### Complete wxHaskell Counter Example

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

A comprehensive example integrating Reactive Banana with wxHaskell to create a GUI counter. It demonstrates setting up the GUI, handling button events, managing a counter behavior, and updating a text display using `sink`.

```haskell
{-# LANGUAGE RecursiveDo #-}

import Graphics.UI.WX hiding (Event)
import Reactive.Banana
import Reactive.Banana.WX

main :: IO ()
main = start $ do
    -- Create GUI
    f      <- frame [text := "Counter"]
    bUp    <- button f [text := "Up"]
    bDown  <- button f [text := "Down"]
    output <- staticText f [text := "0"]

    set f [layout := margin 10 $ column 5
            [ row 5 [widget bUp, widget bDown]
            , widget output
            ]]

    -- Event network
    let networkDescription :: MomentIO ()
        networkDescription = do
            eUp   <- event0 bUp   command
            eDown <- event0 bDown command

            bCounter <- accumB 0 $ unions
                [ (+1)      <$ eUp
                , subtract 1 <$ eDown
                ]

            sink output [ text :== show <$> bCounter ]

    network <- compile networkDescription
    activate network
```

--------------------------------

### Behavior Type Examples in Haskell

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

Illustrates the `Behavior` type in Reactive-banana, representing time-varying values. Shows how Behaviors are `Applicative` for combining, how to create constant Behaviors, and numerical operations.

```Haskell
import Reactive.Banana
import Reactive.Banana.Frameworks

-- Behavior is semantically: Time -> a
-- Behaviors are Applicative
combineBehaviors :: Behavior Int -> Behavior Int -> Behavior Int
combineBehaviors b1 b2 = (+) <$> b1 <*> b2

-- Behaviors can be created from constant values
constantBehavior :: Behavior String
constantBehavior = pure "Hello, World!"

-- Behaviors have Num, Fractional, Floating instances
numericBehavior :: Behavior Double -> Behavior Double -> Behavior Double
numericBehavior b1 b2 = sqrt (b1 * b1 + b2 * b2)
```

--------------------------------

### unionWith Event Combinator Examples in Haskell

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

Shows how to use the `unionWith` combinator to merge two event streams. Examples include adding values for simultaneous events and keeping the first value. Includes a test case with `interpret`.

```Haskell
import Reactive.Banana

-- Merge two integer events, adding values on simultaneous occurrence
mergeAdding :: Event Int -> Event Int -> Event Int
mergeAdding = unionWith (+)

-- Merge keeping the first value on collision
mergeFirst :: Event a -> Event a -> Event a
mergeFirst = unionWith const

-- Example with interpret
main :: IO ()
main = do
    let f e = return $ unionWith (+) (fmap (*2) e) (fmap (*3) e)
    result <- interpret f [Just 1, Nothing, Just 2]
    print result
    -- Output: [Just 5, Nothing, Just 10] (since 2*1+3*1=5, 2*2+3*2=10)
```

--------------------------------

### Compile and Activate Event Network with compile and activate

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

Illustrates the process of compiling an event network using `compile` and then activating it with `activate`. This example shows setting up event sources, accumulating values, and reacting to changes, culminating in firing events to test the network.

```haskell
import Reactive.Banana
import Reactive.Banana.Frameworks

-- Complete example
main :: IO ()
main = do
    -- Create external event sources
    (addHandler, fire) <- newAddHandler

    -- Compile the network
    network <- compile $ do
        eInput <- fromAddHandler addHandler

        bSum <- accumB 0 $ (+) <$> eInput

        eChanges <- changes bSum
        reactimate' $ fmap (putStrLn . ("Running sum: " ++) . show) <$> eChanges

    -- Activate the network
    activate network

    -- Fire events
    mapM_ fire [1, 2, 3, 4, 5]
    -- Output: Running sum: 1, Running sum: 3, Running sum: 6, ...
```

