[luatex] Change in behavior

[Hans Hagen]

On 8/21/2016 8:12 PM, Br. Samuel Springuel wrote:
> As part of the GregorioTeX package, we have a Lua function which
> rescales a particular distance register by multiplying it by an
> arbitrary factor (the factor can be non-integer, so we cannot do this
> math in TeX itself).  Originally we wrote the function as this:
>
> local function scale_space(factor)
>   local skip = tex.getskip('gre at skip@temp at four')
>   skip.width = skip.width * factor
>   -- should skip.stretch and skip.shink also be scaled?
> end
>
> However, we found that after updating our testing process to use TL2016
> (and thus LuaTeX 0.95 instead of 0.80) the rescaling was no longer
> happening.  After some experimentation, we found that the following worked:
>
> local function scale_space(factor)
>   local skip = tex.getskip('gre at skip@temp at four')
>   skip.width = skip.width * factor
>   tex.setskip('gre at skip@temp at four',skip)
>   -- should skip.stretch and skip.shink also be scaled?
> end
>
> This implies to me that the behavior of the function interface for token
> registers changed at some point between these two versions.  In 0.80 a
> variable filled with a token register was linked to that register and
> changes to the variable were reflected in the token register itself.  In
> 0.95 the variable is not linked to the register and thus changes to the
> variable are not communicated to the register unless and until that is
> done so explicitly.  Is my interpretation of the situation correct?  If
> so, when did this change occur and what is the best way for me to stay
> abreast of these sort of changes.

skips (glue) is kind of special in the sense that it's something 
indirect (glue specs)

in the current luatex the glue nodes no longer use these specs (so no 
indirectness) but the registers are still 'specs' so, what you get with
tex.getskip is a gluespec (node) that you can then mess with 
(independently) and you set a skip by passing a gluespec node

(the old model also shared specs with ref counts and such which could be 
quite tricky and have side effects when messing with these specs)

you can use tex.setglue to set the width (or more values)

Hans


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