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Re: [bug-recutils] Value Expressions

From: John Darrington
Subject: Re: [bug-recutils] Value Expressions
Date: Thu, 23 Aug 2012 12:30:14 +0000
User-agent: Mutt/1.5.18 (2008-05-17)

I hadn't really thought about the syntax, but your proposoal seems reasonable.

Some hypotheitical use cases that I can think of are:

# Add a field for a person's retirement date.
recset -e 'Gender = Male' -f Retires -a '/date-of-birth + 65 years'
recset -e 'Gender = Female' -f Retires -a '/date-of-birth + 55 years'

# Add an a second email address to all those who work at foo.com
recset -e 'Email[0] = "address@hidden"'  -f Email -a 

This implies that any FEX element (but not a complete FEX) can be the subject 
of a VEX

but we don't know if the element 0 is the one where the target is, so we need 
kind of positional parameter:

recset -e '\(Email\) ~ "address@hidden"'  -f Email -a 

Here, the \1 represents the first field in the -e expression enclosed in ()
So a more complex example:
recset -e '#(first-name)' -e '#(last-name)' -e '#(company)' -f Email -a 

I can think of more complex cases; One might want to add field/value based upon 
the result of a join.
- But perhaps I shouldn't go there yet.


On Tue, Aug 21, 2012 at 03:50:50PM +0200, Jose E. Marchesi wrote:
     Hi John.
         I think it would be a good idea if the recset and recins commands could
         accept some kind of "value expression" in their -s, -a and -v options.
         That is to say, whereas currently only literal strings can be given 
         recset -f FEX -v "value") it would be nice to allow one to have
         recset -f FEX -v VEX
         For example one might wish to add a new string field which is based 
         another (existing) field: recset -f "field-new" -a 
         Or in the case of numeric fields: recset -f "field-new" -a 
"[field-existing] + 1"
     I like the idea, but what is not clear to me is which syntax to use for
     the expressions.
     We could generalize the usage of sed expression syntax, including the p
     (print) operator, which would be the default.  Examples:
     Simple behaviour with fixed strings:
     recset -f field-new -a foobar
     Usage of sed-like syntax for expressions:
     recset -f field-new -a "/field-existing + 1/p"
     But then 'p' is the default operation, so:
     recset -f field-new -a "/field-existing + 1/"
     recset -f field-new -a "/field-existing/s/foo/bar/"
     Something like that... ideas?
     Jose E. Marchesi         http://www.jemarch.net
     GNU Project              http://www.gnu.org

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