Comparison of APL dialects

All APL dialects share core features, and at least the primitives are backwards compatible with those of APL.SV. However, there are important dialectal differences, the most significant of which are described in this article.

Flat versus Nested
One of the most significant differences between APLs is the choice of array model, namely nested or flat. Originally, all APLs were entirely flat, that is, the only scalar values supported were simple and had to be either numbers and characters, and an array had to be homogeneous, that is, consisting entirely of a single type. SHARP APL introduced the box as a third type, such that a box could hold any other array (including a box array), and this allowed for arrays contain other arrays. NARS introduced the nested array model, where any arrays can be an actual element of another array. Most currently maintained dialects (Dyalog APL, NARS2000, APL2, GNU APL, and others) use the nested array model.

Disclose
Disclose is defined, on scalars, to be the inverse of Enclose, giving the sole element of that scalar. This behavior is extended to arbitrary arrays in one of two ways: Mix, which combines all elements, and First, which returns only the first (in ravel order).

In the SHARP APL family, Disclose is written, and given a function rank of 0, which causes it to behave as Mix. First is not a primitive.

In nested APL dialects, Disclose is written, and it was extended to First by NARS and to Mix by APL2. The other possible meaning is given the glyph  and named "Mix" or "First" (not "Disclose") as appropriate. Dialects might choose either assignment of glyphs, and Dyalog APL and APL*PLUS allow either to be chosen based on Migration Level.

Complex numbers
Most dialects support complex numbers, but some (dzaima/APL, APLX, APL+Win) don't. Dialects with complex numbers also extend Circular to left arguments designed for complex numbers.

Conformability
All APLs implement scalar extension in scalar dyadic functions, and Each if present. However, a few dialects differ in their support for singleton extension. Most support it fully, so that a singleton of any rank is extended. APL2 supports it only for scalars and one-element vectors (and ISO/IEC_13751:2001 specifies only this behavior), and dzaima/APL and Kap support scalar extension only.

Furthermore, dialects that implement the Rank operator differ in their conformability rules for frames. Dyalog APL and SHARP APL implement scalar agreement only, NARS2000 implements singleton agreement, and A+ implements leading axis agreement (in addition to extra options allowing the programmer to specify outer-product-like treatment of some axes).

Depth
Dyalog APL defines Depth to return negative numbers when the depth is uneven. Most other dialects instead define it to return the maximum depth. The original NARS paper assigned the glyph to a function called Simple, which simply answered if the argument was simple or not, without reporting the actual depth. It was thus equivalent to today's.