Reflex FRP is a composable, cross-platform functional reactive programming framework for Haskell. It allows you to build interactive components in pure functional style, working in harmony with established Haskell techniques and improving the quality and elegance of your applications.
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README.md

Reflex Platform CircleCI

The Reflex Platform is a collection of libraries and tools that are useful for developing and deploying Reflex-based applications.

To get started with Reflex development, follow the instructions below.

Try Reflex lets you set up an environment from which you can use Reflex with GHC or GHCJS.

To use Reflex Platform as a build/development system for your own projects, refer to HACKING.md.

Important Notes

OS Compatibility

If you're using one of these platforms, please take a look at notes before you begin:

If you encounter any problems that may be specific to your platform, please submit an issue or pull request so that we can add a note for future users.

Memory Requirements

GHCJS uses a lot of memory during compilation. 16GB of memory is recommended, with 8GB being pretty close to bare minimum.

Setup

This process will install the Nix package manager. If you prefer to install Nix yourself, you may do so any time prior to step 2.

  1. Clone this repository:

    git clone https://github.com/reflex-frp/reflex-platform
  2. Navigate into the reflex-platform folder and run the try-reflex command. This will install Nix, if you don't have it already, and use it to wrangle all the dependencies you'll need and drop you in an environment from which you can use Reflex. Be warned, this might take a little while the first time:

    cd reflex-platform
    ./try-reflex
  3. From this nix-shell, you can compile any haskell source files you like. Replace your-source-file.hs with the name of the file you'd like to compile. For the most part, ghcjs supports the same options as ghc:

    • GHC

      ghc --make your-source-file.hs
      ./your-source-file

      Compilation will produce a your-source-file native executable via WebkitGtk. Simply run it to launch your app.

    • GHCJS

      ghcjs --make your-source-file.hs

      Compilation will produce a your-source-file.jsexe folder containing an index.html file. Open that in your browser to run your app.

Don't use cabal install to install libraries while inside the try-reflex shell - the resulting libraries may not be found properly by ghc or ghcjs. Using Cabal to configure, build, test, and run a particular package, however, should work just fine.

try-reflex and ghcjs --make are not recommended for real-world projects — just as a quick and easy way to install Nix and experiment with reflex-dom. If you need to use additional Haskell libraries (e.g. from Hackage), we recommend using the tools described in project-development.md instead.

Tutorial

In this example, we'll be following Luite Stegemann's lead and building a simple functional reactive calculator to be used in a web browser.

DOM Basics

Reflex's companion library, Reflex-DOM, contains a number of functions used to build and interact with the Document Object Model. Let's start by getting a basic app up and running.

> {-# LANGUAGE OverloadedStrings #-}
> import Reflex.Dom

> main = mainWidget $ el "div" $ text "Welcome to Reflex"

Save this file as source.hs and compile it by running ghcjs source.hs. If you've entered everything correctly, this will produce a folder named source.jsexe in the same directory as source.hs. Navigate to this folder in your file manager and open index.html using your browser. The browser should show a page with the text "Welcome to Reflex".

Most Reflex apps will start the same way: a call to mainWidget with a starting Widget. A Widget is some DOM wrapped up for easy use with Reflex. In our example, we are building the argument to mainWidget, (in other words, our starting Widget) on the same line.

el has the type signature:

el :: MonadWidget t m => Text -> m a -> m a

The first argument to el is a Text, which will become the tag of the html element produced. The second argument is a Widget, which will become the child of the element being produced.

Sidebar: Interpreting the MonadWidget type

FRP-enabled datatypes in Reflex take an argument t, which identifies the FRP subsystem being used. This ensures that wires don't get crossed if a single program uses Reflex in multiple different contexts. You can think of t as identifying a particular "timeline" of the FRP system. Because most simple programs will only deal with a single timeline, we won't revisit the t parameters in this tutorial. As long as you make sure your Event, Behavior, and Dynamic values all get their t argument, it'll work itself out.

In our example, el "div" $ text "Welcome to Reflex", the first argument to el was "div", indicating that we are going to produce a div element.

The second argument to el was text "Welcome to Reflex". The type signature of text is:

text :: MonadWidget t m => Text -> m ()

text takes a Text and produces a Widget. The Text becomes a text DOM node in the parent element of the text. Of course, instead of a Text, we could have used el here as well to continue building arbitrarily complex DOM. For instance, if we wanted to make a unordered list:

> {-# LANGUAGE OverloadedStrings #-}
> import Reflex.Dom

> main = mainWidget $ el "div" $ do
>  el "p" $ text "Reflex is:"
>  el "ul" $ do
>    el "li" $ text "Efficient"
>    el "li" $ text "Higher-order"
>    el "li" $ text "Glitch-free"

Dynamics and Events

Of course, we want to do more than just view a static webpage. Let's start by getting some user input and printing it.

