purescript-formless-aj

Formless-Independent is a general purpose form library for purescript, independent of any UI frameworks. Provide Formless with some initial inputs, and validation to run on those inputs, and it will handle the tedious parts of managing form state, errors, submission, and more.

Formless-Independent is basically Formless, but with all the Halogen or UI specific bits ripped out, and the interface adapted to enable it to work with any framework. It was earlier released as Concur-Formless for use with the Concur UI library, but I realised that it didn't need to have a dependency on Concur either. Now it's truly independent.

Currently the library is released as formless-aj to avoid clashing with an official future lib with the name formless-independent. The creation of such a lib is being tracked here.

Install with Bower:

bower i --save purescript-formless-aj

Or Spago -

spago install purescript-formless-aj

Writing forms is not simple. Handling changes for each field and their validations, and then handling errors can quickly become tedious. Formless helps abstract away most of the messy details of managing form state without imposing any restrictions on how you render your form.

To demonstrate, let's build a signup form in Formless. We'll use Concur for the view layer.

Define the data type we want our form to result in: a User.

type User =
  { id :: Int
  , name :: String
  , email :: Email
  }

Easy-peasy.

Define the Form Type.

Formless doesn't require the type of the form, and the type of the result type to match.

User is the result data type we'll use throughout our application, but our form will have different fields altogether: we want them to provide two email addresses for confirmation purposes, and we don't have an ID for them until the form has been submitted.

Formless requires a specific shape from our Form data type. We are expected to write a newtype that takes two arguments, r and f below, and a row containing the fields in the form.

Every field should use the second argument, f, and provide it with three type arguments:

• an error type, which represents possible validation errors for the field
• an input type, which represents the value the user will provide when interacting with the field
• an output type, which represents the type you'd like to result from successful validation

Here's how we need to define our form type:

-- Note: Common practice to use `Void` to represent "no error possible"
newtype UserForm r f = UserForm (r
  ( name   :: f Error String String -- | String input to String output, or Error on failed validation
  , email1 :: f Error String Email  -- | String input to Email output, or Error on failed validation
  , email2 :: f Error String Email  -- | String input to Email output, or Error on failed validation
  ))
derive instance newtypeUserForm :: Newtype (UserForm r f) _

Formless will use this type to perform all kinds of transformations and track data about your form over time. You simply need to decide what fields will exist and what their error, input, and output types are.

-- This type synonym will throw away most of its arguments, preserving only the last type. Since
-- it takes three arguments, it fits the kind (Type -> Type -> Type -> Type), which is exactly what
-- we need to provide as our `Form` newtype's second argument.
type OutputType e i o = o

-- Let's fill in each occurrence of `f` with `OutputType`
myForm :: Form Record OutputType
myForm = Form
  { name   :: OutputType Error String String
  , email1 :: OutputType Error String Email
  , email2 :: OutputType Error String Email
  }

-- This isn't much less confusing, so let's take things a step further. What if we act as the
-- compiler does and erase the type synonym? After all, OutputType is equivalent to only the
-- third type argument from each field.
myForm2 :: Form Record OutputType
myForm2 = Form
  { name   :: String
  , email1 :: Email
  , email2 :: Email
  }

-- `myForm` and `myForm2` are exactly equivalent! Accepting a type that itself accepts three
-- arguments allows us to represent several different sorts of records and variants from the
-- same underlying row and can result in quite simple data types despite the admittedly
-- complicated-looking original type.

newtype Email = Email String

data Error
  = Required
  | NotEqual String String
  | EmailIsUsed
  | EmailInvalid

We need to define 2 values which will completely specify our form behaviour.

Let's first define some helpers (TODO: Need to move these into Formless).

type UserInputForm = UserForm Record F.InputField
type UserOutputForm = UserForm Record F.OutputField
type UserValidators = UserForm Record (F.Validation UserForm (Widget HTML))
type UserFormState = F.State UserForm (Widget HTML)

Define the initial form input values. These are the values that will be displayed when the form is first rendered. It has the type Form Record F.InputField. Remember: Form is our custom newtype we defined a moment ago, and it was awaiting a type that would be applied to the error, input, and output types we defined for each field -- like F.InputField!