--------------------------------

### Haskell: AccumB for Behavior Accumulation

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

Illustrates the 'accumB' function for accumulating event values into a Behavior. Examples cover creating a counter behavior, a list accumulator, and integrating behavior changes with UI updates.

```haskell
import Reactive.Banana
import Reactive.Banana.Frameworks

-- Counter behavior
counterBehavior :: Event () -> Event () -> Moment (Behavior Int)
counterBehavior eUp eDown = accumB 0 $ unions
    [ (+1) <$ eUp
    , subtract 1 <$ eDown
    ]

-- List accumulator
listAccumulator :: Event a -> Moment (Behavior [a])
listAccumulator e = accumB [] $ (\x xs -> xs ++ [x]) <$> e

-- Example with GUI sink
networkDescription :: MomentIO ()
networkDescription = do
    (eIncrement, _) <- newEvent

    bCounter <- accumB 0 $ (+1) <$ eIncrement

    -- The behavior can be used to update UI elements
    eChanges <- changes bCounter
    reactimate' $ fmap (putStrLn . ("Counter: " ++) . show) <$> eChanges
```

--------------------------------

### apply Event Combinator Example in Haskell

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

Illustrates the `apply` combinator, which applies a time-varying function (Behavior) to incoming event values. This example shows multiplying event inputs by a behavior-controlled multiplier.

```Haskell
import Reactive.Banana
import Reactive.Banana.Frameworks

-- Apply a behavior function to event values
networkDescription :: MomentIO ()
networkDescription = do
    (eInput, fireInput) <- newEvent

    -- Create a behavior that holds a multiplier
    bMultiplier <- stepper 1 never

    -- Apply the multiplier to incoming events
    let eResult = apply ((*) <$> bMultiplier) eInput

    reactimate $ print <$> eResult
```

--------------------------------

### Event Type Examples in Haskell

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

Demonstrates the `Event` type in Reactive-banana, representing discrete occurrences. Shows transformations using `fmap` and merging with `unionWith`. Includes a test case using `interpret` to process a list of potential events.

```Haskell
import Reactive.Banana
import Reactive.Banana.Frameworks

-- Event is semantically: [(Time, a)]
-- Events can be transformed with Functor
example :: Event Int -> Event String
example e = fmap show e

-- Events can be merged with unionWith
mergeEvents :: Event Int -> Event Int -> Event Int
mergeEvents e1 e2 = unionWith (+) e1 e2

-- Testing events with interpret
main :: IO ()
main = do
    result <- interpret (\e -> return $ fmap (*2) e) [Just 1, Nothing, Just 2, Just 3]
    print result
    -- Output: [Just 2, Nothing, Just 4, Just 6]
```

--------------------------------

### Pause and Activate Event Network with pause and activate

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

Shows how to control the execution of an event network using `pause` and `activate`. The example demonstrates pausing the network to stop event processing and then reactivating it to resume.

```haskell
import Reactive.Banana
import Reactive.Banana.Frameworks

-- Network with pause capability
main :: IO ()
main = do
    (addHandler, fire) <- newAddHandler

    network <- compile $ do
        e <- fromAddHandler addHandler
        reactimate $ print <$> e

    activate network
    fire 1  -- Prints: 1

    pause network
    fire 2  -- Nothing printed (network paused)

    activate network
    fire 3  -- Prints: 3
```

--------------------------------

### filterE Event Combinator Examples in Haskell

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

Demonstrates the `filterE` combinator for selectively keeping events that satisfy a given predicate. Examples include filtering for positive and even numbers, with a test case using `interpret`.

```Haskell
import Reactive.Banana

-- Filter to only positive numbers
positiveOnly :: Event Int -> Event Int
positiveOnly = filterE (> 0)

-- Filter to only even numbers
evenOnly :: Event Int -> Event Int
evenOnly = filterE even

-- Testing with interpret
main :: IO ()
main = do
    result <- interpret (\e -> return $ filterE even e) [Just 1, Just 2, Just 3, Just 4]
    print result
    -- Output: [Nothing, Just 2, Nothing, Just 4]
```

--------------------------------

### Create Updatable Behavior with newBehavior

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

Demonstrates how to create a behavior that can be updated programmatically using `newBehavior`. This function returns a behavior and an update handler. The example shows updating the behavior's value and reacting to changes.

```haskell
import Reactive.Banana
import Reactive.Banana.Frameworks

-- Create updateable behavior
networkDescription :: MomentIO ()
networkDescription = do
    (bValue, updateValue) <- newBehavior "initial"

    (eTrigger, _) <- newEvent

    -- Sample behavior when triggered
    reactimate $ (\() -> print =<< return bValue) <$ eTrigger

    -- Update can be called from IO
    liftIO $ updateValue "updated"
```

