Generic type convertion

user_16619794 0
Viewed 54

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.

crystal-lang type-conversion casting generics types
2 Answers

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
user_21996870

What do you really want to do? What is the exact case you're working on? As you hinted already, of course you can't take an arbitrary string and convert that to any other object, as that would be seriously unsafe/unsound. But if we know your use case we can give some better advice.

user_34068810
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