Collecting data from forms is a common requirement in web applications. The Cro::WebApp::Form module aims to take much of the tedium out of doing so. It works as follows:

  • One defines a form by writing a class, using traits to specify the form controls and validation requirements. The class should do the Cro::WebApp::Form role.

  • The Cro::WebApp::Template standard library includes a &form built-in that can be used to render the form without having to write out the form HTML.

  • An empty instance of the form can be created using the empty method on the form class.

  • A form-data router function can be used to obtain an instance of the class constructed with submitted form data. The is-valid method can be called to check if it is valid; if it is not, it can be re-rendered using the template, and validation errors will be displayed.

A basic example§

Defining the form§

Consider the following example:

class Review does Cro::WebApp::Form {
    has Str $.name is required;
    has Int $.rating is required will select { 1..5 };
    has Str $.comment is multiline(:5rows, :60cols) is maxlength(1000);

This defines a form with three fields:

  • A name, which will be rendered as a textbox

  • A rating, which will be a select dropdown box with the options 1 through 5

  • A comment, which will be rendered as a text area of 5 rows and 60 columns

Validation wise, the first two fields are required, while the third has a maximum length of 1000 characters.

A template to render it§

The &form builtin can be used to render a form. For example, we can set up a review.crotmp template to render the form like this:

    <h1>Submit a review</h1>
    <&form(.form, :submit-button-text('Send your review'))>

The routes§

Given the form and template, the routes can be defined as follows:

sub routes() is export {
    route {
        # Render an empty form first
        get -> {
            template 'templates/review.crotmp', { form => Review.empty }

        # When it is submitted, validate it, and render it again with validation
        # errors if there are problems. Otherwise, accept the review.
        post -> {
            form-data -> Review $form {
                if $ {
                    note "Got form data: $form.raku()";
                    content 'text/plain', 'Thanks for your review!';
                else {
                    template 'templates/review.crotmp', { :$form }

Defining forms§

Each attribute in the form class with an accessor (that is, declared like has $.foo) will result in a form field. You may add further private attributes and methods to the form class as you wish; for example, it may be convenient to have the form class carry methods to load and save itself using a database.

Attribute types§

The $ sigil should be used for most attributes, with the exception of the case of a multi-select list, where @ may be used instead.

An attribute with no type is equivalent to one typed with Str. It is also allowed to use the numeric types Int, Rat, and Num. The values will be parsed into these types, and a validation error produced if they are malformed. This will also cause the generation of a number input type in the rendered HTML.

Some further attribute types come with control type defaults too:

  • Bool will result in a checkbox control
  • Date will result in a date control
  • DateTime will result in a datetime-local control

Form controls§

Traits are used to describe the kinds of controls that will be used on a form. The full set of HTML5 control types are available. Remember to check browser support for them is sufficient if needing to cater to older browsers. They mostly follow the HTML 5 control names, however in a few cases alternative names are offered for convenience. Taking care to use is email and is telephone is especially helpful for mobile users.

  • is password - a password input
  • is number - a number input (set implicitly if a numeric type is used)
  • is color - a color input
  • is date - a date input
  • is datetime-local / is datetime - a datetime-local input
  • is email - an email input
  • is month - a month input
  • is multiline - a multiline text input (rendered as a text area); can have the number of rows and columns specified as named arguments, such as is multiline(:5rows, :60cols)

  • is tel / is telephone - a tel input for a phone number
  • is search - a search input
  • is time - a time input
  • is url - a url input
  • is week - a week input
  • will select { ... } - a select input, offering the options specified in the block, for example will select { 1..5 }. If the sigil of the attribute is @, then it will render a multi-select box. While self is not available in such a trait, it is passed as the topic of the block, so one can write a method get-options() { ... } and then do will select { .get-options }. Note that currently there is no assistance with handling situations where the options should depend on another form field.

  • is file - a file upload input; the attribute will be populated with an instance of Cro::HTTP::Body::MultiPartFormData::Part, which has properties filename, body-blob (binary upload) ond body-text (decodes the body-blob to a Str)

  • is hidden - a hidden input

There is no trait for checkboxes; use the Bool type instead.

Labels, help texts, and placeholders §

By default, the label for the control is formed by:

  • Taking the attribute name

  • Replacing each - with a space

  • Calling tclc to title case it

Use the is label('Name') trait in order to explicitly set a label.

For text inputs, one can also set a placeholder using the is placeholder('Text') trait. This text is rendered in the textbox prior to the user filling it.