--------------------------------

### Haskell: Unions for Merging Event Streams

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

Explains the 'unions' function for merging multiple event streams. It shows how to combine different update events to control a behavior and provides a vending machine example demonstrating its use in managing state transitions.

```haskell
import Reactive.Banana
import Reactive.Banana.Frameworks

-- Combine multiple update events
multipleUpdates :: Event () -> Event () -> Event Int -> Moment (Behavior Int)
multipleUpdates eIncr eDecr eSet = accumB 0 $ unions
    [ (+1)      <$
eIncr
    , subtract 1 <$ eDecr
    , const     <$> eSet
    ]

-- Vending machine example
vendingMachine :: Event () -> Moment (Event (), Behavior Int)
vendingMachine eCoin = do
    let price = 50
    rec
        bAmount <- accumB price $ unions
            [ subtract 10 <$ eCoin
            , const price <$ eSold
            ]
        let eSold = whenE ((<= 0) <$> bAmount) eCoin
    return (eSold, bAmount)
```

--------------------------------

### Connect Behaviors to Widget Properties with sink

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

Illustrates using the `sink` function to connect Reactive Banana behaviors to wxHaskell widget properties. This automatically updates the widget whenever the behavior's value changes, as shown in the example with a counter updating a static text label.

```haskell
import Graphics.UI.WX hiding (Event, Behavior)
import Reactive.Banana
import Reactive.Banana.WX

-- Update widget properties from behaviors
networkDescription :: StaticText ()
networkDescription = do
    eClick <- event0 button command

    bCounter <- accumB 0 $ (+1) <$ eClick

    -- Automatically update label text when counter changes
    sink output [ text :== show <$> bCounter ]
```

--------------------------------

### Haskell: MapAccum for Stateful Transformations

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

Introduces 'mapAccum', an efficient function for combining stateful accumulation of events and behaviors. Examples include generating unique IDs and performing stateful transformations on event data with output.

```haskell
import Reactive.Banana
import Reactive.Banana.Frameworks

-- Generate unique IDs
uniqueIdGenerator :: Event a -> Moment (Event Int, Behavior Int)
uniqueIdGenerator e = mapAccum 0 $ (\acc -> (acc, acc + 1)) <$ e

-- Stateful transformation with output
statefulTransform :: Event Int -> Moment (Event String, Behavior Int)
statefulTransform e = mapAccum 0 $ (\x acc ->
    let newAcc = acc + x
    in ("Sum is now: " ++ show newAcc, newAcc)
    ) <$> e
```

--------------------------------

### Haskell: AccumE for Event Accumulation

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

Details the 'accumE' function for accumulating event values using a strict left scan. Examples include counting events and calculating a running sum, as well as a more complex counter with up/down events.

```haskell
import Reactive.Banana
import Reactive.Banana.Frameworks

-- Count events
countEvents :: Event a -> Moment (Event Int)
countEvents e = accumE 0 $ (+1) <$ e

-- Running sum
runningSum :: Event Int -> Moment (Event Int)
runningSum e = accumE 0 $ (+) <$> e

-- Example: Counter with up/down events
networkDescription :: MomentIO ()
networkDescription = do
    (eUp, fireUp) <- newEvent
    (eDown, fireDown) <- newEvent

    eCount <- accumE 0 $ unions
        [ (+1) <$ eUp
        , subtract 1 <$ eDown
        ]

    reactimate $ print <$> eCount
```

--------------------------------

### filterJust Event Combinator in Haskell

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

Shows the `filterJust` combinator, which filters out `Nothing` values from an event stream of `Maybe` types, keeping only `Just` values. Includes an example of parsing strings to integers.

```Haskell
import Reactive.Banana

-- Parse integer events, keeping only successful parses
parseInts :: Event String -> Event Int
parseInts = filterJust . fmap readMaybe
  where
    readMaybe s = case reads s of
        [(x, "")] -> Just x
        _         -> Nothing

-- Testing with interpret
main :: IO ()
main = do
    result <- interpret (\e -> return $ filterJust e) [Just (Just 1), Just Nothing, Just (Just 2)]
    print result
    -- Output: [Just 1, Nothing, Just 2]
```

--------------------------------

### Constructing a Val Expression in Haskell

Source: https://github.com/heinrichapfelmus/reactive-banana/blob/master/reactive-banana/doc/design/design.md

