Comment by antonvs

With a function version of `if`, in general the compiler needs to wrap the alternative in closures ("thunks"), as it does with all function arguments unless optimizations make it unnecessary. That's never needed in the syntactic version. That's one significant optimization.

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.)