⊤) is a monadic function which in the nested array model gives information about the type and structure of its argument array's elements. In normal cases, the result is identical to the argument, with all numbers replaced by zeros and all characters replaced by spaces, and the substitution is done recursively.
Few dialects have Type as a primitive function. Dyalog APL uses
∊ (as in prototypical element), but only when migration level is 0, while NARS2000 uses monadic
⊤ (resembling the Latin letter T for Type), which it inherited from NARS.
However, in all dialects that support nested arrays, the type can be determined by enclosing/boxing the array in an empty array, and then coercing out a fill value. Since the array prototype is used as fill element, the resulting array will be the type of the original array:
↑ 0 ⍴ ⊂ 1 2 'ab' ┌─┬─┬──┐ │0│0│ │ └─┴─┴──┘
⊃ 0 ⍴ ⊂ 1 2 'ab' ┌─┬─┬──┐ │0│0│ │ └─┴─┴──┘
|APL built-ins |
|Monadic||Conjugate ∙ Not ∙ Roll ∙ Type|
|Dyadic||Plus ∙ Equal to (Xnor) ∙ Not Equal to (Xor)|
|Structural||Shape ∙ Reshape ∙ Depth ∙ Ravel ∙ Reverse|
|Selection||Take ∙ Drop ∙ Unique|
|Constants||Index origin ∙ Migration level|
|Other||Zilde ∙ High minus|