scala - pattern match component 'x' is constrained to be isInstanceOf[List[Any]] - but can't treat 'x' as that type on RHS -

to familiar scala lists decided implement flatten, take list like: list(list(2,4), 22, list(1)) list(2, 4, 22, 1).

when attempted run code below

def flatify(xs: list[any]) : list[any] = {   xs match {     case nil=> {       nil     }     case x::rest if x.isinstanceof[list[any]] => {       flatify(x):: flatify(rest)   // bad line     }     case x::rest => {       x:: flatify(rest)     }     case _ => {       throw new illegalstateexception("cant match")     }   } }  var list = list(list(4, 5), 9, 10, list(1, 2)) flatify(list) 

the compiler complained line commented '// bad line', saying:

error:(84, 17) type mismatch;      found   :      required: list[any]             flatify(x):: flatify(rest)                     ^ 

this strikes me odd, since guard condition explicitly requires x isinstanceof[list[any]]. understand any superclass of list[any] have thought once compiler gets right hand side of expression

flatify(x) :: flatify(rest) 

it accept x list[any].

i'm sure implementation might have other problems have not debugged yet, before moved on wanted try understand scala compiler here. tips or pointers gratefully appreciated.

you're matching on list[any], when have

case x :: tail if(x.isinstanceof[list[any]]) => x ...      ^             ^                            ^            true, type of       still                x determined 

x any , tail list[any]. if condition nothing change type of x, if can test x.isinstanceof[list[any]], x still any. need match on type, itself.

def flatify(xs: list[any]) : list[any] = {     xs match {       case nil => nil       case (x: list[any]) :: rest => flatify(x) ::: flatify(rest)       case x :: rest => x :: flatify(rest)       case _ =>  throw new illegalstateexception("cant match")     }   } 

furthermore, there bug. given x correctly matched list[any], following flatify(x) :: flatify(rest) place list[any] @ head of list[any], , therefore no flattening @ all. changed :::.

scala> flatify(list(1,2,3,list(4,5,6), list(7,8,9))) res1: list[any] = list(1, 2, 3, 4, 5, 6, 7, 8, 9)