f. (Fix) Adverb
u f. "compiles" verb u down to its equivalent J primitives.
The term "compiles" is used here figuratively. The analogy is with a classic Pascal compiler, which turns Pascal source statements into p-code.
The name Fix on the other hand is inherited from the APL language, where an analogous system function ⎕FX ("Fix") turns a function definition (expressed as a character matrix) into an APL function.
sum=: +/ count=: # by=: % mean=: sum by count mean sum by count mean f. +/ % #
(u f.) is equivalent in function to u. however it is used
mean i.7 3 (mean f.) i.7 3 meanfdot=: mean f. meanfdot i.7 3
To remove dependencies on named verbs (which might later be changed).