Defines a `val` combinator that constructs a `Val` expression and binds it to a fresh name within a monadic context. It uses `fresh` to get a new name and `bind` to associate the `Val` expression with it.

```haskell
val :: Int -> M Name
val i = do
    name <- fresh
    bind name $ Val i
    pure name
```

--------------------------------

### Implement unionWith for Event in reactive-banana

Source: https://github.com/heinrichapfelmus/reactive-banana/wiki/Multiple-Simultaneous-Events-(Issue-

Defines the `unionWith` function for `Event` types, which combines two events using a provided function. It utilizes `foldr1` and `collect` to process the union of the input events. The example demonstrates its usage with lists of integers.

```haskell
unionWith :: (a -> a -> a) -> Event a -> Event a -> Event a
unionWith f e1 e2 = foldr1 f <$> collect (e1 `union` e2)
```

--------------------------------

### Pure implementation with ST monad

Source: https://github.com/heinrichapfelmus/reactive-banana/blob/master/reactive-banana/TODO.md

Demonstrates how to make a push-driven implementation pure by utilizing the ST monad instead of the IO monad for the Vault data type. This is suggested for applications like MIDI generation.

```haskell
The push-driven implementation can be made pure by putting the `Vault` data type into the `ST` monad instead of the `IO` monad. This might be useful for MIDI, i.e. using one and the same code for both real-time MIDI generation and writing it to files.
```

--------------------------------

### Embedding Arithmetic Expressions with Host Language `let`

Source: https://github.com/heinrichapfelmus/reactive-banana/blob/master/reactive-banana/doc/design/design.md

Illustrates how a simple arithmetic expression language can be embedded into a host language (like Haskell) using smart constructors and the host language's `let` binding. It highlights the potential for expression duplication when `let` is used naively.

```haskell
y :: Exp
y = let x = val 1 + val 2 in (x * x)
```

--------------------------------

### Convert Widgets to Layouts

Source: https://github.com/heinrichapfelmus/reactive-banana/wiki/wx-Control-Overview

Demonstrates the conversion of various widgets into a layout structure. This allows for programmatic arrangement and management of UI elements.

```haskell
widget staticText
widget input1
widget cmb
widget styledTxt
```

--------------------------------

### Create wxHaskell Events with event0 and event1

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

Demonstrates creating Reactive Banana events from wxHaskell widget events using `event0` for events without parameters (like button clicks) and `event1` for events with parameters (like mouse events).

```haskell
import Graphics.UI.WX hiding (Event)
import Reactive.Banana
import Reactive.Banana.WX

-- Create events from button clicks
networkDescription :: Button ()
networkDescription = do
    eClick <- event0 button command
    reactimate $ putStrLn "Button clicked!" <$ eClick

-- Create events with parameters
mouseNetwork :: Panel ()
mouseNetwork = do
    eMouse <- event1 panel mouse
    let eLeftClick = filterJust $ leftDown <$> eMouse
    reactimate $ print <$> eLeftClick
```

--------------------------------

### Using Embedded `Let` for Expression Sharing

Source: https://github.com/heinrichapfelmus/reactive-banana/blob/master/reactive-banana/doc/design/design.md

Demonstrates how to rewrite the previously duplicated expression using the extended `Exp` data type with `Let` and `Var` constructors. This explicitly represents sharing within the embedded DSL.

```haskell
y' = Let "x" (val 1 + val 2) (Var "x" * Var "x")
```

--------------------------------

### Create Combo Box Widget

Source: https://github.com/heinrichapfelmus/reactive-banana/wiki/wx-Control-Overview

Implements a combo box widget, providing a dropdown list of predefined items. It can be initialized with a list of strings.

```haskell
cmb ←  comboBox f [items := ["item1","item2"]]
```

--------------------------------

### Create wxHaskell Behavior with behavior

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

Explains how to create a Reactive Banana behavior by polling a wxHaskell widget's attribute using the `behavior` function. This allows the widget's state to be sampled periodically or in response to other events.

```haskell
import Graphics.UI.WX hiding (Event, Behavior)
import Reactive.Banana
import Reactive.Banana.WX

-- Poll widget state as behavior
networkDescription :: TextCtrl ()
networkDescription = do
    bText <- behavior textCtrl text

    (eTrigger, _) <- newEvent

    -- Sample text at each trigger
    let eTextSample = bText <@ eTrigger
    reactimate $ putStrLn <$> eTextSample
```

