LMTX MkIV difference in expansion

LMTX MkIV difference in expansion

[Rik Kabel]

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Another LMTX/MkIV difference, this time with expansion:

    \define\Align{yes}
    \starttext
    \startalignment[\Align]
         This works with MkIV but fails with LMTX, complaining: {\tt
         tex error on line 3 in file G:/expand.mkvi: The file ended
         when scanning an argument.}
    \blank
        It works in both when \tex{def} or \tex{defineexpandable} is
        used instead of \tex{define}.
    \blank
        What changed?
    \stopalignment
    \stoptext

It may well be that I have been abusing some laxity in MkIV and that 
LMTX is a bit stricter in what it accepts, but I would like to know if 
this is an expected difference.

-- 
Rik


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Re: LMTX MkIV difference in expansion

[Wolfgang Schuster]

Rik Kabel schrieb am 19.11.2020 um 21:20:
> Another LMTX/MkIV difference, this time with expansion:
> 
>     \define\Align{yes}
>     \starttext
>     \startalignment[\Align]
>          This works with MkIV but fails with LMTX, complaining: {\tt
>          tex error on line 3 in file G:/expand.mkvi: The file ended
>          when scanning an argument.}
>     \blank
>         It works in both when \tex{def} or \tex{defineexpandable} is
>         used instead of \tex{define}.
>     \blank
>         What changed?
>     \stopalignment
>     \stoptext
> 
> It may well be that I have been abusing some laxity in MkIV and that 
> LMTX is a bit stricter in what it accepts, but I would like to know if 
> this is an expected difference.

You have to wait for Hans to get an answer but here is a minimal example.

\starttext

\protected\def\testparameter{test}
%\def\testparameter{test}

\def\test[#1]%
   {\expandafter\let\expandafter\testargumentlist\csname#1\endcsname}

\test[\testparameter]

\stoptext

Wolfgang
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Re: LMTX MkIV difference in expansion

[Hans Hagen]

On 11/19/2020 9:41 PM, Wolfgang Schuster wrote:
> Rik Kabel schrieb am 19.11.2020 um 21:20:
>> Another LMTX/MkIV difference, this time with expansion:
>>
>>     \define\Align{yes}
>>     \starttext
>>     \startalignment[\Align]
>>          This works with MkIV but fails with LMTX, complaining: {\tt
>>          tex error on line 3 in file G:/expand.mkvi: The file ended
>>          when scanning an argument.}
>>     \blank
>>         It works in both when \tex{def} or \tex{defineexpandable} is
>>         used instead of \tex{define}.
>>     \blank
>>         What changed?
>>     \stopalignment
>>     \stoptext
>>
>> It may well be that I have been abusing some laxity in MkIV and that 
>> LMTX is a bit stricter in what it accepts, but I would like to know if 
>> this is an expected difference.
> 
> You have to wait for Hans to get an answer but here is a minimal example.
> 
> \starttext
> 
> \protected\def\testparameter{test}
> %\def\testparameter{test}
> 
> \def\test[#1]%
>    {\expandafter\let\expandafter\testargumentlist\csname#1\endcsname}
> 
> \test[\testparameter]
> 
> \stoptext
Often arguments to commands like \startsomething[xx] let the xx end up 
in some \(if)csname expansion. A protected (\unexpanded in context 
speak) macro doesn't expand inside for instance an \edef (or comparable 
expandable situation). Now, from that it makes perfect sense to also not 
let it expand inside a \csname or \ifcsname. One reason is that when it 
does expand, you can get a pretty wild (nested) sequence of nested 
expansions and one can be pretty sure that we then don't have a proper 
csname. This is why in luatex we have a catch for running wild csname 
checking.

The original \ifcsname test was inherited from etex. The \protected 
feature also comes from etex. But \csname is a tex natural.
In pdftex (and luatex) a protected macro inside an \(if)csname does 
expand which to makes no sense and smells like a bug. Or maybe it was 
tricky to catch (the implementation of protected a bit of a hack).

In luametatex protected macros are native and in the process I also 
decided to *not* expand them in a \(if)csname where I expect (as said) 
protected macros to behave like in an edef. I nice side effect is that 
running wild no longer happens (but we still catch it) which can save 
quite some useless backup token list construction (needed because tex 
has to push back stuff in order to be able to report an error).

So, when you still don't understand it (which I can understand) I'm sure 
Wolfgang can explain it better now.

\starttext

           \def\foo{foo}
\protected\def\oof{oof}

\csname foo\endcsname
\csname oof\endcsname
\csname \foo\endcsname

% error in luametatex, ok in pdftex/luatex:

% \csname \oof\endcsname

\ifcsname  foo\endcsname yes\else nop\fi
\ifcsname  oof\endcsname yes\else nop\fi
\ifcsname \foo\endcsname yes\else nop\fi

% nop in luametatex (error intercepted), yes in pdftex/luatex

\ifcsname \oof\endcsname yes\else nop\fi

\stoptext

Now, one can argue that if I consider it a but in the other engines, why 
I don't argue that it should be solved. Well, there is too much legacy 
code already that might use it as feature so it will not change. But in 
luametatex we can 'fix' these things. (We also use the csname in a 
rather predictable way in context so i don't expect issues in the core.)

