> Is this a good idea?

Don't think so: it doesn't help the case where you want to use negation on
a variable rather than a constant, so it introduces a fairly
subtle inconsistency which doesn't seem to be worth the trouble.

I guess in Haskell you could use type class trickery so that

  "f - 3" substracts 3 from f if f is numeric and passes -3 to f if f is
  a function (I leave the case where f is both a function and a member of
  the Number class as an exercise to the user).

that would be even more evil,


        Stefan