--------------------------------

### Haskell: Stepper for Creating Step-Function Behaviors

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

Explains the 'stepper' function, which creates a Behavior from an initial value and a stream of update events. It demonstrates how to create a behavior that changes its value based on incoming events and how to observe these changes.

```haskell
import Reactive.Banana
import Reactive.Banana.Frameworks

-- Create a behavior that steps through values
networkDescription :: MomentIO ()
networkDescription = do
    (eUpdate, fireUpdate) <- newEvent

    -- Behavior starts at 0, updates when events occur
    bValue <- stepper 0 eUpdate

    -- Sample the behavior
    eChanges <- changes bValue
    reactimate' $ fmap print <$> eChanges

-- Testing with interpret
main :: IO ()
main = do
    result <- interpret (\e -> do
        b <- stepper "initial" e
        return $ b <@ e
        ) [Just "first", Nothing, Just "second"]
    print result
    -- Output: [Just "initial", Nothing, Just "first"]
```

--------------------------------

### Haskell: Infix Operators for Applying Behaviors to Events

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

Demonstrates the use of infix operators <@>, <@, and @> for applying behavior values to events or tagging events with behavior values. These operators simplify common patterns in event-driven programming.

```haskell
import Reactive.Banana
import Reactive.Banana.Frameworks

-- <@> is infix apply
applyExample :: Behavior (a -> b) -> Event a -> Event b
applyExample bf ex = bf <@> ex

-- <@ tags events with the current behavior value
tagWithBehavior :: Behavior b -> Event a -> Event b
tagWithBehavior b e = b <@ e

-- @> is flipped <@, useful with ApplicativeDo
tagFlipped :: Event a -> Behavior b -> Event b
tagFlipped e b = e @> b

-- Example: Tag button clicks with current counter value
networkDescription :: MomentIO ()
networkDescription = do
    (eClick, _) <- newEvent
    bCounter <- accumB 0 $ (+1) <$ eClick

    -- Get counter value at each click
    let eCounterAtClick = bCounter <@ eClick
    reactimate $ print <$> eCounterAtClick
```

--------------------------------

### Create Styled Text Control Widget

Source: https://github.com/heinrichapfelmus/reactive-banana/wiki/wx-Control-Overview

Initializes a styled text control, a multi-line textbox with advanced features like highlighting and code completion, suitable for IDE-like applications.

```haskell
styledTxt ←  styledTextCtrl f []
```

--------------------------------

### Create Label Layout

Source: https://github.com/heinrichapfelmus/reactive-banana/wiki/wx-Control-Overview

Generates an immutable label for display purposes within the layout. It takes a string as an argument to set the label's text.

```haskell
label "myLabeltext"
```

--------------------------------

### Create Text Entry Widget

Source: https://github.com/heinrichapfelmus/reactive-banana/wiki/wx-Control-Overview

Creates a text entry widget, also known as TextCtrl, which allows for user input. The content can be programmatically changed.

```haskell
input1    ←  entry f []
```

--------------------------------

### Arithmetic Expression Data Type with Variables and Let Bindings

Source: https://github.com/heinrichapfelmus/reactive-banana/blob/master/reactive-banana/doc/design/design.md

Extends the `Exp` data type to include explicit support for variables (`Var`) and let bindings (`Let`). This allows for direct representation of sharing within the embedded language itself, addressing the duplication issue.

```haskell
data Name = String
data Exp
    = ...
    | Var Name
    | Let Name Exp Exp -- let name = e1 in e2
```

--------------------------------

### Monadic Structure for Variable Binding and Lookup

Source: https://github.com/heinrichapfelmus/reactive-banana/blob/master/reactive-banana/doc/design/design.md

Presents a conceptual monadic structure `M` designed to manage variable bindings and lookups. This monad would store associations between names and expressions, facilitating shared evaluation and providing operations for managing this state.

```haskell
fresh  :: M Name
bind   :: Name -> Exp -> M ()
lookup :: Name -> M (Maybe Exp)
```

--------------------------------

### Create Static Text Widget

Source: https://github.com/heinrichapfelmus/reactive-banana/wiki/wx-Control-Overview

Defines a static text widget that displays read-only text. This text can be modified programmatically but not by user interaction.

```haskell
staticText    ←  staticText f []
```

