You're reading section 9.3.4 "Rank operator deriving monadic function". You also need to look at 9.3.5 "Rank operator deriving dyadic function".
Given g ← f⍤P Q R: P is the monadic rank Q is the left rank R is the right rank So: g Y applies g to the P-cells of Y X g Y applies g to the Q-cells of X and the R-cells of Y The ⌽3⍴⌽y1 stuff is just a too-cute way of saying that you can specify fewer than 3 values in the right operand, and: R is shorthand for R R R Q R is shorthand for R Q R Jay. On 27 April 2016 at 08:28, Elias Mårtenson <loke...@gmail.com> wrote: > About the ISO specification of ⍤ > > In writing the above message, I was reading the ISO specification for the > rank operator, and I find it incredibly confusing. I have quoted the > description below, and based on my reading of this text, the rank parameter > is not just a single value, but can be up to three values. However, no > matter how I read it, I still can't see how any but the the very first value > is actually every used. > > Also, the case where LENGTH ERROR is supposed to be raised does not happen > in GNU APL. > > It seems as the specification for the rank operator is just broken on > several levels in the spec. That seems to me to be reason enough to not pay > attention to the spec in this case and just adapt the way Dyalog does it. > > Here's the spec for the rank operator from the ISO spec: > > Informal Description: > The result of f⍤y is a function which, when applied to B, returns Z, the > result of applying the function f to the rank-y cells of B. > > Evaluation Sequence: > If y is a scalar, set y1 to ,y. Otherwise set y1 to y. > If y1 is not a vector, signal domain-error. > If y1 has more than three elements, signal length-error. > If any element of y1 is not a near-integer, signal domain-error. > Set y2 to ⌽3⍴⌽y1. > Set y3 to the first-item in y2. > Set y4 to the integer-nearest-to y3. > If y4 exceeds the rank of B, set y5 to the rank of B, otherwise set y5 to > y4. > If y5 is negative, set y6 to 0⌈y5 plus the rank of B, otherwise set y6 to > y5. > Apply f to the rank-y6 cells of B. > Conform the individual result cells. Let their common shape after conforming > be q, and let p be the frame of B with respect to f, that is, (rank of B) > minus y6, and > return the overall result with shape p,q.
