Strange Metapost parenthesis bug

Question

When I compile the following Metapost file:

beginfig(1);
def f(expr n) =
  if n=0: 0
  else: 1
  fi;
enddef;
show f(0)+1;
endfig;
end

I expect to get the output 1 (since f(0) is equal to 0, f(0)+1 should be 1!). However, Metapost complains about an Isolated expression.

When I put the expression in parentheses: show (f(0)+1), things get even stranger. The error message becomes : Missing ')' has been inserted. (The first quote should be a backquote, but I couldn't figure out how to escape it). Where on earth was there a mismatched parenthesis??

Thanks for your help!

Answer 1

The def command just expands a name into its definition, so you get literally:

show if 0=0: 0 else: 1;+1;

The semicolon in the middle is what's wrong, so let us remove it:

beginfig(1);
def f(expr n) =
  if n=0: 0
  else: 1
  fi
enddef;
show f(0)+1;
endfig;
end

This produces the correct expansion:

show if 0=0: 0 else: 1+1;

And outputs 1 as expected.

---

On a side note, I'd recommend using begingroup...endgroup for heavier macro definitions, and at least parentheses for lighter ones: for example,

def f = 1 + 2 enddef;
show f * 2;

gives 1 + 2 * 2, which is not the same as what is probably expected:

def f = (1 + 2) enddef;
show f * 2;

which gives (1 + 2) * 2.