RodgerTheGreat has given a great answer; I'd like to add that implicit iteration also lends itself to automatic GPUization, automated SIMD and multithread paral

RodgerTheGreat has given a great answer; I'd like to add that implicit iteration also lends itself to automatic GPUization, automated SIMD and multithread parallelization; the semantics of the "rank/depth" penetration come with no a-priori guarantee about order of evaluation (and it shouldn't matter anyway unless you are extremely naughty with side effects, which the languages greatly discourage). Compare to an vectorizing/parallelizing an explicit loop, about which there have been tens if not hundreds of PhD dissertations, and it is still not a solved problem. This is why "for x in ..." in Python/C#, and even the "map" variants, are still inferior to APL/J/K: the facts that iteration order is predefined, and that the iteration itself my terminate prematurely, make it extremely hard for the compiler to optimize, whereas in most cases, neither consideration matters.