What's up with Html a ?

1-README.md

What's up with Html a?

Ever wonder why sometimes the compiler yells at you for using type Html a ? Why is a ok some times but not others?

It's not unique to Html a. Look at the same problem using a simpler type we all understand: List a.

Here are a few questions and thoughts to ponder:

• Compare the myList and myHtml functions. How are they similar? How are they different?
• Why do you think these functions will compile or not compile?
• Compare the compiler errors for each
• How does making a singature more generic limit what you can do in the implementation?

2-TooGeneric.elm

module TooGeneric exposing (..)

import String
import Html exposing (Html, div)
import Html.Events exposing (onClick)


myList : List a
myList =
    [ String.toUpper "abc" ]


type Msg
    = Foo
    | Bar


myHtml : Html a
myHtml =
    div [ onClick Foo ] []

3-errors.txt

-- TYPE MISMATCH ------------------------------------------------------ Main.elm

The type annotation for `myHtml` does not match its definition.

18| myHtml : Html a
             ^^^^^^
The type annotation is saying:

    Html a

But I am inferring that the definition has this type:

    Html Msg

Hint: A type annotation is too generic. You can probably just switch to the type
I inferred. These issues can be subtle though, so read more about it.
<https://github.com/elm-lang/elm-compiler/blob/0.17.1/hints/type-annotations.md>

-- TYPE MISMATCH ------------------------------------------------------ Main.elm

The type annotation for `myList` does not match its definition.

8| myList : List a
            ^^^^^^
The type annotation is saying:

    List a

But I am inferring that the definition has this type:

    List String

Hint: A type annotation is too generic. You can probably just switch to the type
I inferred. These issues can be subtle though, so read more about it.
<https://github.com/elm-lang/elm-compiler/blob/0.17.1/hints/type-annotations.md>

Detected errors in 1 module.