> {-# LANGUAGE OverloadedStrings #-}
> import Reflex.Dom

> main = mainWidget $ el "div" $ do
>   t <- textInput def
>   dynText $ _textInput_value t

Running this in your browser, you'll see that it produces a div containing an input element. When you type into the input element, the text you enter appears inside the div as well.

textInput is a function with the following type:

textInput :: MonadWidget t m => TextInputConfig t -> m (TextInput t)

It takes a TextInputConfig (given a default value in our example), and produces a Widget whose result is a TextInput. In Reflex.Dom.Widget.Input we can see that a TextInput exposes the following functionality:

data TextInput t
   = TextInput { _textInput_value :: Dynamic t Text
               , _textInput_input :: Event t Text
               , _textInput_keypress :: Event t Int
               , _textInput_keydown :: Event t Int
               , _textInput_keyup :: Event t Int
               , _textInput_hasFocus :: Dynamic t Bool
               , _textInput_builderElement :: InputElement EventResult GhcjsDomSpace t
               }

Here we are using _textInput_value to access the Dynamic Text value of the TextInput. Conveniently, dynText takes a Dynamic Text and displays it. It is the dynamic version of text.

We can also access Events related to the TextInput. For example, consider the following code:

> {-# LANGUAGE OverloadedStrings #-}
> import Data.Text  (pack)
> import Reflex
> import Reflex.Dom

> main = mainWidget $ el "div" $ do
>   t <- textInput def
>   text "Last key pressed: "
>   let keypressEvent = fmap (pack . show) $ _textInput_keypress t
>   keypressDyn <- holdDyn "None" keypressEvent
>   dynText keypressDyn

Here, we are creating a TextInput as we were before. The function _textInput_keypress gives us an Event Int representing the key code of the pressed key. We are using fmap here to apply pack . show to the Int, so the type of keypressEvent is Event Text. Whenever a key is pressed inside the TextInput, the keypressEvent will fire. holdDyn allows us to take create a Dynamic out of an Event. We must provide an initial value for the Dynamic. This will be the value of the Dynamic until the associated Event fires. The type of holdDyn is:

holdDyn :: MonadHold t m => a -> Event t a -> m (Dynamic t a)

We won't go into the details of MonadHold here, but the rest of the type signature should be fairly clear: holdDyn takes an initial value, an Event containing a value of the same type as the initial, and returns a Dynamic containing a value of the same type.

When you run this application, you'll see a textbox and the string "Last key pressed: None" on the screen. Recall that "None" is the initial value we gave holdDyn.

A Number Input

A calculator was promised, I know. We'll start building the calculator by creating an input for numbers.

> {-# LANGUAGE OverloadedStrings #-}
> import Reflex
> import Reflex.Dom
> import Data.Map (Map)
> import qualified Data.Map as Map

> main = mainWidget $ el "div" $ do
>   t <- textInput $ def & textInputConfig_inputType .~ "number"
>                        & textInputConfig_initialValue .~ "0"
>   dynText $ _textInput_value t

The code above overrides some of the default values of the TextInputConfig. We provide a Text value for the textInputConfig_inputType, specifying the html input element's type attribute. We're using "number" here.

Next, we override the default initial value of the TextInput. We gave it "0". Even though we're making an html input element with the attribute type=number, the result is still a Text. We'll convert this later.

Let's do more than just take the input value and print it out. First, let's make sure the input is actually a number:

> {-# LANGUAGE OverloadedStrings #-}
> import Reflex.Dom
> import Data.Map (Map)
> import qualified Data.Map as Map
> import Data.Text (pack, unpack)
> import Text.Read (readMaybe)

> main = mainWidget $ el "div" $ do
>   x <- numberInput
>   let numberString = fmap (pack . show) x
>   dynText numberString

> numberInput :: MonadWidget t m => m (Dynamic t (Maybe Double))
> numberInput = do
>   n <- textInput $ def & textInputConfig_inputType .~ "number"
>                        & textInputConfig_initialValue .~ "0"
>   return . fmap (readMaybe . unpack) $ _textInput_value n

We've defined a function numberInput that both handles the creation of the TextInput and reads its value. Recall that _textInput_value gives us a Dynamic Text. The final line of code in numberInput uses fmap to apply the function readMaybe . unpack to the Dynamic value of the TextInput. This produces a Dynamic (Maybe Double). Our main function uses fmap to map over the Dynamic (Maybe Double) produced by numberInput and pack . show the value it contains. We store the new Dynamic Text in numberString and feed that into dynText to actually display the Text

Running the app at this point should produce an input and some text showing the Maybe Double. Typing in a number should produce output like Just 12.0 and typing in other text should produce the output Nothing.

