formal

Type safe HTML form decoding and validation!

gleam add formal@3

import formal/form

// Define a type that is to be decoded from the form data
pub type SignUp {
  SignUp(email: String, password: String)
}

/// A form that decodes the `Signup` value.
fn signup_form() -> Form(Signup) {
  form.new({
    use email <- form.field("email", {
      form.parse_email
    })
    use password <- form.field("password", {
      form.parse_string
      |> form.check_string_length_more_than(7)
    })
    use _ <- form.field("confirm", {
      form.parse_string
      |> form.check_confirms(password)
    })
    SignUp(email: email, password: password)
  })
}

// This function takes the list of key-value string pairs that a HTML form
// produces. It then decodes the form data into a SignUp value, ensuring that
// all the fields are present and valid.
//
pub fn handle_form_submission(values: List(#(String, String))) {
  let result = 
    signup_form()
    |> form.add_values(values)
    |> form.run

  case result {
    Ok(data) -> {
      // Do something with the SignUp value here
    }
    Error(form) -> {
      // Re-render the form with the error messages
    }
  }
}

Examples

• Backend example with Wisp
• Frontend example with Lustre

Further documentation can be found at https://hexdocs.pm/formal.