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(f g) data => data f (g data)

fn data    NB. function applied to data "NB. is comment in J"

5 + 5   NB. 5 adds_to 5
5 , 6   NB. 5 appended_to 6
4 { 1 2 3 4 5    NB. 4th element_of the_list_12345

(f g) data => data f (g data)

(f g h) data => (f data) g (h data)

avg =: +/ % #

(+/ data) % (# data)
(sum-of data) divided-by (number-of-elems data)
results in average

Falkoff and Iverson, The Design of APL, 1973.
Falkoff and Iverson, The Evolution of APL, 1978.
Iverson, A Personal View of APL, 1991.