Forms & Changesets

The changeset module validates form submissions without reflection or macros — just functions over Dict(String, String).

Casting params

import mistweaver/changeset

let cs =
  form_params           // List(#(String, String)) from the request body
  |> changeset.cast(required: ["email", "password"])

cast marks the changeset invalid if any required field is missing or blank.

Validations

Chain validations on a changeset:

let cs =
  params
  |> changeset.cast(required: ["username", "email", "password"])
  |> changeset.validate_length("username", min: Some(3), max: Some(20))
  |> changeset.validate_length("password", min: Some(8), max: None)
  |> changeset.validate_format("email", with: fn(v) {
    string.contains(v, "@")
  }, message: "must be a valid email address")

Checking validity

case changeset.valid(cs) {
  False -> render_form_with_errors(cs)
  True  -> {
    let username = changeset.get_or(cs, "username", "")
    let email    = changeset.get_or(cs, "email", "")
    create_user(username, email)
  }
}

Rendering errors

let username_errors = changeset.errors_for(cs, "username")
// ["must be at least 3 characters"]

In a Lustre template:

case changeset.errors_for(cs, "email") {
  [] -> html.text("")
  [first, ..] -> html.span([class("error")], [html.text(first)])
}

Available validations

Function	Description
validate_length(field, min:, max:)	String length bounds
validate_format(field, with:, message:)	Predicate on the value
validate_inclusion(field, in:)	Value must be in a list