the scala compiler cannot find the correct generic function parameter -

i have scala code following

object main extends app {    def getfun[t, u](x: array[t] => u, p: array[t]): u = { x(p) }    def getfun2(x: array[int] => double, p: array[int]): double = { x(p) }     val arr = array(1, 2, 3, 4)    println(getfun((x: array[int]) => x.sum, arr))    // println(getfun(_.sum, arr))  // error, why!    // println(getfun(x = _.sum, arr))  // error, why!    println(getfun2(_.sum.todouble, p = arr)) } 

why getfun cannot used simple getfun2. think use "x = _.sum", scala compiler can find correct type, seems failed.

can body explain reason of error.
thanks!

getfun uses type variables whereas getfun2 uses explicit types.

so, getfun2, compiler knows calling sum function on array[int]. knows how sum integers, things work.

for getfun compiler knows there array of something (t stands some type). not how sum something, have compiler here , tell t is. 1 possibility did in first line, specifying array type in function lambda. pass along type parameter values this:

println(getfun[int,double](_.sum, arr))