### Basic Formatting Example

Source: https://fourmolu.github.io/

Demonstrates basic code formatting with Fourmolu. Ensure Fourmolu is installed and accessible in your PATH.

```shell
fourmolu --check src/MyModule.hs
```

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

### Inline let-style Example

Source: https://fourmolu.github.io/config/let-style

Demonstrates the `inline` style for `let` blocks where bindings appear on the same line as the `let`/`in` keywords.

```Haskell
testUser =
    let name = "Alice"
        age = 30
     in User name age

```

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

### Default Indentation (4 spaces)

Source: https://fourmolu.github.io/config/indentation

Example demonstrating default code indentation with 4 spaces.

```Haskell
login :: IO ()
login = do
    name <- getLine
    if name == "admin"
        then putStrLn "Logged in!"
        else do
            putStrLn "Incorrect username"
            login

```

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

### Type Annotation Example

Source: https://fourmolu.github.io/

Demonstrates a type annotation involving MonadIO and a function type.

```Haskell
([],
 (EpaComments
  [])))
 (HsAppTy
  (NoExtField)
  (L
   (EpAnn
    (EpaSpan { <interactive>:25:6-14 })
    (AnnListItem
     [])
    (EpaComments
     []))
   (HsTyVar
    (NoEpTok)
    (NotPromoted)
    (L
     (EpAnn
      (EpaSpan { <interactive>:25:6-12 })
      (NameAnnTrailing
       [])
      (EpaComments
       []))
     (Unqual
      {OccName: MonadIO}))))
  (L
   (EpAnn
    (EpaSpan { <interactive>:25:14 })
    (AnnListItem
     [])
    (EpaComments
     []))
   (HsTyVar
    (NoEpTok)
    (NotPromoted)
    (L
     (EpAnn
      (EpaSpan { <interactive>:25:14 })
      (NameAnnTrailing
       [])
      (EpaComments
       []))
     (Unqual
      {OccName: m})))))))
 (L
  (EpAnn
   (EpaSpan { <interactive>:26:5-15 })
   (AnnListItem
    [])
   (EpaComments
    []))
  (HsFunTy
   (NoExtField)
   (HsUnrestrictedArrow
    (EpUniTok
     (EpaSpan { <interactive>:26:9-10 })
     (NormalSyntax)))
   (L
    (EpAnn
     (EpaSpan { <interactive>:26:5-7 })
     (AnnListItem
      [])
     (EpaComments
      []))
    (HsTyVar
     (NoEpTok)
     (NotPromoted)
     (L
      (EpAnn
       (EpaSpan { <interactive>:26:5-7 })
       (NameAnnTrailing
        [])
       (EpaComments
        []))
      (Unqual
       {OccName: Int}))))
   (L
    (EpAnn
     (EpaSpan { <interactive>:26:12-15 })
     (AnnListItem
      [])
     (EpaComments
      []))
    (HsAppTy
     (NoExtField)
     (L
      (EpAnn
       (EpaSpan { <interactive>:26:12 })
       (AnnListItem
        [])
       (EpaComments
        []))
      (HsTyVar
       (NoEpTok)
       (NotPromoted)
       (L
        (EpAnn
         (EpaSpan { <interactive>:26:12 })
         (NameAnnTrailing
          [])
         (EpaComments
          []))
        (Unqual
         {OccName: m}))))
     (L
      (EpAnn
       (EpaSpan { <interactive>:26:14-15 })
       (AnnListItem
        [])
       (EpaComments
        []))
      (HsTupleTy
       (AnnParens
        (EpTok
         (EpaSpan { <interactive>:26:14 }))
        (EpTok
         (EpaSpan { <interactive>:26:15 })))
       (HsBoxedOrConstraintTuple)
       []))))))))))))
```

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

### Ormolu Indentation (2 spaces)

Source: https://fourmolu.github.io/config/indentation

Example demonstrating code indentation with 2 spaces, similar to Ormolu's style.

```Haskell
login :: IO ()
login = do
  name <- getLine
  if name == "admin"
    then putStrLn "Logged in!"
    else do
      putStrLn "Incorrect username"
      login

```

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

### Newline let-style Example

Source: https://fourmolu.github.io/config/let-style

Illustrates the `newline` style for `let` blocks, where bindings are placed on the line following the `let`/`in` keywords.

```Haskell
testUser =
    let
        name = "Alice"
        age = 30
     in
        User name age

```

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

### Leading Comma Style Example

Source: https://fourmolu.github.io/config/comma-style

Demonstrates the default `leading` comma style for multi-line data declarations and lists.

```Haskell
data User = User
    {
      userName :: String,
      userAge :: Int,
      userActive :: Bool
    }

forbiddenPasswords :: [String]
forbiddenPasswords =
    [
      "password",
      "hunter2",
      "123456"
    ]

