A renderless component to build forms in Halogen
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Formless is a renderless component which helps you build forms in Halogen. Provide Formless with some initial inputs, validation to run on those inputs, and a render function, and the component will handle the tedious parts of managing form state, errors, submission, and more.

You can write a complete Halogen form component with multiple fields, validation, parsing, and errors in less than 100 lines of code (only ~20 lines of which are from Formless).

Have any comments about the library or any ideas to improve it for your use case? Please file an issue, send me an email, or reach out on the PureScript user group.


Install with Bower:

bower i --save purescript-halogen-formless


The default approach to forms in Halogen is to write a component and, for every field in your form, queries to handle changes on those fields and validation. Each form field lives in the state type along with validation results, summary information (like whether fields have been edited), and a possible form output.

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.

Data Types

We'll start with the data type we want our form to result in: a User.

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

This is the 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 your Form data type. You are expected to write a newtype that takes two arguments, r and f below, and a row containing the fields in your form.

Expand to read about these two type arguments The first argument has the kind `(# Type -> Type)` and turns a row of types into a concrete type. For example, you can fill in `Record` to get a record; `Record (name :: String)` is the same as `{ name :: String }`. However, Formless will often fill in `Variant` internally. This lets the library access the entire form at once (`Record`) or a single field (`Variant`) to perform various operations. The important thing is that you make sure this variable is left free in your `Form` newtype.

The second argument has the kind (Type -> Type -> Type -> Type) and will be filled in with one of many types Formless uses internally to manage your form. It expects an error type, an input type, and an output type for the field in question.

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 what our form type looks like:

-- Note: Common practice to use `Void` to represent "no error possible"
newtype Form r f = Form (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 newtypeForm :: Newtype (Form 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.

Expand to read a longer explanation of this form type

This can be a scary type to look at, but it's not so bad once you provide concrete types for r and f. For example, let's try providing Record and the OutputType type from Formless:

-- 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.
Expand to see the definition of the Error and Email types
newtype Email = Email String

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

Component Inputs

Now that we have a form type and an output type we can produce the Input type that the Formless component requires. While we'll take a closer look at each of these types in the next few sections, here's a quick primer on what these types are:

  • initialInputs: Your Form newtype around a record, where each field contains its initial, starting value
  • validators: Your Form newtype around a record, where each field contains a validation function which will process its input value
  • render: The render function the component will use, which is the standard State -> HTML type in Halogen
import Formless as F

type FormlessInput m =
  { initialInputs :: Form Record F.InputField
  , validators :: Form Record (F.Validation Form m)
  , render :: F.State Form m -> F.HTML' Form m

Form Inputs

The first thing Formless requires is a record of the fields in your form with their initial values. 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 :: Form Record F.InputField
inputs = Form
  { name: InputField ""
  , email1: InputField ""
  , email2: InputField ""

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

inputs :: Form Record F.InputField
inputs = F.wrapInputFields
  { name: ""
  , email1: ""
  , email2: ""

In fact, 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 :: Form Record F.InputField
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))

This 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).

The FormField newtype represents the state of every field in the form:

newtype FormField error input output = FormField
  { -- The value the user will input
    input :: input
    -- Whether the field has been modified yet (validators ignore untouched fields)
  , touched :: Boolean
    -- The result of validation, IF validation has been run on this field
  , result :: FormFieldResult error output

A field's result can be in one of several states, represented by the FormFieldResult type:

data FormFieldResult e o
  = NotValidated
  | Validating -- Useful to display a loading spinner during asynchronous / long validations
  | Error e
  | Success o

Let's see some examples of validators written in this style:

-- 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 :: Form Record (F.Validation Form Aff)
validators = Form
  { 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.

Render Function

The last thing you're expected to provide is a render function. Formless is a renderless component, so it provides no rendering at all and expects you to provide an entire render function of the type ∀ m. F.State Form m -> F.HTML' Form m. To learn more about renderless components, see the purescript-halogen-renderless library.

The main things to keep in mind when writing a render function for Formless:

  • You can pass arguments to the function before it is given to Formless (like your parent state). When the parent component re-renders, these values will be given to Formless anew.
  • You can extend Formless' functionality by embedding your own queries in the render function with Raise
  • You can mount external components inside Formless and control them from the parent with send and send'
  • 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
  • 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
  • There are functions to get various parts of a field, given a symbol; these include getInput, getResult, getError, and more.

Let's write a render function using setValidate, asyncSetValidate, and getInput, using symbol proxies we've defined in the where clause:

renderFormless ::  m. F.State Form m -> F.HTML' Form m
renderFormless fstate =
  [ HH.input
    [ HP.value $ F.getInput _name fstate.form
    , HE.onValueInput $ HE.input $ F.setValidate _name
  , HH.input
    [ HP.value $ F.getInput _email1 fstate.form
      -- This will help us avoid hitting the server on every single key press.
    , HE.onValueInput $ HE.input $ F.asyncSetValidate debounceTime _email1
  , HH.input
    [ HP.value $ F.getInput _email2 fstate.form
    , HE.onValueInput $ HE.input $ F.asyncSetValidate debounceTime _email2
    _name = SProxy :: SProxy "name"
    _email1 SProxy :: SProxy "email1"
    _email2 = SProxy :: SProxy "email2"
    debounceTime = Milliseconds 300.0

It can be tedious to write out symbol proxies for every field you want to access in a form. You can instead generate a record of these proxies automatically using the mkSProxies function:

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

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

Now, instead of writing out proxies over and over, you can just import the proxies record!

Mounting The Component

Whew! With those three functions and the Form type, we've now got everything necessary to run Formless. Let's bring it all together by mounting the component and handling its Submitted output message:

import Formless as F

data Query a
  = Formless (F.Message' Form) a

type ChildQuery = F.Query' Form Aff
type ChildSlot = Unit

component :: H.Component HH.HTML Query Unit Void Aff
component = H.parentComponent
  { initialState: const unit
  , render
  , eval
  , receiver: const Nothing


  render :: Unit -> H.ParentHTML Query ChildQuery ChildSlot Aff
  render st =
    [ HH.h1 "My Form"
    , HH.slot unit F.component
        { initialInputs, validators, render: renderFormless }
        (HE.input Formless)

  eval :: Query ~> H.ParentDSL Unit Query ChildQuery ChildSlot Void Aff
  eval (Formless m a) = case m of
    F.Submitted formOutput -> a <$ do
      let form = F.unwrapOutputFields formOutput
      -- Assuming some effectful computation to receive the ID
      id <- registerUser { name: form.name, email: form.email1 }
      let user = { name: form.name, email: form.email, id }
      liftEffect $ Console.log $ "Got a user! " <> show (user :: User)
    _ -> pure a

Next Steps

Ready to move past this simple example? Check out the examples, which vary in their complexity:

If you're curious to learn more about how to use renderless components effectively, or build your own: