purescript-untagged-union

A data type for untagged unions.

Overview

Consider a the following type:

import Untagged.Union

type ISB = Int |+| String |+| Boolean

The type ISB describes values which can either be an Int, a String or a Boolean. Moreover, it is guaranteed that at runtime, values of type ISB is directly stored as an Int, a String or a Boolean without any wrappers. This makes it especially useful for FFI.

Note that |+| is an alias for OneOf.

Creating a OneOf

In order to create a value of OneOf, use asOneOf.

isb1 :: ISB
isb1 = asOneOf 20

isb2 :: ISB
isb2 = asOneOf "foo"


--isb3 :: ISB
--isb3 = asOneOf 3.5

-- isb3 will fail since 3.5 is a Number which is neither
-- an Int, String nor Boolean

Usage with Undefined

The library also defines Undefined. Combined with OneOf, it can represent an optional type:

import Literals.Undefined

type OptionalInt = Undefined |+| Int

An alias UndefinedOr is also provided.

type OptionalInt' = UndefinedOr Int -- Same as `Undefined |+| Int`

Getting a value

To get a purescript value back, use fromOneOf:

import Data.Maybe

valInt :: Maybe Int
valInt = fromOneOf isb1 -- evaluates to `Just 20`

valString :: Maybe String
valString = fromOneOf isb1 -- evalutes to `Nothing` since isb1 contains an Int

Usage with records

Using OneOf, describing JS types as records becomes intuitive:

type Props =
  { text :: String -- a required field

  -- Optional Fields
  , width :: UndefinedOr Number
  , height :: UndefinedOr Number

  -- Optional and Varying types
  , fontSize :: Undefined |+| String |+| Number
  }

A cast helper is made available to convert records with the same runtime value:

import Untagged.Castable (cast)

sampleProps :: Props
sampleProps =
  cast { text: "foo" -- text is required and should be a string

       , width: 30.0 -- width is optional, and may be defined, but should be a Number
       -- height is optional and may be omitted
       , fontSize: "100%" -- fontSize may be defined, and should either be a string or number
       }

Nested Unions

Nesting is possible but requires some hints for the compiler.

Consider the following:

type Props =
  { text :: String -- a required field
  , width :: UndefinedOr Number
  , height :: UndefinedOr Number
  , fontSize :: Undefined |+| String |+| Number
  }


type ContainerProps =
  { titleProps :: UndefinedOr Props
  , opacity :: UndefinedOr Number
  }

In order to create an instance of ContainerProps, the type of titleProps must be specified.

sampleContainerProps :: ContainerProps
sampleContainerProps =
  cast { titleProps: (cast { text: "Foo" } :: Props)
       }

Alternatively, helper constructors may be used to aid type inference.

import Untagged.Castable (Castable)

props :: forall r. Castable r Props => r -> Props
props = cast

containerProps :: forall r. Castable r ContainerProps => r -> ContainerProps
containerProps = cast

sampleContainerProps' :: ContainerProps
sampleContainerProps' =
  containerProps { titleProps: props { text: "Foo" }
                 }