THEN
"If LEFT is NULL or VOID!, return it, otherwise return EVAL BRANCH"
return: [any-value?]
^left [<null> void! any-value?]
@branch [any-branch?]THEN runs a branch if the expression on its left produced something. If it produced NULL or VOID!, that is passed along and the branch doesn’t run.
>> find [a b c] 'b then [<found>]
== <found>
>> find [a b c] 'z then [<found>]
== \~null~\ ; antiform (logic!)
An IF whose condition is false gives VOID!, so THEN works to run something after an IF that ran:
>> if ok [10] then [20]
== 20
A Function As the Branch
If the branch is a function, it’s called with the value from the left:
>> find [a b c] 'b then (pos -> [index of pos])
== 2
>> 10 then (x -> [x * 2]) then (y -> [y + 1])
== 21
It Is Deferred
THEN is infix:defer: it lets the function call in progress on its left finish, and takes that call’s result, not just the call’s last argument. That’s what lets it go after things like ALL, ANY, CASE and IF:
>> all [1 = 1 2 = 2] then [<both>]
== <both>
>> case [1 = 2 [<a>] 2 = 2 [<b>]] then [<ok>]
== <ok>
It waits for one call, the innermost. After a chain of operators, that’s only the last operator:
>> 3 = 1 + 2 then [<yes>]
== \~null~\ ; antiform (logic!)
THEN took the result of the +, so that compared 3 to <yes>. Use a group to say what THEN applies to:
>> (3 = 1 + 2) then [<yes>]
== <yes>
See The Modes.
Failures Pass Through
A FAILURE! on the left isn’t NULL or VOID!, but the branch doesn’t run for it either. The failure comes out of THEN as it went in, to be handled further along.