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Why and what are formlets?

A few years ago I was trying out WebSharper. There was much I didn't understand about WebSharper but I fell in love with the concept of Formlets/Piglets. For years I have been trying to make others see it as I do; an awesome and productive way to build reactive Forms that have validation.

Formlets are not a generic solution to all UI related problems but they are awesome for Forms (as the name implies).

Like parsers combinators like FParsec allows us build complex parsers from simple and generic building blocks Formlets allows us to build forms from simple and generic input elements.

One of the simplest forms is a simple text input element:

let f = text "Enter your name" ""

A single input formlet

We can add a label to it:

let f = text "Enter your name" "" |> withLabel "Name"

A single labeled input formlet

We can create our input formlet with formlet by creating a function

let input lbl hint validation =
  text hint ""                // Text input
  |> validation               // Apply validation
  |> withLabel lbl            // label the text input
  |> withValidationFeedback   // Display validation failures
  |> withFormGroup            // Wrap it in a form-group (Bootstrap)

There is a formlet computation expression which allows us to combine several inputs

let newUser : Formlet<string*string*string> =
  formlet {
    let! firstName  = input "First name" "Enter first name" notEmpty
    let! lastName   = input "Last name"  "Enter last name"  notEmpty
    let! nickName   = input "Nick name"  "Enter nick name"  yes
    return firstName, lastName, nickName
  } |> withCard "New User"

A customer formlet

We can make a slight validation of the Formlet above by showing the user a checkbox that they tick if they like to enter a nick name.

let newUser : Formlet<string*string*string option> =
  formlet {
    let! firstName  = input "First name" "Enter first name" notEmpty
    let! lastName   = input "Last name"  "Enter last name"  notEmpty
    let! hasNick    = checkBox "Do you have a nick name?"
    let! nickName   =
      if hasNick then input "Nick name"  "Enter nick name"  notEmpty |>> Some
      else value None
    return firstName, lastName, nickName
  } |> withCard "New User"

A customer formlet

The cool thing is that the user will see a checkbox that when ticked will display a required text input for the nick. Normally optional behavior in forms require some kind of event handling, not so with formlets.

In order to render a formlet we use Formlet.mkElement

We render the formlet like a React component:

// onCommit is called when user click onCommit with the value produced by the Formlet
// onCancel is called when user click onCancel
let element = Formlet.mkForm newUser onCommit onCancel
ReactDom.render(element, document.getElementById("react-app"))

mkForm adds Commit, Cancel and Reset buttons as well as showing the validations errors in a summary.

The full customer formlet

Test a more complex formlet live.

I think Formlets are a great way to create great forms quickly and I wished more developers talked about them.

What is fable-formlets?

What is great about formlets is that they can be built upon almost any UI frameworks. fable-formlets is a small formlet library to demonstrate formlets running on top of Fable.React.

Currently fable-formlets is merely a functionally demo and not a serious library.

The core type is Formlet<_>

type Formlet<'T> = Ft of (IdGenerator -> FormletPath -> Model -> Dispatcher -> 'T*ViewTree*FailureTree)

That is; a formlet is function that given a id generator, a path that indicates where we are in the model, the current model element and a dispatcher (that allows the view element dispatch a message to update the model). This function produces a value 'T, a view tree and a failure tree.

A formlet always produces a value in order to make bind not shortcut the rest of the formlet. The value is valid if the FailureTree.IsGood x is true.

For formlet we define the usual combinators; bind, apply, map and a few others. While bind is one of the most flexible ways to compose formlets it is preferable to use apply for performance reasons, like so:

let address lbl =
  // <*> is apply
  value Address.New
  <*> input "Carry over"  ""  Validate.yes
  <*> input "Name"        ""  Validate.notEmpty
  <*> input "Street"      ""  Validate.notEmpty
  <*> input "Street"      ""  Validate.yes
  <*> input "Street"      ""  Validate.yes
  <*> input "Zip"         ""  Validate.notEmpty
  <*> input "City"        ""  Validate.notEmpty
  <*> input "County"      ""  Validate.yes
  <*> input "Country"     ""  Validate.notEmpty
  |> withCard lbl

Part of Core.fs is also a set of validators to demonstrate how to validate formlets.


Core.fs contains generic functionality, however in order to build nice looking forms Boostrap is a pretty neat CSS library. Boostrap.fs contains Bootstrap specific elements.

For example this is how the text input is defined:

let text hint initial : Formlet<string> =
  Ft <| fun ig fp m d ->
    let v =
      // Model needs to be deconstructed. If the model isn't a value
      //  the model is being initially constructed or has changed shape
      //  because of other changes to the model, if so assume the initial value
      match m with
      | Model.Value v -> v
      | _             -> initial

    // These are the attributes we like to add to the input element
    let aa : IHTMLProp list =
          Value         v
          OnChange      <| fun v -> update d v.Value
          Placeholder   hint

    // Construct a view element, it needs to be delayed as it possible
    //  that styles and attribute will flow from parent formlets to this
    //  input element
    //  Class and Style attribute are specified separately because they need to
    //  be accumulated
    //  input is normal Fable.React input element constructor
    //  "form-control" is the bootstrap style
    let tvt = delayedElement input aa "form-control" []

    // Returns the value, the view tree and an empty failure tree
    v, tvt, zero ()

Let's also look at how a label is constructed

let withLabel lbl t : Formlet<_> =
  let t = adapt t
  Ft <| fun ig fp m d ->
    //  id is the identifier we given to tie the label and the labeled element
    //  together
    let id            = IdGenerator.Next ig
    // Update the formlet path with label, this will allow validation
    //  formlets append the correct context
    let fp            = (FormletPathElement.Named lbl)::fp
    // Invoke the labeled formlet
    let tv, tvt, tft  = invoke t ig fp m d
    // Create the label element, this time we don't use delayed elements
    //  as we don't want parent formlets to modify the style of the label,
    //  rather the style of the labeled element
    let e             = label [|HTMLAttr.HtmlFor id|] [|str lbl|]
    // We add the `Id` attribute to the labeled element
    let tvt           = ViewTree.WithAttribute (Id id, tvt)
    // The final view tree is the label and labeled element joined
    let tvt           = join (ViewTree.Element e) tvt

    // Done, return the value from the labeled element, view tree and failure tree
    tv, tvt, tft

So it's nice with out of the box input elements but it is possible to extend with new kinds of inputs.


In my opinion, Formlets are great for building forms, they are extendable, they work great with Fable.React.

fable-formlets might not be right implementation of formlets for Fable.React, perhaps someone more clever can pick up the idea and do a proper implementation.

Let's talk about formlets.


  1. WebSharper - Where I first saw the formlet concept
  2. Haskell formlets - Not surprisingly Haskell people has been thinking about Formlets for a long time.