Built-in types

PureScript	JavaScript	Example Values
Number	number	1.0
String	string	"test"
Boolean	boolean	true
Int	int	2
Char	char	'b'
Array	array	[1,2,3] [true, false]
Record	object	{"name": "John", "age": 30}

More types can be found from language reference(https://github.com/purescript/documentation/blob/master/language/Types.md)

Value has a type

> :type 12
Int

> :type 'a'
Char

> :type [1,2,3]
Array Int

info

type name starts with upper case, like Int, Char, etc.

Type has a kind

> :kind Int
> Type

> :kind Char
> Type

info

Type is a special kind which represents the kind of all types which have values, like Number and String.

Static Type System

What is a type

A type is a kind of label that every expression has. It tells us in which category of things that expression fits. The expression True is a boolean, "hello" is a string, etc.

• everything in PureScript has a type
• the type of every expression (vs. statement) is known at compile time. And it only exists at compile time

Type inference

• Infer type by itself, don't have to tell explicitly
• function also has type
  • it's good practice to give them explicit declaration

tip

If you want to give your function a type declaration but are unsure as to what it should be, you can always just write the function without it and then check it with :t (or :type). Functions are expressions too, so :t works on them without a problem.

> hello x = "hello " <> x
> :type hello
String -> String

> double x = x * 2
> :type double
Int -> Int

> add x y = x + y
> :type add
forall (a9 :: Type). Semiring a9 => a9 -> a9 -> a9

Type variable

> :type (+)
forall (@a :: Type). Semiring a => a -> a -> a

> :type (-)
forall (@a :: Type). Ring a => a -> a -> a

> :type (==)
forall (@a :: Type). Eq a => a -> a -> Boolean

what is a?

• is not a type, because types are written in capital case.
• it is a type variable

Functions that have type variables are called polymorphic functions.

pattern matching on data constructor

data Shape = Circle Number | Rectangle Number Number
area :: Shape -> Number
area (Circle r) = 3.14 * r * r
area (Rectangle w h) = w * h

-> just like function

Rows

https://github.com/purescript/documentation/blob/master/language/Types.md#rows Rows are not of kind Type: they have kind Row k for some kind k, and so rows cannot exist as a value