```

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

### Using Fourmolu Programmatically (Haskell)

Source: https://fourmolu.github.io/

Provides an example of how to use Fourmolu's formatting capabilities within a Haskell program. Requires importing the Fourmolu module.

```haskell
import Fourmolu

main :: IO ()
main = do
  let code = "import Data.List\n\nfoo = [1,2,3]"
  formattedCode <- format code
  putStrLn formattedCode
```

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

### Record Brace Space: With Spacing

Source: https://fourmolu.github.io/config/record-brace-space

This example shows the `record-brace-space` option set to `true`, where a space is present before the opening brace of a record.

```haskell
getUsername :: User -> String
getUsername User {..} = username

```

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

### Code with Parentheses

Source: https://fourmolu.github.io/config/single-constraint-parens

Example of a function signature with parentheses around a single constraint.

```Haskell
logAction :: (MonadLogger m) => String -> m ()
logAction msg = logDebugN $ "Action: " <> msg

```

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

### Fixed Point Example for Multiline Constructs in Do Blocks

Source: https://fourmolu.github.io/changelog

This snippet demonstrates a fixed point for formatting multiline constructs within do blocks, illustrating potential odd indentations.

```haskell
foo = do
  do
    (+1)
   1


```

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

### Single-line Haddock Comments

Source: https://fourmolu.github.io/config/haddock-style

Example of single-line Haddock comments using -- | syntax.

```haskell
-- | Get the user with the given username
getUser :: String -> IO (Maybe User)

-- | Same as 'getUsername', except throw
-- an error if the user doesn't exist.
loadUser :: String -> IO User

```

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

### Multi-line Haddock Comments

Source: https://fourmolu.github.io/config/haddock-style

Example of multi-line Haddock comments using {-# ... #-} syntax.

```haskell
-- | Get the user with the given username
getUser :: String -> IO (Maybe User)

{- | Same as 'getUsername', except throw
an error if the user doesn't exist.
-}
loadUser :: String -> IO User

```

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

### Code without Trailing Section Operators

Source: https://fourmolu.github.io/config/trailing-section-operators

Example showing code formatting when trailing section operators are disabled. Each application of the operator starts on a new indented line.

```Haskell
foo :: IO ()
foo =
    bar
        $ baz
        $ bat
        $ quiz x

```

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

### Code With Unicode Syntax

Source: https://fourmolu.github.io/config/unicode

Example of Haskell code with Unicode syntax enabled, demonstrating the use of Unicode characters for operators and type annotations.

```haskell
{-# LANGUAGE UnicodeSyntax #}

loadUser ∷ ∀ m. (MonadIO m) ⇒ String → m User
loadUser username = do
    mUser ← getUser username
    maybe (error "Unknown user") pure mUser


```

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

### Multi-line Compact Haddock Comments

Source: https://fourmolu.github.io/config/haddock-style

Example of multi-line compact Haddock comments using {-# ... #-} syntax on a single line.

```haskell
-- | Get the user with the given username
getUser :: String -> IO (Maybe User)

{-| Same as 'getUsername', except throw
an error if the user doesn't exist.
-}
loadUser :: String -> IO User

```

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

### Function signature with sorted constraints

Source: https://fourmolu.github.io/config/sort-constraints

Example of a function signature where constraints are sorted alphabetically. This occurs when `sort-constraints` is set to `true`.

```haskell
f :: (Eq a, Show a) => a

```

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

### Record Brace Space: Without Spacing

Source: https://fourmolu.github.io/config/record-brace-space

This example shows the `record-brace-space` option set to `false` (default), where no space is present before the opening brace of a record.

```haskell
getUsername :: User -> String
getUsername User{..} = username

```

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

### Code Without Unicode Syntax

Source: https://fourmolu.github.io/config/unicode

Example of Haskell code without enabling Unicode syntax. This is the default behavior.

```haskell
{-# LANGUAGE UnicodeSyntax #}

loadUser :: forall m. (MonadIO m) => String -> m User
loadUser username = do
    mUser <- getUser username
    maybe (error "Unknown user") pure mUser


```

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

### Custom Import Grouping Rule Example

Source: https://fourmolu.github.io/config/import-grouping

Defines a custom import grouping configuration with multiple named groups and specific rules based on glob patterns, scope, and qualification.

```yaml
import-grouping:
  - name: "Text modules"
    rules:
      - glob: Data.Text
  - name: "The rest"
    rules:
      - priority: 100
  - name: "My internals and monads unqualified"
    rules:
      - scope: local
        qualified: no
      - glob: Control.Monad
        qualified: no
  - name: "My internals and monads qualified"
    rules:
      - scope: local
        qualified: yes
      - glob: Control.Monad
        qualified: yes
  - name: "Specific monads"
    rules:
      - glob: Control.Monad.**

