scala - Why is no implicit view found, when eta conversion and specifying type parameters does allow an implicit view to be found? -

the code:

object test {   import scala.language.implicitconversions    case class c1() {}   case class c2() {}    implicit def c1toc2(in: c1): c2 = c2()    def from[a, b](in: a)(implicit f: => b): b = f(in)    def fails(): future[c2] = {     val future: future[c1] = future.successful(c1())     future.map(from) // line fails compile!   }    def compiles1(): future[c2] = {     val future: future[c1] = future.successful(c1())     future.map(x => from(x))   }    def compiles2(): future[c2] = {     val future: future[c1] = future.successful(c1())     future.map(from[c1, c2])   } } 

in example, fails method fails compile. error message is:

error:(23, 16) no implicit view available => b. future.map(from)

i'm confused why no implicit view found. based on compiles1 , compiles2 methods, both compile, seems there is implicit view available => b.

what going on here, , why 2 compilesn methods work, fails not?

my background: i'm still learning scala, case i'm missing pretty obvious. :)

i'm on scala 2.11.8.

the compiler attempts resolve implicits before eta-expansion of from function, type parameters of from not yet inferred when call this:

future.map(from) 

compiles2 works because supply type parameters on own. when call future.map(from[c1, c2]), compiler knows need implicit c1 => c2, because that's you've told it.

with compiles1, difference little more subtle, stems fact future.map(from) , future.map(x => from(x)) different things. former uses eta-expansion, fails aforementioned reasons. future.map(x => from(x)), there no eta-expansion happening. instead, have anonymous function calls from instead of eta-expanding it. therefore, compiler can infer type of x, tells x c1 (in case), , can find implicit conversion c1toc2 satisfies type parameters of from while resolving implicit , final return type of method, future[c2].