_if

Choose what to produce, as directed -- Function

Anything -> _if (predicate, iftrue, iffalse, asList) -> The same Type

Receives: Anything -- this will be passed into the predicate for testing
predicate: The question we're asking -- is this value true?
iftrue: What to produce if the predicate is true
iffalse (optional): What to produce if the predicate isn't true
asList (optional): Whether to treat the context as a whole, or as individual elements
Produces: The result from iftrue if the predicate was true, iffalse (or nothing) otherwise

_if is one of the basic building blocks of programming. It applies the received value to the predicate. If the result is non-empty and is true, it applies the received value to iftrue. Otherwise, if there is an iffalse parameter, it applies the received value to that. (If there is no iffalse, it just produces the empty value.)

For example, take this expression:

[[My List -> _if(_isEmpty, ""Empty"", ""Full"")]]

If My List is empty, this will print "Empty"; if not, it will print "Full".

This is also an easy way to test TrueOrFalse flags. Say that our Space represents a library, and we want to print out whether a given book has the Checked Out flag set on it. We would say:

[[My Book -> _if(Checked Out, ""Checked Out"")]]

So if My Book -> Checked Out is true, it will print "Checked Out". Since we didn't give an iffalse parameter, it doesn't print anything if the Checked Out flag isn't set.

Note that, if predicate is empty, or the wrong type, that is treated as "false". If you're getting false answers and don't understand why, check what your predicate is actually producing.

The syntax of _if is likely to evolve in the future, to something more like most programming languages. For now, though, note that iftrue and iffalse are inside the parentheses, rather than after them as most languages have it.

Normally, _if processes the received context one element at a time: it checks the predicate against each element, and produces either the true or false path depending on the result of the predicate. But if you set the asList parameter to true, then it will run the predicate against the entire context at once, and do the true or false path on the entire context. This is appropriate if you are doing something with the whole list, like checking _isEmpty.

(Note: setting asList to true produces exactly the same result as putting an asterisk before the _if. An asterisk before a function means "do this to the entire List, not one at a time". The asList parameter is provided as an alternative in this common case, though, if you find it clearer.)