typeclass - scala value is not a member of type parameter -


i'm trying hang of scala traits , case classes. below followup this question.

suppose have simple class , object extends it. 

sealed trait operations{     def add(a:double,b:double):double     def multiply(a:double,b:double):double }  case object correctoperations extends operations{     def add(a:double,b:double):double = a+b     def multiply(a:double,b:double):double= a*b }

now have function make use of object of type operations, such as,

def dooperations(a:double,b:double, op:operations)={ op.multiply(a,b) - op.add(a,b)}.

this works well, question how generalize types of trait operations, we're not talking doubles. i'd have generic types trait operations , type specification each object.

using type generics, tried

sealed trait operations[t]{   def add(a:t,b:t):t   def multiply(a:t,b:t):t }  case object correctoperations extends operations[double]{     def add(a:double,b:double):double = a+b     def multiply(a:double,b:double):double= a*b }  def dooperations[t](a:t,b:t, op:operations[t])={ op.multiply(a,b) - op.add(a,b) },

with compile error @ dooperations - "value - not member of type parameter t".

so know op.multiply(a,b) return type t, , error indicate type t has no .- method.

how should thinking achieving generalization of trait operations ? thanks

in context of problem, should introduce subtract method operations trait, can provide evidence t has such method (well doesn't, method subtraction t another).

sealed trait operations[t] {     def add(a: t, b: t): t     def multiply(a: t, b: t): t     def subtract(a: t, b: t): t }  case object correctoperations extends operations[double]{     def add(a: double, b: double): double = + b     def multiply(a: double, b: double): double = * b     def subtract(a: double, b: double): double = - b }  def dooperations[t](a: t, b: t, op: operations[t]) =     op.subtract(op.multiply(a,b), op.add(a,b))

this numeric trait does.


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