Linter Rule: Validate the kind a bound form control holds

Rule: herb-state-valid-bindings

Description

Validates two-way bindings between form controls and declared states. A checked or selected attribute reading a state binds a boolean, so the state must be declared as one. A value attribute or a <textarea>'s content holds text, so the state must be a String, or an Integer for numeric inputs. A derived state cannot be bound at all, since a binding writes back what the user changes and a derived value follows from its sources.

Rationale

A state read into a form control is a two-way binding by construction. The control writes the state back on input or change, so the kinds have to line up. A checkbox writing into a String state would store "true" where a boolean is expected, and a value displaying a Boolean would render the word and then overwrite the flag with arbitrary text.

The type comes from the declaration's default, which is why declaring states with primitive defaults matters. A state seeded from an expression has no static kind and is exempt.

Examples

✅ Good

<%# herb:slots client %>
<%# herb:state (draft: "", agreed: false, attempts: 0) %>

<input value="<%= draft %>" autocomplete="off">
<input type="checkbox" checked="<%= agreed %>">
<textarea><%= draft %></textarea>
<input type="number" value="<%= attempts %>" autocomplete="off">

🚫 Bad

<%# herb:slots client %>
<%# herb:state (draft: "", agreed: false) %>

<input type="checkbox" checked="<%= draft %>">
`checked` binds the String state `draft`, and `checked` holds a boolean. Declare it as one, like `(draft: false)`, or bind a different state. (herb-state-valid-bindings)
<input value="<%= agreed %>" autocomplete="off">
`value` on `<input>` binds the Boolean state `agreed`, and a `value` holds text. Declare it as a String, like `(agreed: "")`. (herb-state-valid-bindings)
<textarea><%= agreed %></textarea>
`<textarea>` binds the Boolean state `agreed`, and a textarea holds text. Declare it as a String, like `(agreed: "")`. (herb-state-valid-bindings)

References

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