Friday, February 5, 2010

Obsolete $_ syntax in <[ expr ]>

Is it to late to revoke a syntax feature ?
I've determined in hindsight that the $_ variable exposed in the<[ xquery expr ]> is not right.
This concatenates all positional parameters into a single sequence. If any of the positional parameters are a sequence > 1 then the result loses information. For example

set <[ 1,2,3 ]> 4 5
echo $#
echo <[ $_[2] ]>



Very non obvious. The fact that $1 is a sequence has been lost and the $_ is a concatenation of 1,2,3,4,5

To fix this I now predeclare distinct variables $_1 $_2 ... for all positional parameters.
This preserves the sequences in the positional parameters.

$ echo <[ $_1 ]>
1 2 3

This means you can access all positional parameters within the <[ ]> expression with no loss of fidelity. I'd like to get rid of $_ as it is now redundant. However someone might be using it ... so I'm keeping it in for now.

A side effect of this, however, is that there is no way to access $# within the <[ ]> or to iterate over all positional parameters in one query. This is a fundamental limit of xquery and XDM which do not allow nested sequences. It also means that if you actually assign a global variable _1 then it is overwritten by the positional parameter $1 within the <[ ]> expression.

So is it too late now that I've published "1.0" of xmlsh ? Even if there is efficiency and usability issues ? Would anyone be affected ?

I have no idea because I have no metrics of how much xmlsh is being used ....
which gets to my next post ...

1 comment:

  1. I discovered I never documented $_ in the <[ ]> syntax so I think its safe to delete it.

    Also time to improve the documentation on <[ ]> which is only 1 paragraph hidden in Core Syntax.


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