scala - Type inference is not happening in polymorphic function -

i have written 2 version of codes below . in 1st version , getting run time error below , not able understand why getting error while passing iterator type function : using. in version 2 running fine while passing type of resource type function : using .

error:(23, 11) inferred type arguments [iterator[string],nothing] not conform method using's type parameter bounds [a <: anyref{def close(): unit},b] control.using(source.fromfile("c:\users\pswain\ideaprojects\test1\src\main\resources\employee").getlines){a => {for (line <- a) { println(line)}}} ^

1st version:-

  /**   * created pswain on 9/22/2016.   */  import java.io.{ioexception, filenotfoundexception}  import scala.io.source  object control {   def using[ <: {def close() : unit},b ] (resource : a) (f: => b) :b =   {     try {        f(resource)     } {       resource.close()     }   } } object filehandling extends app {     control.using(source.fromfile("c:\\users\\pswain\\ideaprojects\\test1\\src\\main\\resources\\employee").getlines){a => {for (line <- a) { println(line)}}} } 

2nd version

/**   * created pswain on 9/22/2016.   */  import java.io.{ioexception, filenotfoundexception}  import scala.io.source  object control {   def using[ <: {def close() : unit},b ] (resource : a) (f: => b) :b =   {     try {        f(resource)     } {       resource.close()     }   } } object filehandling extends app {     control.using(source.fromfile("c:\\users\\pswain\\ideaprojects\\test1\\src\\main\\resources\\employee")){a => {for (line <- a.getlines) { println(line)}}} } 

the first version doesn't compile because you're passing result of getlines, of type iterator[string] first argument. argument must have def close(): unit method (as bounded a <: {def close() : unit}), , iterator[string] not have such method.

the second version works because source passed a, fits bound (has matching close method)