Generic type convertion

Question

I am trying to create a method which gets input from the user and converts that to a specified type. Is there a way to do this without rewriting for each type.

Something like this:

struct Input(InputType)
    def self.get_from_stdin(msg_fail : String = "Error! Wrong type, please reenter: ")
        input = gets
        begin
            input = input.to_s.to(InputType)
        rescue
            puts msg_fail
            input = Input(InputType).get_from_stdin
        end
        input
    end
end

age = Input(Int32).get_from_stdin("Age must be a number, please reenter: ")

Basically, I want to achieve something akin to this:

foo = foo.to(MyType)

I imagine this would be difficult to do without macros.

Answer 1

Instead of messing with generics or macros I would look into ways to reduce boilerplate. That is extract common code into a yielding method and then write specific methods for each case you want to handle on top of that.

def prompt_user(prompt)
  print prompt
  print ' '
  loop do
    input = gets
    if input && !input.strip.empty?
      break yield input
    else
      print "Please enter a value: "
    end
  rescue
    print "Wrong type, please reenter: "
  end
end

def prompt_int32(prompt)
  prompt_user(prompt, &.to_i32)
end

def prompt_my_thing(prompt)
  prompt_user(prompt) {|value| MyThing.parse value }
end