Use attributes instead of properties

[lydell]

Intro

This PR is a companion to elm/virtual-dom#187. That elm/virtual-dom PR works without this elm/html PR, but virtualization won’t work 100 % without it.

This PR removes the stringProperty helper function, and uses attributes for the exposed functions that used it instead. The boolProperty function is kept, and I added a comment for why.

Details

Most functions in Html.Attributes that take a String used this helper function:

stringProperty : String -> String -> Attribute msg
stringProperty key string =
  Elm.Kernel.VirtualDom.property key (Json.string string)

For example, href:

href : String -> Attribute msg
href url =
  stringProperty "href" (Elm.Kernel.VirtualDom.noJavaScriptUri url)

In other words, lots of the Html.Attributes functions were implemented by setting properties.

This PR removes stringProperty, and instead prefers attributes over properties. Here’s href in this PR:

href : String -> Attribute msg
href url =
  Elm.Kernel.VirtualDom.attribute "href" (Elm.Kernel.VirtualDom.noJavaScriptUri url)

In short, attributes are preferred because:

1. Attributes can be removed, while properties often cannot. Closes elm/html#228, closes elm/html#148, closes elm/virtual-dom#122, closes elm/virtual-dom#169
2. Virtualization is way easier when most Html.Attributes functions are attributes. Closes elm/virtual-dom#144
3. Some properties are read-only and therefore throw errors if you try to set them.
4. Attributes are easier to diff.

I explain those points more in the properties-vs-attributes.md file (which I updated in this PR).

[lydell]

This was unused (not exposed).

[lydell]

value is an exception where we have to use a property, not an attribute.

[lydell]

Properties are diffed against the actual DOM, not the virtual DOM, so this ensures that the DOM is up-to-date with the model.

This refers to the implementation in elm/virtual-dom#187