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Message-ID:  <8516B615-51AA-4D90-BB7D-A9E122AA0335@gmail.com>
Date:         Tue, 11 Aug 2009 00:21:23 +0930
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From: Will Robertson <wspr81@GMAIL.COM>
Subject: Re: xparse
To: LATEX-L@LISTSERV.UNI-HEIDELBERG.DE
In-Reply-To:  <4A7AD930.2090106@residenset.net>
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Status: R

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Hi Lars,

On 06/08/2009, at 10:52 PM, Lars Hellstr=F6m wrote:

> The xdoc2l3 preprocessing is more about doing things to an argument =20=

> after you've grabbed it, than about changing the conditions under =20
> which it is grabbed.

[...]

> and refer to the scratch variables \l_tmpa_tl, \l_tmpb_tl, etc. =20
> where the values are needed, we can have #1, #2, etc. be these =20
> cleaned-up values throughout the body.

[...]

> Just bear in mind that what seems esoteric today may be standard =20
> practice for the programming technique you'll learn tomorrow.

Sorry for the slow reply; your idea finally clicked in my brain a few =20=

days ago and the idea has kinda stuck.

So let's say we want to define \foo that takes an argument but has to =20=

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

 =46rom my not in-depth understanding of xdoc2l3, this can be =20
dramatically simplified (I imagine) with something like

     \DeclareDocumentCommand \foo { ?{\my_sanitise:n} } {
       % do whatever \foo is supposed to do with #1
     }

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 =20=

lot of code that's just shuffling arguments around. (In the same way =20
that manual optional argument processing consists of lots of =20
intermediate functions that simply pass their arguments around.)

Best regards,
Will


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