks.
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@table @asis
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@item Creative literal shell comment
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@example
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AC_DEFUN([my_case],
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[case $file_name in #(
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*.c) echo "C source code";;
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esac])
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@end example
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@noindent
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This version provides balanced parentheses to several editors, and can
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be copied and pasted into a terminal as is. Unfortunately, it is still
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unbalanced as an Autoconf argument, since @samp{#(} is an M4 comment
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that masks the normal properties of @samp{(}.
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@item Quadrigraph shell comment
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@example
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AC_DEFUN([my_case],
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[case $file_name in @@%:@@(
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*.c) echo "C source code";;
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esac])
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@end example
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@noindent
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This version provides balanced parentheses to even more editors, and can
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be used as a balanced Autoconf argument. Unfortunately, it requires
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some editing before it can be copied and pasted into a terminal, and the
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use of the quadrigraph @samp{@@%:@@} for @samp{#} reduces readability.
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@item Quoting just the parenthesis
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@example
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AC_DEFUN([my_case],
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[case $file_name in
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*.c[)] echo "C source code";;
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esac])
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@end example
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@noindent
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This version quotes the @samp{)}, so that it can be used as a balanced
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Autoconf argument. As written, this is not balanced to an editor, but
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it can be coupled with @samp{[#(]} to meet that need, too. However, it
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still requires some edits before it can be copied and pasted into a
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terminal.
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@item Double-quoting the entire statement
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@example
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AC_DEFUN([my_case],
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[[case $file_name in #(
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*.c) echo "C source code";;
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esac]])
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@end example
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@noindent
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Since the entire macro is double-quoted, there is no problem with using
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this as an Autoconf argument; and since the double-quoting is over the
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entire statement, this code can be easily copied and pasted into a
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terminal. However, the double quoting prevents the expansion of any
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macros inside the case statement, which may cause its own set of
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problems.
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@item Using @code{AS_CASE}
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@example
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AC_DEFUN([my_case],
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[AS_CASE([$file_name],
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[*.c], [echo "C source code"])])
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@end example
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@noindent
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This version avoids the balancing issue altogether, by relying on
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@code{AS_CASE} (@pxref{Common Shell Constructs}); it also allows for the
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expansion of @code{AC_REQUIRE} to occur prior to the entire case
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statement, rather than within a branch of the case statement that might
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not be taken. However, the abstraction comes with a penalty that it is
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no longer a quick copy, paste, and edit to get back to shell code.
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@end table
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underquoted argument to an Autoconf macro. @xref{Balancing
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Parentheses}, for tradeoffs involved in various styles of dealing with
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unbalanced @samp{)}.
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Zsh handles pattern fragments derived from parameter expansions or
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command substitutions as though quoted:
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