--------------------------------

### Create Events from External Handlers with fromAddHandler (Haskell)

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

The fromAddHandler function integrates external event sources into the Reactive Banana framework. It takes an AddHandler (a function to register callbacks) and converts it into an Event, allowing external systems to trigger events within the reactive network. This is crucial for bridging the gap between imperative and reactive code.

```Haskell
import Reactive.Banana
import Reactive.Banana.Frameworks
import Control.Event.Handler

-- Create event from external callback
networkDescription :: AddHandler Int -> MomentIO ()
networkDescription addHandler = do
    eExternal <- fromAddHandler addHandler
    reactimate $ print <$> eExternal

-- Full example with handler setup
main :: IO ()
main = do
    (addHandler, fireEvent) <- newAddHandler

    network <- compile $ do
        e <- fromAddHandler addHandler
        reactimate $ putStrLn . ("Received: ") . show <$> e

    activate network

    -- Fire events
    fireEvent 1
    fireEvent 2
    fireEvent 3
```

--------------------------------

### Representing Events as a List of Time-Tagged Values

Source: https://github.com/heinrichapfelmus/reactive-banana/blob/master/reactive-banana/doc/design/design.md

This snippet shows a simple, though problematic, way to represent a sequence of discrete events with their timestamps. It uses a lazy list to allow future events to be added, but suffers from potential time leaks if not managed carefully.

```haskell
type Events a = [(Time, a)]
```

--------------------------------

### Dynamically Switch Behaviors with switchB (Haskell)

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

The switchB function enables dynamic switching between different behavior sources. It takes an initial behavior and an event that emits new behaviors, allowing the system to change its underlying state representation. This is valuable for adapting to changing data sources or configurations.

```Haskell
import Reactive.Banana
import Reactive.Banana.Frameworks

-- Switch between different value sources
switchValueSource :: Behavior a -> Event (Behavior a) -> Moment (Behavior a)
switchValueSource initial eSwitchTo = switchB initial eSwitchTo

-- Example: Switch between data sources
networkDescription :: MomentIO ()
networkDescription = do
    (eSwitch, _) <- newEvent

    let bSource1 = pure "Source 1"
        bSource2 = pure "Source 2"

    bCurrent <- switchB bSource1 $ bSource2 <$ eSwitch

    eChanges <- changes bCurrent
    reactimate' $ fmap print <$> eChanges
```

--------------------------------

### Shared and Unshared Expression Construction in Haskell

Source: https://github.com/heinrichapfelmus/reactive-banana/blob/master/reactive-banana/doc/design/design.md

Demonstrates two ways to construct expressions using monadic combinators. `y` uses the host language's `let` for immediate binding, while `y'` uses monadic assignment `<-` for delayed binding, illustrating the difference between immediate and shared variable binding.

```haskell
y :: M Name
y = do
    let x = val 1 + val 2
    x * x

y' :: M Name
y' = do
    x' <- val 1 + val 2
    let x = var x'
    x * x
```

--------------------------------

### Dynamically Switch Event Streams with switchE (Haskell)

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

The switchE function allows dynamic switching between different event sources. It takes an initial event stream and an event that emits new event streams, effectively changing the source of events over time. This is useful for scenarios where the event generation logic needs to adapt based on external triggers.

```Haskell
import Reactive.Banana
import Reactive.Banana.Frameworks

-- Switch to new event source when signal occurs
dynamicEventSource :: Event a -> Event (Event a) -> Moment (Event a)
dynamicEventSource initial eSwitchTo = switchE initial eSwitchTo

-- Example: Mode switching
networkDescription :: MomentIO ()
networkDescription = do
    (eInput, _) <- newEvent
    (eMode, _) <- newEvent

    let eDouble = fmap (*2) eInput
        eTriple = fmap (*3) eInput

    -- Switch between doubling and tripling based on mode events
    eResult <- switchE eDouble $ eTriple <$ eMode
    reactimate $ print <$> eResult
```

--------------------------------

### Observe Moment Values at Event Occurrences with observeE (Haskell)

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

The observeE function allows executing moment computations at the time events occur. It takes an event that emits moment computations and returns an event that yields the results of those computations. This is useful for sampling behavior values or performing side effects within the reactive network.

```Haskell
import Reactive.Banana

-- Execute moment computations when events occur
observeAtEvents :: Event (Moment a) -> Event a
observeAtEvents = observeE

-- Sample a behavior at specific events
sampleAtEvent :: Behavior a -> Event b -> Event a
sampleAtEvent b e = observeE $ valueB b <$ e
```

