MetaFun - Pragma ADE

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often, you may expect draw and fill to be primitives, but they are not. They are macros defined
in terms of primitives.
Given a path pat, you can consider a draw to be defined in terms of:
addto currentpicture doublepath pat
The fill command on the other hand is defined as:
addto currentpicture contour pat
Both macros are actually a bit more complicated but this is mainly due to the fact that they also
have to deal with attributes like the pen and color they draw with.
You can use doublepath and contour directly, but we will use draw and fill whenever possible.
Given a picture pic, the following code is valid:
addto currentpicture also pic
You can add pictures to existing picture variables, where currentpicture is the picture that is
flushed to the output file. Watch the subtle difference between adding a doublepath, contour or
picture.
Here is another nice example of what happens when you fill a path that is partially reversed.
fill fullsquare rotated 45 scaled 2cm
withcolor .625 red ;
fill fullcircle scaled 2cm -- reverse fullcircle scaled 1cm -- cycle
withcolor .625 yellow;
The inner circle is indeed not filled:
1.6 Variables
At this point you may have noted that METAPOST is a programming language. Contrary to some
of today's languages, METAPOST is a simple and clean language. Actually, it is a macro language.
Although METAPOST and TEX are a couple, the languages differ in many aspects. If you are using
both, you will sometimes wish that features present in one would be available in the other. When
using both languages, in the end you will understand why the conceptual differences make sense.
Being written in PASCAL, it will be no surprise that METAPOST has some PASCAL--like features, although
some may also recognize features from ALGOL68 in it.
Welcome to MetaPost Variables