Adding

Now that we have numberInput we can put together a couple inputs to make a basic calculator.

> {-# LANGUAGE OverloadedStrings #-}
> import Reflex
> import Reflex.Dom
> import Data.Map (Map)
> import qualified Data.Map as Map
> import Data.Text (pack, unpack)
> import Text.Read (readMaybe)
> import Control.Applicative ((<*>), (<$>))
>
> main = mainWidget $ el "div" $ do
>   nx <- numberInput
>   text " + "
>   ny <- numberInput
>   text " = "
>   let result = zipDynWith (\x y -> (+) <$> x <*> y) nx ny
>       resultString = fmap (pack . show) result
>   dynText resultString

> numberInput :: MonadWidget t m => m (Dynamic t (Maybe Double))
> numberInput = do
>   n <- textInput $ def & textInputConfig_inputType .~ "number"
>                        & textInputConfig_initialValue .~ "0"
>   return . fmap (readMaybe . unpack) $ _textInput_value n

numberInput hasn't changed here. Our main function now creates two inputs. zipDynWith is used to produce the actual sum of the values of the inputs. The type signature of zipDynWith is:

    Reflex t => (a -> b -> c) -> Dynamic t a -> Dynamic t b -> Dynamic t c

You can see that it takes a function that combines two pure values and produces some other pure value, and two Dynamics, and produces a Dynamic.

In our case, zipDynWith is combining the results of our two numberInputs (with a little help from Control.Applicative) into a sum.

We use fmap again to apply pack . show to result (a Dynamic (Maybe Double)) resulting in a Dynamic Text. This resultText is then displayed using dynText.

Supporting Multiple Operations

Next, we'll add support for other operations. We're going to add a dropdown so that the user can select the operation to apply. The function dropdown has the type:

dropdown :: (MonadWidget t m, Ord k) => k -> Dynamic t (Map k Text) -> DropdownConfig t k -> m (Dropdown t k)

The first argument is the initial value of the Dropdown. The second argument is a Dynamic (Map k Text) that represents the options in the dropdown. The Text values of the Map are the strings that will be displayed to the user. If the initial key is not in the Map, it is added and given a Text value of "". The final argument is a DropdownConfig.

Our supported operations will be:

data Op = Plus | Minus | Times | Divide deriving (Eq, Ord)

ops = Map.fromList [(Plus, "+"), (Minus, "-"), (Times, "*"), (Divide, "/")]

We'll use this as an argument to dropdown:

d <- dropdown Times (constDyn ops) def

We are using constDyn again here to turn our Map of operations into a Dynamic. Using def, we provide the default DropdownConfig. The result, d, will be a Dropdown. We can retrieve the Dynamic selection of a Dropdown by using _dropdown_value.

> {-# LANGUAGE OverloadedStrings #-}
> import Reflex
> import Reflex.Dom
> import Data.Map (Map)
> import qualified Data.Map as Map
> import Data.Text (pack, unpack, Text)
> import Text.Read (readMaybe)
> import Control.Applicative ((<*>), (<$>))
>
> main = mainWidget $ el "div" $ do
>   nx <- numberInput
>   d <- dropdown Times (constDyn ops) def
>   ny <- numberInput
>   let values = zipDynWith (,) nx ny
>       result = zipDynWith (\o (x,y) -> runOp o <$> x <*> y) (_dropdown_value d) values
>       resultText = fmap (pack . show) result
>   text " = "
>   dynText resultText
>
> numberInput :: MonadWidget t m => m (Dynamic t (Maybe Double))
> numberInput = do
>   n <- textInput $ def & textInputConfig_inputType .~ "number"
>                        & textInputConfig_initialValue .~ "0"
>   return . fmap (readMaybe . unpack) $ _textInput_value n
>
> data Op = Plus | Minus | Times | Divide deriving (Eq, Ord)
>
> ops :: Map Op Text
> ops = Map.fromList [(Plus, "+"), (Minus, "-"), (Times, "*"), (Divide, "/")]
>
> runOp :: Fractional a => Op -> a -> a -> a
> runOp s = case s of
>             Plus -> (+)
>             Minus -> (-)
>             Times -> (*)
>             Divide -> (/)

This is our complete program. We've added an uninteresting function runOp that takes an Op and returns an operation. The keys of the Map we used to create the Dropdown had the type Op. When we retrieve the value of Dropdown, we'll use runOp to turn the Dropdown selection into the function we need to apply to our numbers.

After creating the two numberInputs, we combine them using zipDynWith applying (,), making a tuple of type Dynamic (Maybe Double, Maybe Double) and binding it to values.

