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where is upvar

Cary O'Brien cobrien at Radix.Net
Fri Sep 22 09:08:56 EDT 2000 
In article <39C8FEAC.1668A1C4 at sage.att.com>,
Garry Hodgson  <garry at sage.att.com> wrote:
>Harald Kirsch wrote:
>> 
>> Jon Ribbens <jon+python-list at unequivocal.co.uk> writes:
>> > Accessing variables in other peoples' scopes directly is nasty anyway,
>> > use function arguments ;-).
>> 
>> Not if it is documented and if the sole purpose of a certain function
>> is to do that in a defined way. Those functions are sometimes called
>> `control structures'. It seems like Tcl is a bit ahead of Python here,
>> as it allows to create functions which look, smell and work like
>> custom made control structures.
>
>it's arguable whether this is a good thing or not.  i've used languages
>which allowed this, including tcl.  it's cute, but of limited utility.
>and the readability cost can be high.
>

One reason to have upvar in TCL is to implement call-by-reference,
rather than TCL call-by-value.  Passing a variable name is
common in the builtin functions (i.e. append) and upvar lets
you implement routines that expect a variable name.  I use this
a lot when there are lots of parameters to a function.  Rather
than have 20 parameters, I create an array with the information
and pass the name.  An upvar in the called function makes this
array available during function execution.   I don't think
there is a readability cost, frankly.

Since python is, i guess, call-by-reference, you don't need this
at all.

One TCL example of a good use of "upvar" as a control structure is
something that we call sql_map, and a couple of other people have
implemented independantly.  Basically it is a for loop over all the
rows in a select from a database with variable assignments.  The body
runs in the context of of the caller.  Oops -- This uses uplevel, not
upvar.  Ok, but it is the same idea.

sql_map $sql(h) {name phone age} {
	if {$age > $max_age} {
	   whataver...
	}
}

uplevel lets you create new control structures.

You could iterate over a result set in python, but at the
cost of a throw-away function.



-- cary
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