MetaFun - Pragma ADE

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the language. Occasionally you run into situations where you wished that you could reach over a
group (for instance in order to pass a value).
In METAPOST grouping behaves quite different. First of all, it provides the mechanism that hides
processing from the current flow. The previously mentioned vardef is implicitly grouped. Contrary
to TEX, in METAPOST all assignments are global by default, even in a group. If you assign a
variable inside a group it is persistent unless you first save the variable (or macro) using the save
operator.
So, in the next code snippet, the value of \value inside the box is no but after the box is typeset, it
will be yes again.
\def\value{yes} \hbox{\def\value{no}\value} \value
To make a value local in METAPOST, the following code is needed.
string value ; value := "yes" ;
def intermezzo
begingroup ;
save value ; string value ; value := "no" ;
endgroup ;
enddef ;
Once you start writing your own METAPOST macros, you will appreciate this ‘always global' behaviour.
As with other differences between the two languages, they make sense if you look at what
the programs are supposed to do.
2.13 Internals and Interims
Related to grouping is the internal numeric datatype. When numeric variables are defined as
interim, you can quickly overload them inside a group.
newinternal mynumber ; mynumber := 1 ;
begingroup ; ... interim mynumber := 0 ; ... ; endgroup ;
You can only interim a variable if it is already defined using newinternal.
Among the METAPOST macros is one called drawdot. This macro is kind of redundant because, at
least at first sight, you can use draw to achieve the same result. There is however a very subtle
difference: a dot is slightly larger than a drawn point. We guess that it's about the device pixel, so
you may not even notice it. It may even be due to differences in accuracy of the methods to render
them.
pickup pencircle scaled 50pt ;
drawdot origin shifted (-120pt,0) ; draw origin shifted (-60pt,0) ;
drawdot origin ; draw origin withcolor white ;
setbounds currentpicture to boundingbox currentpicture enlarged 1pt ;
A few more details Internals and Interims