Incidentally, I find strange misuses of "call" ("calling a command", "calling a button") one of the more grating phrases used by ESL CS students.
My favourite (least favourite?) is using “call” with “return”. On more than one occasion I’ve heard:
“When we call the return keyword, the function ends.”
In an imperative programming language with eager evaluation, i.e. where arguments are evaluated before applying the function, implementing `if` as a function will evaluate both the "then" and "else" alternatives, which will have undesirable behavior if the alternatives can have side effects.
In a pure but still eager functional language this can work better, if it's not possible for the alternatives to have side effects. But it's still inefficient, because you're evaluating expressions whose result will be discarded, which is just wasted computation.
In a lazy functional language, you can have a viable `if` function, because it will only evaluate the argument that's needed. But even in the lazy functional language Haskell, `if` is implemented as built-in syntax, for usability reasons - if the compiler understands what `if` means as opposed to treating it as an ordinary function, it can optimize better, produce better messages, etc.
In a language with the right kind of macros, you can define `if` as a macro. Typically in that case, its arguments might be wrapped in lambdas, by the macro, to allow them to be evaluated only as needed. But Scheme and Lisp, which have the right kind of macros, don't define `if` as a macro for similar reasons to Haskell.
One language in which `if` is a function is the pure lambda calculus, but no-one writes real code in that.
The only "major" language I can think of in which `if` is actually a function (well, a couple of methods) is Smalltalk, and in that case it works because the arguments to it are code blocks, i.e. essentially lambdas.
tl;dr: `if` as a function isn't practical in most languages.
if' :: Bool -> a -> a -> a
if' True x _ = x
if' False _ y = y
The compiler could substitute this if it knew the first argument was a constant.
Maybe it was needed in early versions. Or maybe they just didn't know they wouldn't need it yet. The early versions of Haskell had pretty terrible I/O, too.
In GHC, `if` desugars to a case statement, and many optimizations flow from that. It's pretty central to the compiler's operation.
> Maybe it was needed in early versions. Or maybe they just didn't know they wouldn't need it yet.
Neither of these are true. My comment above was attempting to explain why `if` isn't implemented as a function. Haskell is a prime example of where it could have been done that way, the authors are fully aware of that, but they didn't because the arguments against doing it are strong. (Unless you're implementing a scripting-language type system where you don't care about optimization.)