Add a function to coerce Cmd Never to Cmd msg

[the-g-man]

This is more a suggestion than a pull request but it seemed easiest to illustrate the idea by writing the code.

There should be an idiom for expressing the idea of a sink effect - i.e. one that isn't expected to emit any value when it is called. A possible example might be writing to the storage API. The obvious way to do this is to use a Cmd Never. But as things stand there isn't an idiomatic way to incorporate this into an application loop.

Implementing the idiom is trivial but might be surprising if it appeared naked in application code. It seems like it might be better to wrap it up in a core function to create a common idiom for these cases.

[jvoigtlaender]

The strict (instead of lazy) semantics of Elm will hit you in the face as soon as you try to use the sink-function in your program. Hint: the Debug.crash call there is not a good idea.

[jvoigtlaender]

The strict (instead of lazy) semantics of Elm will hit you in the face as soon as you try to use the sink-function in your program. Hint: the Debug.crash call there is not a good idea.

[the-g-man]

Actually, as it is, it won't even type-check. It was written in haste at work while waiting for something to compile.

How about...

map (\_ -> Debug.crash "Cmd Never passed to Cmd.sink resulted in a value")

?

[the-g-man]

Actually, as it is, it won't even type-check. It was written in haste at work while waiting for something to compile.

How about...

map (\_ -> Debug.crash "Cmd Never passed to Cmd.sink resulted in a value")

?

[jvoigtlaender]

Actually, as it is, it won't even type-check.

Oh, of course it would have type-checked. The type of Debug.crash is String -> a, polymorphic in a, so it would have been instantiable to an arbitrary function type as return type well enough.

How about...

Yes, that's better in that it won't immediately crash. Whether it does exactly what you want depends on the definition of Cmd.map, which I haven't looked into.

[jvoigtlaender]

Actually, as it is, it won't even type-check.

Oh, of course it would have type-checked. The type of Debug.crash is String -> a, polymorphic in a, so it would have been instantiable to an arbitrary function type as return type well enough.

How about...

Yes, that's better in that it won't immediately crash. Whether it does exactly what you want depends on the definition of Cmd.map, which I haven't looked into.

[jvoigtlaender]

Yep, should work.

[jvoigtlaender]

Yep, should work.

[andersk]

What do you think about adding absurd : Never -> a (#593), thereby solving this with Cmd.map absurd? It’s more general than Cmd Never -> Cmd msg, and safer than making the user write a Debug.crash call.

[andersk]

What do you think about adding absurd : Never -> a (#593), thereby solving this with Cmd.map absurd? It’s more general than Cmd Never -> Cmd msg, and safer than making the user write a Debug.crash call.

[evancz]

I'd rather not add something like this. There is no motivating example. No reason to add it then.

[evancz]

I'd rather not add something like this. There is no motivating example. No reason to add it then.

[andersk]

I added my motivating example to #593.

[andersk]

I added my motivating example to #593.