Stop
⊣

Stop (⊣
) (in SHARP APL and APLX), Hide (in GNU APL and Kap), or Null (in A+) is a primitive monadic function that ignores its argument and returns a nonprinting value. It shares the left tack glyph with Left Identity. The nature of the returned value can vary: for example an empty matrix is used in SAX and APLX, while GNU APL uses the scalar 0 but returns it as a shy result (or "committed"). In SHARP APL's initial implementation of ⊣
, Stop returned "no result", so that attempting to use its result would be an error.^{[1]}
In other dialects, monadic ⊣
is the identity function, exactly like ⊢
. This redundancy means that it can be redefined without losing any primitive functionality. However, pairing both dyadic functions with the identity may be useful in constructing ambivalent functions, particularly in tacit programming. In April, monadic ⊣
("Empty") returns its argument is returned as a shy result, giving it the properties of both Stop and the identity.
In dialects where ⊣
does not return the argument when used monadically, the effect can be achieved with ⊢⍨
using the commute operator.
Documentation
References
 ↑ "Language Extensions of May 1983". SATN45, 19830502.