How to specify type parameter

Question

How to specify type parameter using sorbet?

For example, I want to annotate a method with an argument of type A returning generic type T[A].

def build_array(value) 
  [value]
end

The output type depends on the input type:

build_array(42) #=> return Array[Integer]
build_array('42') #=> return Array[String]

Answer 1

You can accomplish this using type_parameters:

# typed: true
extend T::Sig

sig do
  type_parameters(:T)
  .params(value: T.type_parameter(:T))
  .returns(T::Array[T.type_parameter(:T)])
end
def build_array(value) 
  [value]
end

x = build_array(5)
T.reveal_type(build_array(42))   # T::Array[Integer]
T.reveal_type(build_array('42')) # T::Array[String]

Here's a sorbet.run link with the above code.