Boolean array in If guard

According to the Dyalog APL language reference

the test condition to the right of :If must return a single element Boolean value 1 (true) or 0 (false)

Is there any particular reason why the value is allowed to be an array rather than requiring it to be a scalar? Is it only for convenience? Do other other APL dialects which implements the If statement also allow non-scalar guards?


  • (A scalar is a 0-dimensional array.)

    A few more details to expand on Jürgen's answer:

    It is very common to have 1-element vectors where a scalar is actually appropriate. This is especially true in APLs that do not have the monadic (Tally) function (including Dyalog prior to version 14.0), prompting people to use (Shape) instead. For the same reason, allowing a 1-element array (singleton) is widely allowed in many contexts where, strictly speaking, a scalar would be required. This is known as singleton extension. Here are some examples of constructs that give a 1-element vector when one would actually want a scalar:

    Indeed, it was traditionally quite awkward to get a scalar, and this leniency was therefore a very welcome convenience. Some APLs didn't (and still don't!) have for the empty numeric vector, but was lenient about types and allowed '' so the shortest contruct was the unintuitive ''⍴expression. Today we have or as first (depending on dialect).

    Before control structures were added to various APLs (some still don't have them!), the only flow control was (Branch) which was defined as "go to the first line number in the array on the right, if any; do nothing if empty", allowing various computational methods to select branching destination. For example:

    ⍝ code for non-scalar arrays
    ⍝ code for scalars

    The new control structures had to allow easy conversion of existing code and support common patterns:

     :If 0≠⍴⍴myArray
    ⍝ code for non-scalar arrays
    ⍝ code for scalars

    I don't have access to APL+, but APLX also allows 1-element vectors — not other singletons. As far as I know, it is unique to Dyalog to also accept higher-rank singletons.