MetaFun - Pragma ADE

vardef spring (expr a, b, w, h, n) =
pair vec ; path pat ; numeric len ; numeric ang ;
vec := (b-a) ;
pat := for i=1 upto n-1: (if odd(i):-fi w/2,i)--endfor (0,n) ;
pat := (0,0)--(0,h)-- pat shifted (0,h)--(0,n+h)--(0,n+2h) ;
len := (xpart vec ++ ypart vec)/(n+2h) ;
ang := -90+angle(vec) ;
( pat yscaled len rotatedaround(origin,ang) shifted a )
enddef ;
If you use vardef, then the last statement is the return value. Here, when p := spring (z1,
z2, .75cm, 2, 10) is being parsed, the macro is expanded, the variables are kept invisible for
the assignment, and the path at the end is considered to be the return value. In a def the whole
body of the macro is ‘pasted' in the text, while in a vardef only the last line is visible. We will
demonstrate this with a simple example.
def one = (n,n) ; n := n+1 ; enddef ;
def two = n := n + 1 ; (n,n) enddef ;
Now, when we say:
pair a, b ; numeric n ; n= 10 ; a := one ; b := two ;
we definitely get an error message. This is because, when macro two is expanded, METAPOST sees
something:
b := n := n + 1 ;
By changing the second definition in
vardef two = n := n + 1 ; (n,n) enddef ;
the increment is expanded out of sight for b := and the pair (n,n) is returned.
We can draw a slightly better looking spring by drawing twice with a different pen. The following
commands use the spring macro implemented by the vardef.
path p ; p :=
(0,0)--spring((.5cm,0),(2.5cm,0),.5cm,0,10)--(3cm,0) ;
draw p withpen pencircle scaled 2pt ;
draw p withpen pencircle scaled 1pt withcolor .8white;
This time we get:
Since the spring macro returns a path, you can do whatever is possible with a path, like drawing
an arrow:
Simple drawings A few applications
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