Compiler: programWithFlags : … -> Program Never produces a value of uninhabited type Never

Created on 25 May 2016 · 4Comments · Source: elm/compiler

The compiler magically provides a flags value of type Never to a program of type Program Never. Most constructors of Program Never automatically ignore this “value”, but a program constructed with programWithFlags can receive it in init and use it to break the safety of APIs expecting Never to be uninhabited.

Demo:

import Html
import Html.App

shouldNeverHappen : Never -> String
shouldNeverHappen n = "Got a value of type Never: " ++ toString n

main : Program Never
main =
  Html.App.programWithFlags
    { init = \flags -> (flags, Cmd.none)
    , update = \_ model -> (model, Cmd.none)
    , subscriptions = \_ -> Sub.none
    , view = \model -> Html.text (shouldNeverHappen model)
    }

displays Got a value of type Never: <internal structure>.

The Never type is supposed to be uninhabited and should not be used for this purpose. A program taking no flags should be marked with type Program (), not Program Never.

Source

andersk

👍5

Most helpful comment

Or better (I think?), can we just remove the flags parameter from Program entirely, changing

Program Never to Program,
Program flags to flags -> Program, and
programWithFlags { init = \flags -> …, … } to \flags -> program { init = …, … }?

andersk on 25 May 2016

👍4

All 4 comments

Or better (I think?), can we just remove the flags parameter from Program entirely, changing

Program Never to Program,
Program flags to flags -> Program, and
programWithFlags { init = \flags -> …, … } to \flags -> program { init = …, … }?

andersk on 25 May 2016

👍4

Assuming it's doable, I like how that suggested API would remove the type variable from Program.

Also, I like how it'd let you wire things up differently based on flags. I'd use that feature for webworkers. (Accept a flag for running things in parallel vs not, depending on whether the current browser supports it, then use a different update accordingly.) I can work around it, of course, but it'd be nicer to do it that way. 😄