CODEWITHSUNDEEP
elm
djangodeveloper
djangodeveloper

Reputation: 213

Elm main function type annotation

What is the meaning of this Elm example's main function type annotation?

main : Html msg
main =
    div [] [ text "1" ]

What is Html and msg?

Upvotes: 2

Views: 1334

Answers (1)

0xc14m1z
0xc14m1z

Reputation: 3725

First of all, that one is not a function, because it doesn't take any parameter.

However, type annotation is saying that the main is of type Html with a not yet known variant msg.

Html is the type, like Int or String, and msg is the type variant.

Some types can have variants, take List a as an example.

List a means that you are declaring a List where the type variant is irrelevant or not yet known. For instance, the type annotation for List.length method is:

length : List a -> Int

When you'll use this method, you'll certainly have a type instead of a, like String or Int or whatever, by the behaviour of length doesn't change.

Don't be confused then by the msg addition to Html. It has the same meaning. In Elm you render html code that may dispatch some messages, and you usually define them as a union type like:

type Msg
  = ClickedSave
  | EnteredText String

So, your main declaration can remain Html msg, or Html a until you dispatch a message.

If you, for instance, add an onClick handler to your div like:

main =
    div [ onClick ClickedSave ] [ text "1" ]

the type annotation becomes (whether you annotate it or not) Html Msg, because now the message you are dispatching is of type Msg.

Hope it's clear, and sorry for my english, it's not my native language.

Upvotes: 9

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