what does the elm signature mean in “Program Never Model Msg”?

问题

I'm just starting on elm and without understanding Haskell and its compiler I'm trying to grasp what the signature mean in Html.program

func: (a -> String) -> String -- this means expects a function and return a string
main: Program Never Model Msg -- What does this mean?

回答1:

Program is a type parameterized by three type variables: flags, model, and msg. Never is a type that cannot have any value (see the link for a good explanation of what this means and how it differs from the unit type ()).

Program Never Model Msg therefore is the type of a program that doesn't have any flags (Never), has a model of type Model, and passes messages of type Msg.

来源：https://stackoverflow.com/questions/47845714/what-does-the-elm-signature-mean-in-program-never-model-msg