Elm in Action

Appendix C. Html.Lazy’s change check

This appendix adds details about the Html.Lazy function first used in chapter 8

• How lazy works with strings and numbers
• How it works for other values
• How it works when memory locations change

In chapter 8, we used the Html.Lazy.lazy function to skip running view code if certain values did not change on repeated calls. How does lazy decide whether a value has changed? The answer to this question involves two important design goals:

• lazy must never skip calling the function if there is any chance the function could return a different value. At worst, lazy should be an ineffective performance optimization; it should never cause bugs under any circumstances.
• Because it’s a tool for making things run faster, ...