Finally, one may use the is help('...') trait in order to provide help text. This is displayed beneath the form field.


The various standard HTML5 validations are available and can be set up using traits on the form attributes. Some have been given aliases so as to allow for more Raku-ish code.

  • is min-length(5) / is minlength(5) - set the minimum length of a text input
  • is max-length(500) / is maxlength(500) - set the maximum length of a text input
  • is min(1) - set the minimum value of a numeric input
  • is max(100) - set the maximum value of a numeric input
  • is required - indicates that the form field is required

All of these are validated both server side and result in the appropriate client side attributes being placed on the form fields during form rendering.

Further server-side validation at the field level can be specified by using the is validated($match-me, 'Message') trait, which will use the given validation error message if the input values fails to smartmatch against the condition. For example, it could be used with a regex:

has $.username is validated(/^<[A..Za..z0..9]>+$/, 'Only alphanumerics are allowed');

Or code (whatever code or block):

has Int $.places is validated(* %% 2, 'Must be an even number of places');
has Str $.title is validated({ !.contains('TODO') }, 'Title must not be still TODO');

Form-level validation is implemented by writing a validate-form method, which calls add-validation-error with each problem it finds:

my class BlogPost does Cro::WebApp::Form {
    has Str $.title is required is minlength(5) is maxlength(100)
            is placeholder('Enter a title') is help('5 to 100 chars');
    has Str $.content is required is multiline(:5rows, :80cols)
            is minlength(5) is maxlength(100000);
    has Str $.category will select { 'Coding', 'Photography', 'Trains' };

    method validate-form(--> Nil) {
        if $!title eq $!content {
            self.add-validation-error("Cannot just repeat title as post body");

The validity of the form can be tested by calling is-valid on the form. Until this is called, the validation errors will not be populated.

Creating form instances§

The form can be:

  • Created empty using TheForm.empty (use this instead of .new, since that will give an error if is required fields are missing)

  • Parsed from a application/x-www-form-urlencoded form body (TheForm.parse($body)); any values that don't parse into numeric types will be retained in a shadow storage so they can be rendered back to the browser along with the validation error, and missing required values also do not prevent construction

  • Created like a normal object (|%values)); this will enforce is required and type constraints

Typically, however, one shall not use TheForm.parse, but rather the form-data router function:

post -> {
    form-data -> Review $form {
        if $ {
            # Process it
        else {
            # Render it with the errors

This function will:

  • Look at the expected type of form by introspecting the type of the parameter to the block

  • Obtain the request body, doing the await for you

  • Pass it to the parse method of the appropriate form (in this case, Review.parse($body))

  • Invoke the block with the result of that

It does not trigger validation; this should be done with an explicit is-valid call.


The &form built-in template function can be used to render the form. It requires the form object to be passed as a single named argument:


It also takes a wide range of named arguments. Some control form behavior:

  • action sets the action attribute of the form (where it submits to); by default it is not set, so the form submits back to the current URL

  • method sets the HTTP method for submitting the form. It defaults to post, and may be set to get or post

  • novalidate sets the novalidate attribute of the form, disabling the browser's built-in client side validation

Some control texts used in the form:

  • submit-button-text - the text placed on the form submit button
  • form-errors-text - text that comes before form-level errors are rendered

Others allow CSS classes to be placed on form elements to style them:

  • input-group-class - goes on the div enclosing the form label and control for most inputs, but not check boxes

  • input-label-class - goes on the label element for an input, except check boxes
  • input-control-class - goes on the input or select element for an input, except check boxes

  • check-group-class - goes on the div enclosing a check box and its label
  • check-label-class - goes on the label element for a check box
  • check-control-class - goes on the input element for a check box
  • was-validated-class - applied to to form only if it was validated (so not applied when an empty form is rendered)

  • is-invalid-class - applied to form controls that fail validation
  • invalid-feedback-class - applied to the validation message for a control
  • form-errors-class - applied to the div rendered above any form controls when there are form-level validation errors; this contains a ul which in turn has an li for each error

  • help-class - applied to the element containing a control's help text
  • submit-button-class - applied to the submit button

CSRF protection§

Provided the form is processed and rendered in the dynamic scope of a route handler, CSRF protection will automatically be applied. This is achieved by use of the Double Submit Cookie approach: a session cookie is set with a random token (if one is already present, it will be re-used), and then this token is also placed into a hidden field in the form.