ALEN / @#

(alen var-name)               ; return the array length
(alen array-var e1 .. eN)     ; a nested array reference
(@# var-name)                 ; return the array length short-form
(@# foo :names 0)             ; short-form, nested array reference

This requests that the back-end coder do whatever the target language permits to return the length of the variable, assumed to contain an 'array' of some type or other.

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Return values

Most languages return an integer value if the variable that is passed is an Array type and if not, the behaviour is wide, varied and usually going to result in some kind of run-time error or warning being displayed somewhere in the output stream.