Derp.

https://gist.github.com/1336856

For the +, – and other operators  in F# there are two things:
You can simply use it like in a + b, in this case, it is infixed, and you have to provide both operands.
When using it between parentheses, it becomes a simple function, hence prefixed and curryable.

You can define almost any operator like ??, |?, , +-, ~> by defining it between () like I did for ??.

Amazing how I can now read F# and know what its doing, functional FTW! SICP is opening my eyes a lot!

BTW how did you post code in disqus like that?

let (++) a b = (if a > 0 then (+) else (-)) a b

You can then use the infix operator ++ :
3 ++ -5

or : if you want an operator for if then else :
let (!?) x f e = if x then f else e
let (++) a b = (!? (b > 0) (+) (-)) a b

Have F#un !

I know in lisp that you have to be careful because the (if  predicate then else) acts as a special construct that behaves slightly differently. 

I like this method I want to see what will happen if you turned that into some kind of recursive function.  Something really contrived like adding the numbers by incrementing a while decrementing b

Kind of interesting actually