MetaFun - Pragma ADE

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There are more cases where the result may surprise you. Take the following code:
drawarrow ((0,0)--(10,0))
withpen pencircle scaled 2pt
withcolor red randomized (.4,.9) ;
currentpicture := currentpicture scaled 8 ;
The arrow is made up out of two pieces and each piece gets a different shade of red. This is because
the attributes are collected and applied to each of the components that make up the arrow. Because
for each component the attribute code is expanded again, we get two random colors. One way
around this is to apply the color afterwards.
draw
image (drawarrow ((0,0)--(10,0)) withpen pencircle scaled 2pt)
scaled 8 withcolor red randomized (.4,.9) ;
Here the image macro creates a picture and as you can see, this provides a way to draw within a
draw operation.
Once you see the benefits of image, you will use it frequently. Another handy (at first sight strange)
macro is hide. You can use this in situations where you don't want code to interfere.
def mydraw text t =
boolean error ; error := false ;
def withpencil expr p = hide (error := true) enddef ;
draw t ;
if error : message "pencils are not supported here" fi ;
enddef ;
mydraw fullcircle scaled 10cm withpencil sharp ;
Here, setting the boolean normally interferes with the draw operation, but by hiding the assignment,
this code becomes valid. This code will bring the message to your terminal and log file.
Once you start using expressions you have a good chance of encountering messages with regards
to redundant expressions. The following code is for instance a recipe for problems:
z1 = (1,0) ; z1 = (2,0) ;
Changing the = into := helps, but this may not be what you want.
A few more details Pitfalls