It should be valid. for a function declaration, it goes

"fun" ... ":" type

, where

type

then is

modifier typeReference

, where in your case

typeReference

matches

functionType

, where the

"("parameter{","}?")"

has the

parameter

simply matching

SimpleName : type

. So the

f

in which case would be the

SimpleName

Although I am not very sure of what the significance of this type name is