elm-typed-value

For a type with just 1 variant, Typed is a convenient, safe replacement (↑ limits)

Attach a tag so a Typed .. Meters .. Float isn't accepted as Typed .. Kilos .. Float anymore.

type boilerplate ↓ is covered by Typed

quantity =
    \(Meters quantity) ->
        quantity

alter quantityAlter =
    \(Meters quantity) ->
        quantity |> quantityAlter |> Meters

and other helpers like mapping multiple etc.

Plus you don't have to spell out the obvious:

3.2 |> Meters.fromFloat

prime |> Prime.toInt
height |> Meters.toFloat

(oneHeight |> Meters.toFloat)
    + (otherHeight |> Meters.toFloat)
    |> Meters.fromFloat

with Typed

3.2 |> tag Meters

prime |> untag
height |> untag

oneHeight
    |> Typed.and otherHeight
    |> Typed.map (\( h0, h1 ) -> h0 + h1)

Kinds of Typed

• Tagged → attach a label things like when you'd use

  -- module Cat exposing (Cat(..))
  
  type Cat
      = -- variant can be used anywhere
        Cat { name : String, mood : Mood }

  Users can create & alter new Cats everywhere

  A type(..) can't expose the variant for creating & altering without allowing access as well. Typed can, as we'll see in section Tagged Internal

• Checked → only "validated" things like when you'd use

  -- module Prime exposing (Prime)
  
  type Prime
      = -- nobody outside this module can use this variant
        Prime Int

  creating & altering Primes will only be possible inside that module

  An opaque type can't expose the variant for destructuring only. Typed can, as we'll see in section Checked Public

• Public → everyone can access → untag

• Internal → only those with the tag can access → internal

Tagged Public

import Typed exposing (Typed, Tagged, Public, tag)

type alias Cat =
    Typed Tagged CatTag Public { name : String, mood : Mood, napsPerDay : Float }

type CatTag
    = Cat

type alias Dog =
    Typed Tagged DogTag Public { name : String, mood : Mood, barksPerDay : Float }

type DogTag
    = Dog

sit : Dog -> Dog
sit =
    Typed.map (\d -> { d | mood = Neutral })

howdy : Cat
howdy =
    { name = "Howdy", mood = Happy, napsPerDay = 2.2 }
        |> tag Cat

howdy |> sit -- error

Another example:

-- module Pixels exposing (Pixels, PixelsTag(..))

import Typed exposing (Typed, Tagged, Public, tag)

type alias Pixels =
    Typed Tagged PixelsTag Public Int

type PixelsTag
    = Pixels

-- in another module using Pixels

innerWidth : Pixels
innerWidth =
    700 |> tag Pixels

borderWidth : Pixels
borderWidth =
    5 |> tag Pixels

defaultWidth : Pixels
defaultWidth =
    innerWidth
        |> Typed.and borderWidth
        |> Typed.map
            (\( inner, border ) -> inner + border * 2)

defaultWidth |> Typed.untag
--> 710

Checked Public

-- module Even exposing (Even, n0, n2, add, multiplyBy)

import Typed exposing (Typed, Checked, Public, tag)


type alias Even =
    Typed Checked EvenTag Public Int


-- don't expose(..) its variant
type EvenTag
    = Even


multiplyBy : Int -> Even -> Even
multiplyBy factor =
    \even ->
        even
            |> Typed.map (\int -> int * factor)
            |> Typed.toChecked Even


add : Even -> Even -> Even
add toAddEven =
    \even ->
        even
            |> Typed.and toAddEven
            |> Typed.map
                (\( int, toAddInt ) -> int + toAddInt)
            |> Typed.toChecked Even


n0 : Even
n0 =
    0 |> tag Even


n2 : Even
n2 =
    2 |> tag Even

-- in another module using Even

cakeForEven : Even -> { cake : () }
cakeForEven _ =
    { cake = () }

n0 |> Typed.map (\n -> n + 1) |> cakeForEven
--→ compile-time error: is Tagged but expected Checked

n2 |> multiplyBy -5 |> cakeForEven
--> { cake = () }

Above example is just for illustration! In practice, prefer a narrow type

type Even
    = Times2 Int

Checked Internal

A validated thing that can't be directly accessed by a user.

A module that only exposes randomly generated unique Ids:

-- module Id exposing (Id, random, toBytes, toString)

import Typed exposing (Typed, Checked, Internal, tag)
import Random

type alias Id =
    Typed Checked IdTag Internal (List Int)

type IdTag
    = Id

random : Random.Generator Id
random =
    Random.list 4
        (Random.int 0 (2 ^ 32 - 1))
        |> Random.map (tag Id)

-- the API stays the same even if the implementation changes
toBytes --...
toString --...

