Strict locals
Strict locals are a Rails feature. A magic comment at the top of a partial declares which locals it takes, and Rails raises when a caller passes one it does not declare or leaves out one it requires. Herb reads the same comment, so the linter can check every render call against it before the page runs.
<%# locals: (post:, compact: false) %>
<article class="<%= "compact" if compact %>">
<h2><%= post.title %></h2>
</article>locals:
Declares the locals a template takes.
<%# locals: (name:, name: default, **rest) %>The signature is a Ruby keyword parameter list. A keyword without a default is required, and a keyword with one is optional. A ** splat accepts any other local, and () declares that the template takes none.
Put the declaration on the first line of the partial, followed by a blank line. Rails reads only the first declaration in a template.
What Herb rejects
Rails passes locals by name, so only keyword parameters mean anything in the signature. Herb reports anything else as a parse error with the strict_locals parser option on.
| Template | Error |
|---|---|
<%# locals: (title) %> | StrictLocalsPositionalArgumentError |
<%# locals: (*items) %> | StrictLocalsSplatArgumentError |
<%# locals: (&block) %> | StrictLocalsBlockArgumentError |
<%# locals: title: %> | StrictLocalsMissingParenthesisError |
A second <%# locals: (...) %> | StrictLocalsDuplicateDeclarationError |
Strict locals only support keyword arguments. Positional argument `title` is not allowed. Use keyword argument format: `title:`.Checking callers
The linter resolves each render call to its partial and compares the locals it passes with the declaration. It reports a required local that is not passed and a local the partial does not declare. It also reports a literal whose type contradicts the default, such as compact: "yes" for compact: false.
<% @posts.each do |post| %>
<%= render "posts/card", post: post, compact: true %>
<% end %>Strict locals and state
A state may take its starting value from a strict local, as in <%# herb:state (open: open_initially) %>. A name cannot be both a strict local and a state, since a local comes from the caller and a state is owned by the client. To share a state with the caller instead of copying its value, bind it with state: on the render call.