[NTG-pdftex] \ifprimitive and \ifinternal/\iffeature (design proposal)

Hans Hagen pragma at wxs.nl
Mon Jul 4 13:18:49 CEST 2005


Taco Hoekwater wrote:

> As promised, some proposals for the symbol table. No code yet,
> because careful thinking is needed first, to avoid unexplainable
> side-effects.

interesting -)

> Let me propose two rather straightforward new primitives first
> (please imagine the \ptex prefix yourself):
> 
>   \ifprimitive <csname>
> 
>     True if the <csname> has the same meaning as it would in initex,
>     (but gives an error if <csname> is currently undefined).

why not false instead of an error?

>   \executeprimitive <csname>
> 
>     Executes the primitive that would be known as <csname> in initex,
>     even if the <csname> is currently undefined or defined to mean
>     something else. Raises an error if there is no matching internal
>     command (of course).
> 
> These are quite straightforward, and I do not assume this needs
> much discussion. But the next ones are related to "if feature ...",
> and those are bit harder to define.
> 
> I propose to make it possible to 'predefine' primitives in the
> executable. All program code is completely ready, except that the
> hash table entry is never created, making it impossible to use
> that primitive in initex without extra work.
> 
>   \ifinternal <csname>
> 
>     True if there is code in the pdtex executable to implement the
>     internal command <csname>, regardless of whether or not it is
>     currently reachable from macros.
> 
>   \executeinternal <csname>
> 
>     Executes the internal that is registered as <csname>, even
>     if that internal is currently obscured (disable) or the <csname>
>     is defined to mean something else. Raises an error if there is
>     no matching internal command.
> 
>   \exposeinternal <csname>
> 
>     This does the final step of the primitive() function: it creates
>     a hash entry for <csname> that maps it to its primitive meaning.
>     An error is raised if the <csname> is currently defined.
>     (perhaps this primitive should accept the \global prefix?)
> 
>   \obscureinternal <csname>
> 
>     The inverse of \exposeinternal, this 'undefines' the primitive
>     related to <csname>. If <csname> is currently a macro, nothing
>     needs to be done. Control sequences that are \let to the meaning
>     of this csname are left as is.
>     (perhaps this primitive should accept the \global prefix?)
> 
> 
> I chose \expose... and \obscure... instead of \disable... \enable
> because this is closer to the truth: in this proposal it is not
> possible to unconditionally disable a primitive (because that
> would require much more work than this).

sounds ok to me; concerning the global, i think this is indeed a needed feature 
   (unless it messes up the code too much)

> Side note: The pascal implementation can be rather straightforward:
> a pair of shadow_hash and shadow_eqtb has to be created that contains
> only the equivalents of the internal control sequences, nothing else.

in the same area, how about a primitive that marks such a primitive (or maybe 
any macro but that may be harder to do) as being an 'if'

\def\ifMyTest#1#2%
   {..... \iftrue}

\exposeif\ifMyTest

\ifnum\a>\b
   \ifMyTest\c\d
      ...
   \fi
\fi

(no missing fi messages and such)

Hans

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