→ Outside of this module, the only way to create an Id is Id.random

Again, above example is just for illustration! In practice, prefer a narrow type as shown in elm-bits

type alias Id =
    ArraySized (Exactly N128) Bit

Combined with Tagged Internal

-- module Password exposing (PasswordUnchecked, PasswordGood, toChecked, length, unchecked)

import Typed exposing (Typed, Tagged, Checked, Internal, tag, internal)

type alias Password goodOrUnchecked =
    Typed goodOrUnchecked PasswordTag Internal String

type PasswordTag
    = -- don't expose the tag variant
      Password

type alias PasswordGood =
    Password Checked

type alias PasswordUnchecked =
    Password Tagged

-- ! annotates the result as `Tagged` ↓
unchecked : String -> PasswordUnchecked
unchecked =
    tag Password

toChecked : PasswordUnchecked -> Result String PasswordGood
toChecked =
    \passwordToTest ->
        let
            passwordString =
                passwordToTest |> internal Password
        in
        if (passwordString |> String.length) < 10 then
            Err "Use at lest 10 letters & symbols."

        else if commonPasswords |> Set.member passwordString then
            Err "Choose a less common password."

        else
            passwordToTest |> Typed.toChecked Password |> Ok

commonPasswords =
    Set.fromList
        [ "password1234", "secret1234"
        , "c001_p4ssw0rd", "1234567890"
        --...
        ]

You can then decide that only a part of the information should be accessible.

-- doesn't expose too much information
length : Password goodOrUnchecked_ -> Int
length =
    \password ->
        password
            |> internal Password
            |> String.length

used in

-- module Register exposing (State, Event, ui, reactTo, stateInitial)

import Password exposing (PasswordUnchecked)

type alias State =
    { -- accessing user-typed password is impossible
      passwordTyped : PasswordUnchecked
    , loggedIn : LoggedIn
    }

stateInitial : State
stateInitial =
    { passwordTyped =
        "" |> Password.unchecked
    , loggedIn = NotLoggedIn
    }

type LoggedIn
    = -- no user can have an unchecked password
      LoggedIn { userPassword : PasswordGood }
    | NotLoggedIn


type Event
    = PasswordEdited PasswordUnchecked
    | PasswordConfirmed PasswordGood

reactTo : Event -> (Model -> Model)
reactTo event =
    case event of
        PasswordEdited uncheckedPassword ->
            \model ->
                { model
                    | passwordTyped = uncheckedPassword
                }
        
        PasswordConfirmed passwordGood ->
            \model ->
                { model
                    | passwordTyped =
                        "" |> Password.unchecked
                    , loggedIn =
                        LoggedIn { userPassword = passwordGood }
                }

ui =
    \{ passwordTyped } ->
        [ [ "register" |> Html.text ] |> Html.div []
        , Html.input
            [ onInput
                (\text ->
                    text
                        |> Password.unchecked
                        -- not accessible from now on
                        |> PasswordEdited
                )
            , String.repeat
                (passwordTyped
                    |> Password.length
                )
                "·"
                |> Html.value
            ]
            []
        , case passwordTyped |> Password.toChecked of
            Ok passwordGood ->
                Html.button
                    [ onClick (PasswordConfirmed passwordGood) ]
                    [ "Create account" |> Html.text ]
                
            Err message ->
                message |> Html.text
        ]
            |> Html.div []

passwordTyped |> untag |> leak
userPassword |> untag |> leak

→ compile-time error: expected Public but found Internal

prior art

This package wouldn't exist without inspiration:

• Punie/elm-id

especially

• joneshf/elm-tagged
• IzumiSy/elm-typed

limits

the type of the Public untagged thing is not obvious but used often

In that case expose more descriptive API and leave the rest as "safe internals"!

If you strictly want to avoid allowing untag under all circumstances, make it Internal

toDescriptiveValue : TypedThing -> DescriptiveValue
toDescriptiveValue =
    Typed.internal ThingTag

always prefer narrow type over Checked

More often than not, there's already a type with the same promises even when created directly by users:

Instead of

type alias StringFilled =
    Typed Checked StringFilledTag Public String

type alias PasswordLongEnough =
    Typed Checked PasswordLongEnoughTag Public String

make it safe

type alias StringFilled =
    { head : Char, tail : String }

type alias PasswordLongEnough =
    ArraySized (Min (Fixed N10)) Char

Here using typesafe-array

Use those! Extensively. No opaque type or Checked necessary

packages: unnecessary major version bumps

All ↓ aren't breaking in practice but result in a major version bump

• Checked → Tagged
• Internal → Public
• Checked Internal thing type change

For many package authors, this is a deal-breaker.

Be explicit and choose a type for parts of information that could be added or removed in the future.

can't be defined recursively

type alias Comment =
    Typed
        Tagged
        CommentTag
        Public
        { message : String
        , responses : List Comment
        }

elm:

This type alias is recursive, forming an infinite type

recursive alias hint:

Somewhere in that cycle, you need to define an actual type to end the infinite expansion.

In this instance: try tree structures like zwilias/elm-rosetree:

type alias Comments =
    Maybe (Tree { message : String })

From the outside, recursive aliases seem like a problem solvable at the language level. Let's watch how elm handles them in the future.