Somebody asked this question, I just thought I should post the answer on NL

Why don't programmers use IFF statements?
The easy answer is we do

We just spell it “IF.”

The more complex answer:

IFF is a mathematical term used in theorem proving. It’s not a “flow control” construct.

So if a mathematician writes:

“x = y iff a = b”, they mean…

if a = b, then x = y, AND,
if a != b, then x != y
If a programmer writes:

“IF (a == b) { x = y}” then they mean something very different.

They mean:

If a = b, then set the value of x to the value of y
If a != b, then do nothing, leave the value of x unchanged
In terms of IF vs IFF, that’s basically the same logic as the IFF version… which is why I wrote the “easy answer” they way I did.

The most complex answer:

There are a tiny number of languages in which you can use IFF.

They are languages like OBJ3, which is used for theorem proving.

Unless you are a mathematician you are unlikely ever to come across them.

Eh? It's not just part of the syntax for god's sake.