Lazy.md

Module Html.Lazy

Since all Elm functions are pure we have a guarantee that the same input will always result in the same output. This module gives us tools to be lazy about building Html that utilize this fact.

Rather than immediately applying functions to their arguments, the lazy functions just bundle the function and arguments up for later. When diffing the old and new virtual DOM, it checks to see if all the arguments are equal. If so, it skips calling the function!

This is a really cheap test and often makes things a lot faster, but definitely benchmark to be sure!

lazy

lazy :: forall msg a. Eq a => (a -> Html msg) -> a -> Html msg

A performance optimization that delays the building of virtual DOM nodes.

Calling (view model) will definitely build some virtual DOM, perhaps a lot of it. Calling (lazy view model) delays the call until later. During diffing, we can check to see if model is referentially equal to the previous value used, and if so, we just stop. No need to build up the tree structure and diff it, we know if the input to view is the same, the output must be the same!

lazy_

lazy_ :: forall msg a. (a -> Html msg) -> a -> Html msg

Like lazy, but without the Eq constraint.

lazy2

lazy2 :: forall msg a b. Eq a => Eq b => (a -> b -> Html msg) -> a -> b -> Html msg

Same as lazy but checks on two arguments.

lazy2_

lazy2_ :: forall msg a b. (a -> b -> Html msg) -> a -> b -> Html msg

Like lazy2, but without the Eq constraint.

lazy3

lazy3 :: forall msg a b c. Eq a => Eq b => Eq c => (a -> b -> c -> Html msg) -> a -> b -> c -> Html msg

Same as lazy but checks on three arguments.

lazy3_

lazy3_ :: forall msg a b c. Eq a => Eq b => Eq c => (a -> b -> c -> Html msg) -> a -> b -> c -> Html msg

Like lazy3, but without the Eq constraint.