Scala automatic type inference in polymorphic functions -

how can make work? task @ hand little bit more complicated boils down this:

object z {   class b extends function1[int, int] {     def apply(i: int): int =   }    def compose[t <: function1[x, x], x](fcts: list[t]): function1[x, x] = {     fcts.reduce(_ andthen _)   }    def test() = {     val fcts = list.empty[b]      // unspecified type parameter x     val composed: function1[int, int] = compose[b](fcts)   } } 

i don't know how define "compose" function able receive concrete class b , automatically infer dependent types x

the scala compiler not when trying infer multiple levels of type parameters have. instead, simpler remove t <: function1[x, x] , require single type parameter represents argument , return type of function1.

def compose[a](fcts: list[function1[a, a]]): function1[a, a] = {   fcts.reduce(_ andthen _) } 

the compiler have easier time inferring a, instead of trying figure out t , x are, when x part of type t.

val a: int => int = _ + 10 val b: int => int = _ * 2 val c: int => int = _ - 3  scala> val f = compose(list(a, b, c)) f: int => int = scala.function1$$lambda$1187/930987088@531ec2ca  scala> f(2) res1: int = 21 

note reduce throw exception empty list of functions.