Working with Types

Please note that there are far better resources on the web for learning about type systems and what advantages they provide. This article describes how types are used within this wiki.

This article covers:

• What a Type Is
  • On Cardinality
  • Further Examples
• Type Notation
• [Basic Examples]
• [When to Write a New Type]

What a Type Is

A type describes data. It can describe a little bit about data, or a lot. Cardinality, valid values, operations, and even intended purpose can all be communicated with a good type. boolean is a very simple example.

boolean has a cardinality of 2

It has two valid values: true or false

With respect to intended purpose, boolean is meant to be used in boolean algebra – we know that intuitively because of its name. There is absolutely no reason you can't write a binary number as a boolean[] though! The boolean type communicates its purpose through its name, and its value names.

On Cardinality

Cardinality is the magnitude of possible values of a type. It is important with respect to the complexity of a function, class, interface, type alias, or an application as a whole. Generally speaking, low cardinality results in better understanding. Use the right type for the job, but consider a type that limits cardinality where possible.

Further Examples

number

number has a cardinality of 18014398509481982 (Number.MAX_SAFE_INTEGER - Number.MIN_SAFE_INTEGER)

Every value from Number.MIN_SAFE_INTEGER to Number.MAX_SAFE_INTEGER is a valid number value

number has a fairly obvious purpose... to be a number. Specifically, real numbers in the given range. Imaginary and complex numbers are not included.

IPerson

export interface IPerson {
  readonly name: string
  readonly age: number
  sayHi(): string
}

IPerson has an very, very large cardinality; basically infinity. The cardinality of string multiplied by the cardinality of number multiplied by the cardinality of string again for the return value of sayHi().

Any object having properties name, age, and sayHi, where name is a string, age is a number, and sayHi is a function which takes no arguments and returns a string, is a valid IPerson.

IPerson clearly describes its intent as a description of a person, and how that person might greet another person.

Type Notation

This wiki uses a type notation that, I think, was popularized by Haskell. Haskell behaves very differently than Javascript, so the type notation used here is an adaptation. The particular adaptation used is defined in the fantasy-land readme. This section will introduce examples of how the notation is translated to Typescript, but it will not explain concepts like type variables, generics, or currying.

Some novel concepts exist within this notation, like "Type Constructors". If one of these concepts demand further explanation, just open an issue.

Small note: All types and type variables are capitalized in this notation.

WIP