newtype InputField error input output = InputField input

Applied to our form, an InputField represents the input type only. We can give Formless a valid record of inputs by just supplying concrete input values for each field:

inputs :: UserInputForm
inputs = UserForm
  { name: InputField ""
  , email1: InputField ""
  , email2: InputField ""
  }

OR It's a little tedious writing out all those newtypes, so Formless.Spec.Transform provides helper functions to generate them for you:

inputs :: UserInputForm
inputs = F.wrapInputFields
  { name: ""
  , email1: ""
  , email2: ""
  }

OR you don't even have to do this: if your input types belong to the Formless.Initial type class (all monoidal values do), it can generate the values for you from a proxy for your form:

proxy = F.FormProxy :: F.FormProxy Form

inputs :: UserInputForm
inputs = F.mkInputFields proxy

The next thing Formless requires is a record of validators: functions that will be run on the form to validate the inputs and produce the specified output types. Every field in this record ought to use the Formless Validation type:

newtype Validation form m error input output
  = Validation (form Record FormField -> input -> m (Either error output))

A Validation type represents a function which takes your entire form, the input for this particular field, and produces either an error or result.

• This function can be monadic, so you can do things like confirm with a server that an email is not already in use.
• This function takes your entire form as an argument, so you can use the values of other fields during validation. For example, you could verify that two password fields are equal to one another.
• If you are using purescript-validation and already have a composed validation function that results in V, then you can convert it into a Formless validator with hoistFnE_ <<< Data.Validation.Semigroup.toEither (or the Semiring module).

-- This helper function lets you take any function from `input` to `output` and turns it into
-- the Validation type from Formless.
hoistFn_ :: ∀ form m e i o. Monad m => (i -> o) -> Validation form m e i o

-- For example, this validator simply transforms the input `Int` into a `String` using `hoistFn_`
-- output.
myStringValidator :: ∀ form m. Monad m => Validation form m Void Int String
myStringValidator = hoistFn_ show

-- This helper function lets you take any function from `input` to `Either error output` and turns
-- it into the Validation type from Formless.
hoistFnE_ :: ∀ form m e i o. Monad m => (i -> Either e o) -> Validation form m e i o

-- For example, this validator makes sure that the string is not empty
isNonEmpty :: ∀ form m. Monad m => Validation form m Error String String
isNonEmpty = hoistFnE_ $ \str ->
  if null str
     then Left Required
     else Right str

-- This validator transforms the input into an `Email` type if successful.
validEmail :: ∀ form m. Monad m => Validation form m Error String Email
validEmail = hoistFnE_ $ \str ->
  if contains (Pattern "@") str
     then Right (Email str)
     else Left EmailInvalid

-- Continuing the trend, this helper takes a function from `input` to a monad `m (Either error output)` and
-- turns it into the Validation type from Formless.
hoistFnME_ :: ∀ form m e i o. Monad m => (i -> m (Either e o)) -> Validation form m e i o

-- For example, this validator makes sure that an email address is not in use. Notice how it relies
-- on the input value already being an `Email` -- we'll see how to chain validators together so this
-- can be used with `validEmail` in a moment.
emailNotUsed :: ∀ form. Validation form Aff Error Email Email
emailNotUsed = hoistFnME_ $ \email -> do
  isUsed <- checkEmailIsUsed :: Email -> Aff Boolean
  pure $
    if isUsed
      then Right email
      else Left EmailIsUsed

-- Now, let's do something a little more complex. Let's validate that two fields are equal to one another.

-- This time, we want to rely on our existing `Form` as an argument for our validation, so instead of using
-- `hoistFnE_` we'll reach for `hoistFnE`, which doesn't throw away the form argument.
hoistFnE :: ∀ form m e i o. Monad m => (form Record FormField -> i -> Either e o) -> Validation form m e i o

-- We'll use `getInput` from Formless to retrieve the input value of the field "email1" from the form, and then
-- we'll validate that the current field is equal to it. Formless can prove that a "email1" field exists using
-- your form row, so you'll never access a value you don't have.
equalsEmail1 :: ∀ m. Monad m => Validation Form m Error String String
equalsEmail1 = hoistFnE $ \form str ->
  let e1 = F.getInput (SProxy :: SProxy "email1") form
   in if str == e1
        then Right str
        else Left $ NotEqual str e1

These validators are building blocks that you can compose together to validate any particular field. Now that we've got some validation functions we can provide our validators record to Formless:

validators :: UserValidators
validators = UserForm
  { name: isNonEmpty
  , email1: isNonEmpty >>> validEmail >>> emailNotUsed
  , email2: isNonEmpty >>> equalsEmail1 >>> emailNotUsed
  }

Note how validators can be composed: validEmail takes a String and produces an Email, which is then passed to emailNotUsed, which takes an Email and produces an Email. You can use this to build up validators that change a field's output type over time. Composition with >>> will short-circuit on the first failure.