Hans

-----------------------------------------------------------------
                                           Hans Hagen | PRAGMA ADE
               Ridderstraat 27 | 8061 GH Hasselt | The Netherlands
        tel: 038 477 53 69 | www.pragma-ade.nl | www.pragma-pod.nl
-----------------------------------------------------------------
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Re: LMTX MkIV difference in expansion

[Rik Kabel]

On 11/19/2020 17:03, Hans Hagen wrote:
> On 11/19/2020 9:41 PM, Wolfgang Schuster wrote:
>> Rik Kabel schrieb am 19.11.2020 um 21:20:
>>> Another LMTX/MkIV difference, this time with expansion:
>>>
>>>     \define\Align{yes}
>>>     \starttext
>>>     \startalignment[\Align]
>>>          This works with MkIV but fails with LMTX, complaining: {\tt
>>>          tex error on line 3 in file G:/expand.mkvi: The file ended
>>>          when scanning an argument.}
>>>     \blank
>>>         It works in both when \tex{def} or \tex{defineexpandable} is
>>>         used instead of \tex{define}.
>>>     \blank
>>>         What changed?
>>>     \stopalignment
>>>     \stoptext
>>>
>>> It may well be that I have been abusing some laxity in MkIV and that 
>>> LMTX is a bit stricter in what it accepts, but I would like to know 
>>> if this is an expected difference.
>>
>> You have to wait for Hans to get an answer but here is a minimal 
>> example.
>>
>> \starttext
>>
>> \protected\def\testparameter{test}
>> %\def\testparameter{test}
>>
>> \def\test[#1]%
>> {\expandafter\let\expandafter\testargumentlist\csname#1\endcsname}
>>
>> \test[\testparameter]
>>
>> \stoptext
> Often arguments to commands like \startsomething[xx] let the xx end up 
> in some \(if)csname expansion. A protected (\unexpanded in context 
> speak) macro doesn't expand inside for instance an \edef (or 
> comparable expandable situation). Now, from that it makes perfect 
> sense to also not let it expand inside a \csname or \ifcsname. One 
> reason is that when it does expand, you can get a pretty wild (nested) 
> sequence of nested expansions and one can be pretty sure that we then 
> don't have a proper csname. This is why in luatex we have a catch for 
> running wild csname checking.
>
> The original \ifcsname test was inherited from etex. The \protected 
> feature also comes from etex. But \csname is a tex natural.
> In pdftex (and luatex) a protected macro inside an \(if)csname does 
> expand which to makes no sense and smells like a bug. Or maybe it was 
> tricky to catch (the implementation of protected a bit of a hack).
>
> In luametatex protected macros are native and in the process I also 
> decided to *not* expand them in a \(if)csname where I expect (as said) 
> protected macros to behave like in an edef. I nice side effect is that 
> running wild no longer happens (but we still catch it) which can save 
> quite some useless backup token list construction (needed because tex 
> has to push back stuff in order to be able to report an error).
>
> So, when you still don't understand it (which I can understand) I'm 
> sure Wolfgang can explain it better now.
>
> \starttext
>
>           \def\foo{foo}
> \protected\def\oof{oof}
>
> \csname foo\endcsname
> \csname oof\endcsname
> \csname \foo\endcsname
>
> % error in luametatex, ok in pdftex/luatex:
>
> % \csname \oof\endcsname
>
> \ifcsname  foo\endcsname yes\else nop\fi
> \ifcsname  oof\endcsname yes\else nop\fi
> \ifcsname \foo\endcsname yes\else nop\fi
>
> % nop in luametatex (error intercepted), yes in pdftex/luatex
>
> \ifcsname \oof\endcsname yes\else nop\fi
>
> \stoptext
>
> Now, one can argue that if I consider it a but in the other engines, 
> why I don't argue that it should be solved. Well, there is too much 
> legacy code already that might use it as feature so it will not 
> change. But in luametatex we can 'fix' these things. (We also use the 
> csname in a rather predictable way in context so i don't expect issues 
> in the core.)
>
> Hans
>
You are right about not quite understand.

Does this mean that I can have the same definitions in MkIV and LMTX 
(after some future update), or should I hunt down the \defines in both, 
or that I should fork (or mode test) my source environment files, one 
set for LMTX and one for MkIV?

-- 
Rik

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Re: LMTX MkIV difference in expansion

[Hans Hagen]

On 11/20/2020 12:18 AM, Rik Kabel wrote:
> You are right about not quite understand.
> 
> Does this mean that I can have the same definitions in MkIV and LMTX 
> (after some future update), or should I hunt down the \defines in both, 
> or that I should fork (or mode test) my source environment files, one 
> set for LMTX and one for MkIV?
Kind of

\protected\def\MyCommand#1{\tricky{?}\stuff{!}}

\def\MyKeyword{tricky stuff}

because you want to always expand MyKeyword and in controlled cases 
\MyCommand.

