Monday, June 15, 2009

Expanding sequences into positional parameters.

I'm debauting over a syntax to convert a sequence into positional parameters.

The problem is this. Suppose I have a sequence expression, say a variable
a=(a b c d e)

then want to pass that sequence as separate parameters to a command. Using $a does NOT expand the sequence. For example
command $a

passes 1 argument of type sequence (unless command is an external program in which case the sequence is converted to args).

this is typically desirable. Sequence expressions should be able to be passed to commands without "flatening". Particularly useful if you want to pass multiple sequences for example as say xquery or xslt parameters
xquery -q ... -v value1 $a -v value2 $b

a and b can be sequence values. But suppose I actually want a sequence to be flattened. I haven't figured out a clean way to do it in the current syntax. Posix shells don't help with this as they don't have sequences. Bash has arrays which are similar but you cant pass an array around to a command.

Eval can do this if the sequence contains simple unquoted strings like above
eval set $a
sets $1,2,3 to a,b,c BUT

a=<[ <foo/> , 2 , <bar>spam</bar> ]>
eval set $a

is a syntax error.
I believe this works but haven't tried it in all cases
set --
for arg in $a ; do
set -- "$@" $arg

ugly though !

I'm considering the "all array" syntax to do this like ${a[*]}
which would instead of returning all elements of the sequence as a single sequence value, returning them as multiple positional values similar to $*

That might do the trick but I'm not sure I like it. The odd thing is that this feature is only really useful when in the context of command argument expansion.

would do nothing different then

This is a bit subtle but the difference is that single variables can only hold 1 level of sequence whereas positional parameters hold 2 levels. That is "$*" is a List of Values where each value can be a sequence.

also considering both a prefix and suffix modifier such as ${+a} or ${a:+}

Of course none of these syntax would be obvious at all what they do until you learn them !

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