Y must be a simple integer scalar or a 1 or 2 element vector containing the file tie number followed by an optional passnumber. If the passnumber is omitted it is assumed to be zero. Subject to a few restrictions, X may be any array.
The shy result R is the number of the component to which X is written, and is 1 greater than the previously highest component number in the file, or 1 if the file is new.
(1000?1000) ⎕FAPPEND 1
⎕←(2 3⍴⍳6) 'Geoff' (⎕OR'FOO') ⎕FAPPEND 1
12
⎕←A B C ⎕FAPPEND¨1
13 14 15
Dump←{
tie←⍺ ⎕FCREATE 0 ⍝ create file.
(⎕FUNTIE tie){}⍵ ⎕FAPPEND tie ⍝ append and untie.
}