--------------------------------

### Creating a Union of Pulses in the Build Monad

Source: https://github.com/heinrichapfelmus/reactive-banana/blob/master/reactive-banana/doc/design/design.md

This function demonstrates how to combine two 'Pulse' values within the 'Build' monad, which represents changes to a mutable graph. This is a mid-level operation in the reactive-banana library's implementation.

```haskell
union :: Pluse a -> Pulse a -> Build (Pulse a)
```

--------------------------------

### Schedule and delay events

Source: https://github.com/heinrichapfelmus/reactive-banana/blob/master/reactive-banana/TODO.md

Schedules a sequence of events, delaying them but cutting them off if a new event arrives. This is considered very useful when used recursively.

```haskell
-- delay a sequence of events, but cut them off when a new one comes
-- very useful when used recursively
schedule :: Event (a,Duration) -> Event a
```

--------------------------------

### Monoid Instance for Event (a -> a) in reactive-banana

Source: https://github.com/heinrichapfelmus/reactive-banana/wiki/Multiple-Simultaneous-Events-(Issue-

Removes the general `Monoid` instance for `Event` and introduces a specific `Monoid` instance for `Event (a -> a)`. The `mempty` is `never`, and `mappend` is implemented using `unionWith` with function composition.

```haskell
instance Monoid (Event (a -> a)) where
    mempty  = never
    mappend = unionWith (flip (.))
```

--------------------------------

### Create New Events with newEvent (Haskell)

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

The newEvent function creates a pair: an Event and a function to fire that event. This allows for the creation of internal events within a reactive network, enabling feedback loops and complex event generation. The fire function can be used within reactimate or other parts of the network description.

```Haskell
import Reactive.Banana
import Reactive.Banana.Frameworks

-- Create event/handler pair
networkDescription :: MomentIO ()
networkDescription = do
    (eInput, fireInput) <- newEvent

    -- Use fireInput in reactimate to create feedback
    let eTransformed = fmap (*2) eInput

    reactimate $ (\x -> do
        print x
        when (x < 100) $ fireInput (x + 1)
        ) <$> eTransformed
```

--------------------------------

### Wrap an action in a caching mechanism using cache in Haskell

Source: https://github.com/heinrichapfelmus/reactive-banana/blob/master/reactive-banana/doc/design/design.md

The `cache` function takes an arbitrary monadic action `m a` and wraps it in the `Cached` monad. This allows the action to be executed only once, with subsequent calls returning the cached result, similar to combining `unsafePerformIO` and `lookup`.

```haskell
cache :: m a -> Cached m a
```

--------------------------------

### Arithmetic Expression Data Type

Source: https://github.com/heinrichapfelmus/reactive-banana/blob/master/reactive-banana/doc/design/design.md

Defines a simple data type `Exp` for representing arithmetic expressions, including integer values, addition, and multiplication. This serves as a basis for demonstrating expression evaluation and embedding.

```haskell
data Exp
    = Val Int     -- plain `Int`
    | Add Exp Exp -- addition
    | Mul Exp Exp -- multiplication
```

--------------------------------

### Time behavior in reactive systems

Source: https://github.com/heinrichapfelmus/reactive-banana/blob/master/reactive-banana/TODO.md

Represents the current time within a reactive system, likely as a behavior that updates over time. This is crucial for enabling continuous time behaviors.

```haskell
time :: Behavior Time
```

--------------------------------

### Observable Sharing Addition Combinator in Haskell

Source: https://github.com/heinrichapfelmus/reactive-banana/blob/master/reactive-banana/doc/design/design.md

Implements an addition combinator `(+)` with observable sharing. It uses `unsafePerformIO` to conditionally generate a fresh name. If the name is already bound, it reuses the existing name; otherwise, it binds a new `Add` expression. This ensures shared sub-expressions are evaluated only once.

```haskell
-- with observable sharing
(+) :: M Name -> M Name -> M Name
(+) = \a b -> unsafePerformIO $ do
    name <- fresh
    pure $ do
        mexp <- lookup name
        case mexp of
            Nothing -> do
                x <- a
                y <- b
                bind name $ Add (Var x) (Var y)
            Just _  ->
                pure $ Var name
```

