generics - Scala error: missing parameter type for expanded function -

i trying write query library scala. here code far:

class query[telement](source: traversable[telement]) {     def join[tother](other: traversable[tother]) = new {         def on[tkey](keyselector1: telement => tkey) = new {             def equals(keyselector2: tother => tkey) = new {                 def into[tresult](resultselector: (telement, tother) => tresult): query[tresult] = {                     val map = source.map(e => (keyselector1(e), e)).tomap                     val results = other                         .map(e => (keyselector2(e), e))                         .filter(p => map.contains(p._1))                         .map(p => (map(p._1), p._2))                         .map(p => resultselector(p._1, p._2))                     new query[tresult](results)                 }             }         }     } }  object query {     def from[telement](source: traversable[telement]): query[telement] = {         new query[telement](source)     } } 

...

val results = query.from(users)     .join(accounts).on(_.userid).equals(_.owneruserid).into((_, _)) 

i following error when go compile:

error: missing parameter type expanded function ((x$2) => x$2.owneruserid) 

i little confused why error on non-generic function equals. generic parameters come outer scope, i'd think. know fix have explicitly parameter type writing (a: account) => a.owneruserid. however, trying make pretty fluent library, , making messy.

the problem quite simple. there ambiguity existing method equals inherited any. simple example:

scala> class x[a, b] { def equals(f: => b) = f } defined class x  scala> val x = new x[int, string] x: x[int,string] = x@52d455b8  scala> x.equals((x: int) => x.tostring) res0: int => string = <function1>  scala> x.equals((x: string) => x.tostring) // uh-oh res1: boolean = false 

as 1 can see in last example, when wrong function type passed, compiler has choose def equals(any): boolean. when don't specify type, compiler has infer one, can't in example.

simply rename method else , problem gone.