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Date:         Tue, 11 Aug 2009 15:03:45 +0200
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From: =?ISO-8859-1?Q?Lars_Hellstr=F6m?= <Lars.Hellstrom@RESIDENSET.NET>
Subject: Re: xparse
To: LATEX-L@LISTSERV.UNI-HEIDELBERG.DE
In-Reply-To:  <8516B615-51AA-4D90-BB7D-A9E122AA0335@gmail.com>
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Status: R

Will Robertson skrev:
> Hi Lars,
> 
> On 06/08/2009, at 10:52 PM, Lars Hellström wrote:
> 
>> The xdoc2l3 preprocessing is more about doing things to an argument 
>> after you've grabbed it, than about changing the conditions under 
>> which it is grabbed.
> 
> [...]
> 
>> and refer to the scratch variables \l_tmpa_tl, \l_tmpb_tl, etc. where 
>> the values are needed, we can have #1, #2, etc. be these cleaned-up 
>> values throughout the body.
> 
> [...]
> 
>> Just bear in mind that what seems esoteric today may be standard 
>> practice for the programming technique you'll learn tomorrow.
> 
> Sorry for the slow reply; your idea finally clicked in my brain a few 
> days ago and the idea has kinda stuck.

Well, I've been slow on reading as well, mostly for weather-related 
reasons.

> So let's say we want to define \foo that takes an argument but has to 
> sanitise it in some way before processing it:
> 
>     \DeclareDocumentCommand \foo {m} {
>       \my_sanitise:Nn \l_sanitised_tl {#1}
>       \foo_internal:V \l_sanitised_tl
>     }
>     \cs_new:Nn \my_sanitise:Nn {
>       % do something with #1 and save it in \l_sanitised_tl

The #1 of \foo, yes.

>     }
>     \cs_new:Nn \foo_internal:n {
>       % do whatever \foo is supposed to do with #1

Which may make use of #1 in several places, some of which may be smack 
in the middle of a complex call of some other user-level command, whose 
implementation we know very little about...

>     }
>     \cs_generate_variant:Nn \foo_internal:n {V}

I thought O was the appropriate variant for expanding a token list 
control sequence?

>  From my not in-depth understanding of xdoc2l3, this can be dramatically 
> simplified (I imagine) with something like
> 
>     \DeclareDocumentCommand \foo { ?{\my_sanitise:n} } {
>       % do whatever \foo is supposed to do with #1
>     }

Yes. The way I coded it, you would have to declare something as a 
"processor", and then be free to combine said processor with other 
processors in the specification of an argument; the reason for this was 
partly that processors would leave their result "up ahead" as a 
mandatory argument that the next processor could operate on.

For example, the processing of an (x,y) argument (xparse "c") would 
probably be in two steps:

     (x,y)
grab and handle delimiters
     {x,y}
split at comma
     {{x}{y}}

The most general-purpose processor I defined was "x", but unfortunately 
the in-source description of that seems to be wrong (copied from "t" 
and only partially rewritten). I believe I gave a more user-oriented 
description in a posting to LATEX-L, however.

> Just to confirm -- have I got this thing sort of right? :)
> 
> With this sort of example, it seems pretty clear that this can save a 
> lot of code that's just shuffling arguments around. (In the same way 
> that manual optional argument processing consists of lots of 
> intermediate functions that simply pass their arguments around.)

Exactly!

I might add, that with some processing at argument-grabbing time, it 
also becomes interesting to do some argument checking and error 
reporting there: it is easier to understand _where_ a mandatory 
coordinate argument was expected if you get an error message where the 
token stream is cut off at precisely the position where the coordinate 
was expected.

Lars Hellström

PS: Just to be clear, I don't particularly like (x,y) arguments, but in 
this posting they made a good example.