The last thing you're expected to do is compose the form fields together into a rendered form. Formless allows you to handle rendering in any way you want, and provides a few parts to plumb pieces together.

While rendering the form, you can use helper functions to get various parts of a field, given a field label; these include getInput, getResult, getError, and more.

User events on your form should return values of the type Query form which represents all the actions you can take on a form data in response to user input. You can use the various helper functions defined in Formless.Query to construct them.

Here's some more information on the query helpers:

• You should use F.set to set a field's value, F.modify to modify a field with a function, F.validate to validate fields, and F.setValidate or F.modifyValidate to do both at the same time
• You should use F.submit to submit the form
• If you want to avoid running expensive or long-running validations on each key press, use the asynchronous versions (F.asyncSetValidate, etc.) and provide a number of milliseconds to debounce. You can use getResult to show a loading spinner when the result is Validating.
• If you need to chain multiple operations, you can use F.andThen to provide multiple Formless queries

Let's render the form we have built using the Concur UI framework.

Notice how the result type of the renderer is the Query.

Also note how we fetch and display the error messages associated with each field.

renderFormWidget :: UserFormState -> Widget HTML (Query UserForm)
renderFormWidget st = do
  query <- D.div'
    [ D.input
      [ P.value $ F.getInput _name fstate.form
      , (F.set _name <<< P.unsafeTargetValue) <$> P.onChange
      ]
    , errorDisplay $ F.getError _name fstate.form
    , D.input
      [ P.value $ F.getInput _email1 fstate.form
        -- This will help us avoid hitting the server on every single key press.
      , (F.asyncSetValidate debounceTime _email1 <<< P.unsafeTargetValue) <$> P.onChange
      ]
    , errorDisplay $ F.getError _email1 fstate.form
    , D.input
      [ P.value $ F.getInput _email2 fstate.form
      , (F.asyncSetValidate debounceTime _email2 <<< P.unsafeTargetValue) <$> P.onChange
      ]
    , errorDisplay $ F.getError _email2 fstate.form
    ]
  where
    _name = SProxy :: SProxy "name"
    _email1 = SProxy :: SProxy "email1"
    _email2 = SProxy :: SProxy "email2"
    debounceTime = Milliseconds 300.0
    errorDisplay = maybe mempty (\err -> D.div [P.style {color: "red"}] [D.text $ V.toText err])

prx :: F.SProxies Form
prx = F.mkSProxies (F.FormProxy :: F.FormProxy Form)

-- These are now equivalent
x = SProxy :: SProxy "name"
x = prx.name

When we obtain a Query from the rendering function in response to a user event, we must use the F.eval function to evaluate the query and update the form state data.

type Query form
eval :: Query form -> m (Maybe (form Record OutputField))

eval returns Right outputForm if the form was successfully submitted, or Left state if the state changed, but the form was not submitted, or the submission was not successful. When we have an updated state, we can rerender the form with the updated state.

We can extract the fields from the outputForm using a helper function F.unwrapOutputFields.

Once again, using Concur, we can wrap this render, update, loop into a nice little Widget -

formWidget :: UserFormState -> Widget HTML User
formWidget st = do
  query <- renderFormWidget st
  res <- F.eval query st
  case res of
    Left st' -> formWidget st'
    Right form -> do
      let values = F.unwrapOutputFields form
      -- Assuming some effectful computation to receive the ID
      id <- registerUser { name: values.name, email: values.email1 }
      let user = { id, name: values.name, email: values.email1 }
      pure user

Whew! Now we have a nicely encapsulated form which can be used on other parts of our application.

import Formless as F

main :: Effect Unit
main = runWidgetInDom "root" page

page :: Widget HTML Unit
page = do
  user <- D.div'
    [ D.h1' [D.text "Contact Form"]
    , formWidget (initState initialInputs validators)
    ]
  liftEffect $ Console.log $ "Got a user! " <> show (user :: User)