Hans


-----------------------------------------------------------------
                                           Hans Hagen | PRAGMA ADE
               Ridderstraat 27 | 8061 GH Hasselt | The Netherlands
        tel: 038 477 53 69 | www.pragma-ade.nl | www.pragma-pod.nl
-----------------------------------------------------------------
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Re: LMTX MkIV difference in expansion

[Wolfgang Schuster]

Rik Kabel schrieb am 20.11.2020 um 00:18:

> You are right about not quite understand.

There are cases where you want to pass a command to another command as 
it is without replacing it with its content, e.g. when you store the 
\TeX logo in the table of content the \TeX command should be written in 
the register and not the content of the command.

In the following example the first line prints the definition of the 
\TeX logo but in many cases you ant to preserve the command as in the 
second line.

\starttext

\tex{TeX} = \detokenize\expandafter{\TeX}

\blank

\tex{TeX} = \detokenize{\TeX}

\stoptext


To make it easier to keep the command eTeX added a new command 
\protected which can be used before \def to achieve this (ConTeXt 
provides the same thing under the name \unexpanded).

The following example shows how you can use \protected\def to keep 
always the current meaning of \foo when you print the content of \bar.

\starttext

\def\foo{foo}

\edef\bar{\foo}

\def\foo{bar}

\startlines
bar=\bar
foo=\foo
\stoplines

\blank

\protected\def\foo{foo}

\edef\bar{\foo}

\protected\def\foo{bar}

\startlines
bar=\bar
foo=\foo
\stoplines

\stoptext


A problem in older TeX engines is that \csname ...\endcsname didn't 
respect this protection and replaced the protected command with its 
content, recently Hans changed this behavior in LMTX which lead to the 
error message in your document.


> Does this mean that I can have the same definitions in MkIV and LMTX 
> (after some future update), or should I hunt down the \defines in both, 
> or that I should fork (or mode test) my source environment files, one 
> set for LMTX and one for MkIV?


When you use \define to store arguments which are passed as arguments to 
other command you have to change this to \defineexpandable but its best 
to do this in MkIV and LMTX because protected commands are the wrong 
thing in this case. Even though it would work in MkIV in some cases you 
run into problems when you pass argument to Lua.

Wolfgang
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Re: LMTX MkIV difference in expansion

[Rik Kabel]

On 11/21/2020 10:05, Wolfgang Schuster wrote:
> Rik Kabel schrieb am 20.11.2020 um 00:18:
>
>> You are right about not quite understand.
>
> There are cases where you want to pass a command to another command as 
> it is without replacing it with its content, e.g. when you store the 
> \TeX logo in the table of content the \TeX command should be written 
> in the register and not the content of the command.
>
> In the following example the first line prints the definition of the 
> \TeX logo but in many cases you ant to preserve the command as in the 
> second line.
>
> \starttext
>
> \tex{TeX} = \detokenize\expandafter{\TeX}
>
> \blank
>
> \tex{TeX} = \detokenize{\TeX}
>
> \stoptext
>
>
> To make it easier to keep the command eTeX added a new command 
> \protected which can be used before \def to achieve this (ConTeXt 
> provides the same thing under the name \unexpanded).
>
> The following example shows how you can use \protected\def to keep 
> always the current meaning of \foo when you print the content of \bar.
>
> \starttext
>
> \def\foo{foo}
>
> \edef\bar{\foo}
>
> \def\foo{bar}
>
> \startlines
> bar=\bar
> foo=\foo
> \stoplines
>
> \blank
>
> \protected\def\foo{foo}
>
> \edef\bar{\foo}
>
> \protected\def\foo{bar}
>
> \startlines
> bar=\bar
> foo=\foo
> \stoplines
>
> \stoptext
>
>
> A problem in older TeX engines is that \csname ...\endcsname didn't 
> respect this protection and replaced the protected command with its 
> content, recently Hans changed this behavior in LMTX which lead to the 
> error message in your document.
>
>
>> Does this mean that I can have the same definitions in MkIV and LMTX 
>> (after some future update), or should I hunt down the \defines in 
>> both, or that I should fork (or mode test) my source environment 
>> files, one set for LMTX and one for MkIV?
>
>
> When you use \define to store arguments which are passed as arguments 
> to other command you have to change this to \defineexpandable but its 
> best to do this in MkIV and LMTX because protected commands are the 
> wrong thing in this case. Even though it would work in MkIV in some 
> cases you run into problems when you pass argument to Lua.
>
> Wolfgang

Thank you, Wolfgang, for the explanation and examples.

I have in fact already gone through and replaced the impacted 
occurrences of \define with \defineexpandable. LMTX made it easy to 
identify them.

-- 
Rik

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