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Comment by jamesfinlayson

6 days ago

I remember someone in university talking about the if function (which ostensibly takes one boolean argument).

31 comments

jamesfinlayson

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Delphiza  5 days ago

In Excel formulas everything is a function. IF, AND, OR, NOT are all functions. It is awkward and goes against what software devs are familiar with, but there are probably more people familiar with the Excel IF function than any other forms. Here is an example taken from the docs... =IF(AND(A3>B2,A3<C2),TRUE,FALSE)

weinzierl  6 days ago

Sounds like something Prof. John Ousterhout would say:-; The place where this was literally accurate would be Tcl.

I don't know enough Smalltalk to be sure but I think to remember it has a similar approach of everything is an object and I wouldn't be surprised if they'd coerced control flow somehow into this framework.

Also Forth comes to mind, but that would probably be a stretch.

Zambyte  5 days ago

If takes two or three arguments, but never one. The condition is the one made syntactically obvious in most languages, the consequent is another required argument, and the alternative is optional.

  • dylan604  5 days ago

    Huh? if (true) {} takes precisely one argument.

    • mjburgess  5 days ago

      That's an application of `if` with one of the arguments empty.

      The semantics of `if` requrie at least, `if(cond, clause)`, though more generally, `if(cond, clause, else-clause)`

      6 replies →

    • andriamanitra  5 days ago

      Depends on the language! If "if" wasn't a keyword, in Ruby that would be calling a method that takes one positional argument and one block argument, such as `def if(cond, &body) = cond && body.call`. In PureScript that could be a call to a function with signature `if :: Boolean -> Record () -> _`.

      But I assume the comment you were replying to was not referring to the conditional syntax from C-like languages, instead referring to a concept of an if "function", like the `ifelse` function in Julia [1] or the `if` form in Lisps (which shares the syntax of a function/macro call but is actually a special form) [2], neither of which would make sense as one argument function.

      [1] https://docs.julialang.org/en/v1/base/base/#Base.ifelse

      [2] https://www.gnu.org/software/emacs/manual/html_node/elisp/Co...

    • Zambyte  3 days ago

      I count two: true and void. This becomes obvious in languages that have consistent application syntax like Lisp, which would write this as

          (if true '())

      Or if you wanted to capture the exact same semantics (rather than returning a null value to the continuation of the if)

          (if true (values))

      Now it's obvious that if takes two (or three) arguments :)

antonvs  6 days ago

Try implementing that in most languages and you'll run into problems.

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.

  • immibis  5 days ago

    I don't think Haskell needs 'if' to be a construct for compiler optimization reasons; it could be implemented easily enough with pattern matching:

    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.

    • antonvs  5 days ago

      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.)

  • igouy  5 days ago

    Isn't practical in Smalltalk either, so the compiler does something special:

        ifFalse: alternativeBlock
        "Answer the value of alternativeBlock. Execution does not actually
         reach here because the expression is compiled in-line."

        ^alternativeBlock value

    • antonvs  5 days ago

      Oh thanks, I didn't know that. I thought it just relied on the explicit code blocks.

      But yeah, this is a pretty critical point for optimizations - any realistic language is likely to optimize this sooner or later.

ahartmetz  5 days ago

I frequently see people treating if as if it was "taking a comparison", so: if (variable == true) ...

lmm  5 days ago

if should be a function, though sadly many languages aren't good enough to express it and have to make it a builtin.