--------------------------------

### Execute IO Actions on Event Occurrence with reactimate (Haskell)

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

The reactimate function allows executing arbitrary IO actions whenever an event occurs. It takes an Event and an IO action that consumes the event's value, enabling side effects like printing, file writing, or network communication within the reactive system. Multiple reactimate calls can observe the same event.

```Haskell
import Reactive.Banana
import Reactive.Banana.Frameworks

-- Execute IO on event occurrence
networkDescription :: MomentIO ()
networkDescription = do
    (eInput, fireInput) <- newEvent

    -- Print each event
    reactimate $ print <$> eInput

    -- Multiple reactimates can observe the same event
    reactimate $ putStrLn . ("Logged: ") . show <$> eInput

-- Example with side effects
networkWithSideEffects :: MomentIO ()
networkWithSideEffects = do
    (eSave, _) <- newEvent

    reactimate $ (\content -> do
        writeFile "output.txt" content
        putStrLn "File saved!"
        ) <$> eSave
```

--------------------------------

### Addition Combinator for Monadic Expressions in Haskell

Source: https://github.com/heinrichapfelmus/reactive-banana/blob/master/reactive-banana/doc/design/design.md

Implements an addition combinator `(+)` for monadic expressions. It executes two argument 'macros' (monadic actions), retrieves their resulting names, generates a fresh name for the addition, and binds an `Add` expression using the retrieved names.

```haskell
(+) :: M Name -> M Name -> M Name
(+) a b = do
    x <- a
    y <- b
    name <- fresh
    bind name $ Add (Var x) (Var y)
    pure name
```

--------------------------------

### Define a one-time event with duration

Source: https://github.com/heinrichapfelmus/reactive-banana/blob/master/reactive-banana/TODO.md

Defines an event that occurs once at a specific duration relative to the UNIX epoch. This primitive is noted as not very useful without dynamic event switching.

```haskell
type Duration = -- time difference in UNIX epoch

-- event that happens once (not very useful without dynamic event switching)
once :: a -> Duration -> Event era a
```

--------------------------------

### Observe Behavior Changes with changes (Haskell)

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

The changes function adapts a behavior to produce an event stream that fires whenever the behavior's value changes. This is useful for reacting to state updates in a granular way. It typically works in conjunction with reactimate' to process these change events.

```Haskell
import Reactive.Banana
import Reactive.Banana.Frameworks

-- Observe behavior changes
networkDescription :: MomentIO ()
networkDescription = do
    (eUpdate, _) <- newEvent

    bValue <- stepper "initial" eUpdate

    -- Get events when behavior changes
    eChanges <- changes bValue

    -- Use reactimate' with Future values from changes
    reactimate' $ fmap (putStrLn . ("New value: ") ++) <$> eChanges
```

--------------------------------

### never Event Combinator in Haskell

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

Demonstrates the `never` combinator in Reactive-banana, which creates an event that never occurs. It's shown as a base case for switching event streams.

```Haskell
import Reactive.Banana

-- An event that never fires
emptyEvent :: Event a
emptyEvent = never

-- Useful as a base case for switching
example :: Event Int -> Moment (Event Int)
example e = switchE never (const never <$> e)
```

--------------------------------

### Define Behavior and Events types using Cached monad in Haskell

Source: https://github.com/heinrichapfelmus/reactive-banana/blob/master/reactive-banana/doc/design/design.md

Defines the `Behavior` and `Events` types as `Cached` actions within the `Moment` monad. This caching ensures that the underlying computations, such as building `Pulse` or `Latch` values, are executed only once.

```haskell
type Behavior a = Cached Moment (Latch a, Pulse ())
```

```haskell
type Events a   = Cached Moment (Pulse a)
```

--------------------------------

### Keep Only the First Event Occurrence with once (Haskell)

Source: https://context7.com/heinrichapfelmus/reactive-banana/llms.txt

The once function filters an event stream, ensuring that only the first occurrence of an event is propagated. Subsequent events are discarded. This is commonly used for initialization logic or actions that should only happen once.

```Haskell
import Reactive.Banana
import Reactive.Banana.Frameworks

-- Get only the first event
firstEventOnly :: Event a -> Moment (Event a)
firstEventOnly = once

-- Example: Initialize on first click
networkDescription :: MomentIO ()
networkDescription = do
    (eClick, _) <- newEvent

    eFirstClick <- once eClick
    reactimate $ putStrLn "First click received!" <$ eFirstClick
```

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