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Specular is a library for building Web-based UIs in PureScript, based on Functional Reactive Programming (FRP).

The API and DOM interaction is heavily inspired by Reflex and Reflex-DOM. The FRP implementation is based on Incremental (although the algorithm differs in some important ways).


FRP types

To use Specular effectively, you need to be familliar with some basic types.


Dynamic a represents a read-only reference to a changing value of type a.

-- | Read the current value of a `Dynamic`.
readDynamic :: forall m a. MonadEffect m => Dynamic a -> m a

-- | Execute the given action for the current value, and each new value when it changes.
subscribeDyn_ :: forall m a. MonadEffect m => MonadCleanup m => (a -> Effect Unit) -> Dynamic a -> m a

Dynamic is a Monad.

-- `pure` creates a Dynamic that never changes.
pure "foo" :: Dynamic String

-- An applicative combination of Dynamics changes whenever one of them changes.
d1 :: Dynamic Int
d2 :: Dynamic Int
(+) <$> d1 <*> d2 :: Dynamic Int

-- Using the power of Monad we can choose which Dynamic to observe.
which :: Dynamic Bool
(which >>= if _ then d1 else d2) :: Dynamic Int

We can introduce new Dynamics using newDynamic:

-- | Construct a new root Dynamic that can be changed from `Effect`-land.
newDynamic :: forall m a. MonadEffect m => a -> m { dynamic :: Dynamic a, read :: Effect a, set :: a -> Effect Unit, modify :: (a -> a) -> Effect Unit }


Event a represents a source of occurences. Each occurence carries a value of type a.

Event is a Functor.

We can construct a trivial event never :: forall a. Event a, which never occurs.

Events can be combined:

-- | An Event that occurs when any of the events occur. If some of them occur simultaneously, the occurence value is that of the leftmost one.
leftmost :: forall a. Array (Event a) -> Event a

Events can be transformed:

-- | Retain only the occurences of the event for which the given predicate function returns `true`.
filterEvent :: forall a. (a -> Boolean) -> Event a -> Event a

-- | Map the given function over an Event, and retain only the occurences for which it returned a Just value.
filterMapEvent :: forall a b. (a -> Maybe b) -> Event a -> Event b

-- | Retain only the occurences of the Event which contain a Just value.
filterJustEvent :: forall a. Event (Maybe a) -> Event a

We can observe Events by being notified of their occurences.

-- | Execute the given action for each occurence of the Event.
subscribeEvent_ :: forall m a. MonadEffect m => MonadCleanup m => (a -> Effect Unit) -> Event a -> m a


Callback a represents an action handler which consumes a value of type a. Think of it as a -> Effect Unit.

-- | Trigger the action in Effect.
triggerCallback :: forall a. Callback a -> a -> Effect Unit


Ref a represents a read-write reference to a mutable observable variable.

We can think of a Ref as of Effect.Ref, but with additional functions:

  • the ability to notify subscribers about changes to the value,
  • the ability to focus using a lens.

Ref a consists of:

  • Ref.value :: Ref a -> Dynamic a to observe the value
  • Ref.modify :: Ref a -> Callback (a -> a) to modify the value using a function

As a shortcut we have Ref.set :: Ref a -> Callback a to replace the value completely.

Creating a Ref:

newRef :: forall m a. MonadEffect m => a -> m (Ref a)

Ref is not a Functor, because it's read-write. It's Invariant, that is, it can be mapped over using a bijection.

Building DOM content


Widget a is a computation which can perform Effects, produce DOM nodes, subscribe to Events and Dynamics and returns a value of type a.

Widgets can be executed using runMainWidgetInBody - their contents will be inserted into the document.body element.


Prop is a modifier attached to a DOM element. Specific ways to construct a Prop are presented below.


Attrs is a map of HTML attributes.

-- A singleton map can be constructed using the `:=` operator.
"type":="button" :: Attrs

-- Attrs can be combined using the Monoid instance.
"type":="button" <> "name":="btn" :: Attrs

Static DOM

import Specular.Dom.Element

-- | Produce a text node.
text :: String -> Widget Unit

-- | `el tag props body` - Produce a DOM Element.
-- |
-- | The elements produced by the `body` widget will be inserted as children of the element.
el :: forall a. TagName -> Array Prop -> Widget a -> Widget a

-- | `el tag props body` - Produce a DOM Element with no props.
el_ :: forall a. TagName -> Widget a -> Widget a

-- | Attach static attributes to the element.
attrs :: Attrs -> Prop

-- | Attach a static attribute to the element.
attrs :: AttrName -> AttrValue -> Prop

