Multiple type parameters on a scala method
generics, methods, scala
Solution
Suppose I want to write a generic method that operates on any old `Map[K, V]`—I'd have to include both type parameters in the method type parameter list:
def mapToList[K, V](m: Map[K, V]): List[(K, V)] = m.toList
Three parameters works the same way. Suppose I have a function `A => B` and another `B => C`, and I want to compose them to get a function from `A` to `C`—I need all three types in my parameter list:
def andThen[A, B, C](f: A => B, g: B => C): A => C = (a: A) => g(f(a))
Both of these examples are trivial, since we already have `m.toList` and `f andThen g`, but they should make the use case for multiple type parameters clear.
Problem
In Java, a `Map` could be parameterized as `Map<K, V>`, but in Scala, I don't know what's the meaning of multiple type parameters on a method, for example: ``` def foo[T, U, R] ``` It is easy to understand when a method parameterized by one type parameter. such as `def f[T](t: T)` .