Package

purescript-untagged-union

Repository
jvliwanag/purescript-untagged-union
License
MIT
Uploaded by
jvliwanag
Published on
2020-09-04

A data type for untagged unions.

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 = Int |+| Undefined

An alias UndefinedOr is also provided.

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

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 :: String |+| Number |+| Undefined
  }

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

import Untagged.Coercible (coerce)

sampleProps :: Props
sampleProps =
  coerce { 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
         }
Modules
Untagged.Coercible
Untagged.TypeCheck
Untagged.Union
Dependencies