Got an idea: functions (in FP) could be composed similar was as components in OOP. For example with dependency injection. For components in OOP we use interfaces. For functions we could use delegates. To achieve decomposition and modularity.
I tried to find something about it. No luck. Probably because there are no functional programs big enough to need this ?
While searching for enterprise scale applications written in FP I found this list.
Functional Programming in the Real World and this paper
I hope I just missed the killer application for FP, which would be big enough to deserve decomposition. Maybe someone could find me answer at stack overflow.