The APL2 manual (p35) says "The right operand of a
dyadic operator is the function or array to its immediate
right." This is their way of saying that operator expressions
are left-associative. For example, in ∘.∘.+ the right operand of
the leftmost dot is ∘ (not ∘.+); so ∘.∘.+ is parsed as (∘.∘).+
The fact that operator-operand expressions are
left-associative while function-array expressions are
right-associative is one of the beautiful symmetries of APL!
Some more examples demonstrating the left-associativity of
⎕FX'R←(F L G)Y' 'R←F Y'
⎕FX'R←(F R G)Y' 'R←G Y'
+ L × R - 33
(+ L ×) R - 33
+ L (× R -) 33