"The GNU APL interpreter is an (almost) complete implementation of ISO standard 13751 aka. Programming Language APL, Extended." [1]
GNU APL is closely following the APL2 standard [2], Dyalog has lots of (proprietary) extensions.
APL does not have its roots in the FOSS world, all the more the efforts to provide and maintain a powerful, free option here are to be supported. It is not self-evident that there are always free software alternatives available.