Next, we call zipDynWith again, combining the _dropdown_value and values. Now, instead of applying (+) to our Double values, we use runOp to select an operation based on the Dynamic value of our Dropdown.

Running the app at this point will give us our two number inputs with a dropdown of operations sandwiched between them. Multiplication should be pre-selected when the page loads.

Dynamic Element Attributes

Let's spare a thought for the user of our calculator and add a little UI styling. Our number input currently looks like this:

numberInput :: MonadWidget t m => m (Dynamic t (Maybe Double))
numberInput = do
  n <- textInput $ def & textInputConfig_inputType .~ "number"
                       & textInputConfig_initialValue .~ "0"
  return . fmap (readMaybe . unpack) $ _textInput_value n

Let's give it some html attributes to work with:

numberInput :: MonadWidget t m => m (Dynamic t (Maybe Double))
numberInput = do
  let attrs = constDyn ("style" =: "border-color: blue")
  n <- textInput $ def & textInputConfig_inputType .~ "number"
                       & textInputConfig_initialValue .~ "0"
                       & textInputConfig_attributes .~ attrs
  return . fmap (readMaybe . unpack) $ _textInput_value n

Here, we've created a Dynamic (Map Text Text). This Map represents the html attributes of our inputs. Because we're using constDyn again, this Dynamic will never change. If you load this in the browser, you'll see that the inputs now have a blue border.

Unchanging attributes are useful and quite common, but attributes will often need to change. Instead of just making the TextInput blue, let's change it's color based on whether the input successfully parses to a Double:

{-# LANGUAGE RecursiveDo #-}
...
numberInput :: (MonadWidget t m) => m (Dynamic t (Maybe Double))
numberInput = do
  let errorState = "style" =: "border-color: red"
      validState = "style" =: "border-color: green"
  rec n <- textInput $ def & textInputConfig_inputType .~ "number"
                           & textInputConfig_initialValue .~ "0"
                           & textInputConfig_attributes .~ attrs
      let result = fmap (readMaybe . unpack) $ _textInput_value n
          attrs  = fmap (maybe errorState (const validState)) result
  return result

Note that we need to add a language pragma here to enable the RecursiveDo language extension. We've defined two Maps of attributes for our inputs: one to represent bad input and one to represent valid input. Next, you'll see that the code for actually making the number input is now inside of a rec block. This is because the attributes we apply depend on the value of the input.

In the first line of the rec, we have supplied the argument attrs, of type Dynamic (Map Text Text). The Dynamic value of the input is bound to result. The code for parsing this value has not changed.

After we bind result, we use fmap again to apply a switching function to result. The switching function checks whether the value was successfully parsed. If it was, we get the Map of attributes representing the valid state, otherwise we get the Map representing the error state. The result is a Dynamic (Map Text Text), which is the type textInputConfig_attributes expects to receive.

The complete program now looks like this:

> {-# LANGUAGE OverloadedStrings #-}
> {-# LANGUAGE RecursiveDo       #-}
> import Reflex
> import Reflex.Dom
> import Data.Map (Map)
> import qualified Data.Map as Map
> import Data.Text (pack, unpack, Text)
> import Text.Read (readMaybe)
> import Control.Applicative ((<*>), (<$>))
>
> main = mainWidget $ el "div" $ do
>   nx <- numberInput
>   d <- dropdown Times (constDyn ops) def
>   ny <- numberInput
>   let values = zipDynWith (,) nx ny
>       result = zipDynWith (\o (x,y) -> runOp o <$> x <*> y) (_dropdown_value d) values
>       resultText = fmap (pack . show) result
>   text " = "
>   dynText resultText
>
> numberInput :: (MonadWidget t m) => m (Dynamic t (Maybe Double))
> numberInput = do
>   let errorState = "style" =: "border-color: red"
>       validState = "style" =: "border-color: green"
>   rec n <- textInput $ def & textInputConfig_inputType .~ "number"
>                            & textInputConfig_initialValue .~ "0"
>                            & textInputConfig_attributes .~ attrs
>       let result = fmap (readMaybe . unpack) $ _textInput_value n
>           attrs  = fmap (maybe errorState (const validState)) result
>   return result
>
> data Op = Plus | Minus | Times | Divide deriving (Eq, Ord)
>
> ops :: Map Op Text
> ops = Map.fromList [(Plus, "+"), (Minus, "-"), (Times, "*"), (Divide, "/")]
>
> runOp :: Fractional a => Op -> a -> a -> a
> runOp s = case s of
>             Plus -> (+)
>             Minus -> (-)
>             Times -> (*)
>             Divide -> (/)

The input border colors will now change depending on their value.