specify type of object's keys

I propose the generic object type Dictionary[Key, Value] to define that all keys of an object should be of type Key and all values should be of type Value.

Example from #98

Type of

{
  joe: {
    skating: {time: 1000, money: 300},
    cooking: {time: 9999, money: 999}
  },
  emily: {
    running: {time: 300, money: 0}
  }
}

can be described as

Dictionary[String, Dictionary[String, { time: Number, money: Number }]]

which can be splitted in smaller definitions using type aliases to provide more informative type names:

interface Name: String

interface Hobby: String

interface Expense {
    time: Number, money: Number
}

interface ExpBy_Hobby: Dictionary[Hobby, Expense]

interface ExpBy_Hobby_Person: Dictionary[Name, ExpBy_Hobby]

[maiermic]

TypeScript calls it indexable types and uses this syntax

interface ExpBy_Hobby_Person {
  [name: string]: {
    [hobby: string]: {
      time: number,
      money: number
    }
  }
}

var obj: ExpBy_Hobby_Person = {
  joe: {
    skating: {
      time: 1000,
      money: 300
    },
    cooking: {
      time: 9999,
      money: 999
    }
  },
  emily: {
    running: {
      time: 300,
      money: 0
    }
  }
};

which looks similar to computed property names in object literals.

[Mouvedia]

I think the nesting doesn’t hurt the readability. Since we kept the keyword interface in place of type to help the developers familiar with typescript transition easily, we could also adopt this syntax if it meets all of our requirements.

The only additions that I would introduce are:

• a default syntax if the type of the key is a string and a label exists
• only 2 types allowed for the property key: String, Symbol cf https://github.com/ericelliott/rtype/issues/97#issuecomment-230130868
• only 3 types allowed for the property name: String, Number and regex literal cf https://github.com/ericelliott/rtype/issues/97#issuecomment-230132233

I am probably gonna do a PR that addresses all of these issues at once which would hopefully close #97, #98 and #99 if there’s an agreement.