-- | Attach CSS classes to the element
classes :: [ClassName] -> Prop

-- | Attach a CSS class to the element
class_ :: ClassName -> Prop

For example, to produce the following HTML:

<div class="alert alert-warning alert-dismissible fade show" role="alert">
  <strong>Holy guacamole!</strong> You should check in on some of those fields below.
  <button type="button" class="close" data-dismiss="alert" aria-label="Close">
    <span aria-hidden="true">&times;</span>

One would write the following Specular code:

el "div" [classes ["alert", "alert-warning", "alert-dismissible", "fade", "show"], attr "role" "alert"] do
  el_ "strong" $ text "Holy guacamole!"
  text " You should check in on some of those fields below."
  el "button" [class_ "close", attrs ("type":="button" <> "data-dismiss":="alert" <> "aria-label":="Close")] do
    el "span" [attr "aria-hidden"  "true"] do
      text "×"

Dynamic text, attributes and classes

Most of the Prop constructors have their dynamic counterparts. As a convention, their names end in D. For example:

-- | Attach dynamic attributes to the element.
attrsD :: Dynamic Attrs -> Prop

-- | Attach dynamic CSS classes to the element
classesD :: Dynamic [ClassName] -> Prop

text also has a dynamic counterpart:

-- | Create a text node whose text will reflect the value of the given Dynamic.
dynText :: Dynamic String -> Widget Unit

For convenience, utilities for common cases are provided such as:

attrWhenD :: Dynamic Boolean -> AttrName -> AttrValue -> Prop
classWhenD :: Dynamic Boolean -> ClassName -> Prop

For example: assume you have name :: Dynamic String. The code:

let isLong nm = String.length nm >= 5
el "div" [class_ "name", classWhenD (isLong <$> name) "long"] do
  text "Your name is: "
  dynText name

when name has value "Jan", would produce

<div class="name">Your name is Jan</div>

whereas when name has value "Titelitury", would produce

<div class="name long">Your name is Titelitury</div>

Dynamic DOM structure

Sometimes changing text and attributes is not enough. For that there's withDynamic_:

withDynamic_ :: forall a. Dynamic a -> (a -> Widget Unit) -> Widget Unit

Whenever the Dynamic changes, it will re-render a new Widget based on the latest value.


-- Assume loading :: Dynamic Boolean

withDynamic_ loading $
  if _ then
    el "div" [class_ "loading"] $ text "Loading..."
    el_ "div" do
      el_ "h1" $ text "Content"
      el_ "p" $ text "Bla bla bla"

Warning: Re-rendering a whole DOM block on each change has performance implications. Use with care.

Handling events

-- | Connect a DOM event on the node to a Callback.
on :: EventType -> Callback DOM.Event -> Prop

-- | Shorthand: `on "click"`
onClick :: Callback DOM.Event -> Prop

-- | Like `onClick`, but takes a callback which ignores the DOM event.
onClick_ :: Callback Unit -> Prop


-- Assume save :: Callback Unit

el "button" [attr "type" "button", onClick_ save] do
  text "Save"

Why not just use Reflex and GHCJS?

In short: code size. Specular demos are 240K unminified (with DCE - pulp build -O), or 19K minified with uglifyjs -c -m and gzipped. In contrast, a a GHCJS ( program that prints Hello World (no DOM bindings included, just base) weighs 1.1M unminified, or 62K minified with Closure Compiler's ADVANCED_OPTIMIZATIONS and gzipped. Supporting Haskell semantics has a cost.

There are also other reasons, of course.

Why not use other PureScript UI libraries?

See Motivation.


Some of the cons of Specular:

  • No good way to do server-side rendering. Local state complicates this.

  • Performance may be sometimes bad, because it does not use any Virtual DOM - the element placement instructions you write translate pretty much directly to createElement/appendChild. There are no (representative) benchmarks yet.

  • Time travel debugging, as known from Elm, is not possible.

  • Currently no way to bind to React Native.

  • Programs written with Specular may be harder to understand for some people who prefer the single state variable approach.

  • Compared to Reflex, it has way less FRP combinators.

  • Creating recursive data flows is more cumbersome than in Reflex, because PureScript has eager evaluation and no RecursiveDo.

  • It's immature and not popular, and may have bugs.

If you think there are more, please open an issue. They should be listed.

Who's using it?

  • Restaumatic - used in production for a signification portion of online ordering frontend, as well as for backoffice apps and our mobile app for restaurants.


If you discover bugs, want new features, or have questions, please post an issue using the GitHub issue tracker.

You can also contact @mbieleck on FP Chat, if you want to chat about Specular.