```

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

### Enable OverloadedRecordDot and OverloadedRecordUpdate

Source: https://fourmolu.github.io/changelog

These GHC language extensions are supported by Ormolu starting from version 9.2. They are disabled by default.

```haskell
OverloadedRecordDot
OverloadedRecordUpdate
```

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

### Haddock Single-line Module Docstring

Source: https://fourmolu.github.io/config/haddock-style-module

Example of a Haddock module docstring using line comment syntax. This style is suitable for concise module descriptions.

```haskell
-- |
-- Module: MyModule
-- Author: Joe Haskell
--
-- The primary API for my library.
module MyModule where


```

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

### Code without Parentheses

Source: https://fourmolu.github.io/config/single-constraint-parens

Example of a function signature without parentheses around a single constraint.

```Haskell
logAction :: MonadLogger m => String -> m ()
logAction msg = logDebugN $ "Action: " <> msg

```

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

### Aligned Record Style

Source: https://fourmolu.github.io/config/record-style

Demonstrates the 'aligned' style for record declarations and default values. Braces are aligned with the start of the record definition.

```Haskell
data User = User
    {
      userName :: String,
      userAge :: Int,
      userActive :: Bool
    }

defaultUser =
    User
        {
          userName = "",
          userAge = -1,
          userActive = false
        }

```

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

### Haddock Multi-line Compact Module Docstring

Source: https://fourmolu.github.io/config/haddock-style-module

Example of a compact multi-line Haddock module docstring using block comment syntax. This style minimizes whitespace within the comment block.

```haskell
{-|
Module: MyModule
Author: Joe Haskell

The primary API for my library.
-}
module MyModule where


```

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

### Deriving with Parentheses

Source: https://fourmolu.github.io/config/single-deriving-parens

Example of a deriving clause formatted with parentheses. Use this when you want to enforce the presence of parentheses around single deriving clauses.

```Haskell
data Foo = Foo
    deriving stock (Show)

```

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

### Configure Indentation in YAML

Source: https://fourmolu.github.io/config

Specify the `indentation` option within the `fourmolu.yaml` file to set the number of spaces per indentation step. This is an example of how to set a specific configuration option in the YAML file.

```yaml
indentation: 2

```

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

### Example of Sorted Deriving Clauses

Source: https://fourmolu.github.io/config/sort-deriving-clauses

This snippet demonstrates a `newtype` declaration with alphabetically sorted deriving clauses. This is the result when `sort-deriving-clauses` is `true`.

```haskell
newtype A = A Int
    deriving (ToJSON)
    deriving stock (Show, Eq, Ord, Generic)
    deriving newtype (Num)

```

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

### Haddock Multi-line Module Docstring

Source: https://fourmolu.github.io/config/haddock-style-module

Example of a multi-line Haddock module docstring using block comment syntax. This style is often used for more detailed module descriptions.

```haskell
{- |
Module: MyModule
Author: Joe Haskell

The primary API for my library.
-}
module MyModule where


```

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

### Code with Trailing Section Operators

Source: https://fourmolu.github.io/config/trailing-section-operators

Example demonstrating the use of trailing section operators, where the expression continues indented below the operator.

```Haskell
foo :: IO ()
foo =
    bar $
        baz $
            bat $
                quiz x

```

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

### Example of Unsorted Deriving Clauses

Source: https://fourmolu.github.io/config/sort-deriving-clauses

This snippet shows a `newtype` declaration with deriving clauses in their original, unsorted order. Use this when `sort-deriving-clauses` is `false`.

```haskell
newtype A = A Int
    deriving newtype (Num)
    deriving (ToJSON)
    deriving stock (Show, Eq, Ord, Generic)

```

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

### Function signature without sorted constraints

Source: https://fourmolu.github.io/config/sort-constraints

Example of a function signature where constraints are not sorted alphabetically. This is the default behavior when `sort-constraints` is `false`.

```haskell
f :: (Show a, Eq a) => a

```

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

### Deriving without Parentheses

Source: https://fourmolu.github.io/config/single-deriving-parens

Example of a deriving clause formatted without parentheses. Use this when you want to enforce the absence of parentheses around single deriving clauses.

```Haskell
data Foo = Foo
    deriving stock Show

```

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

### Haskell Import Statement with Explicit Imports

Source: https://fourmolu.github.io/

Demonstrates a Haskell import declaration for the 'Person' module, explicitly importing 'Person', 'name', and 'loadPerson'. It also shows an example of importing a module with a qualified name.

```haskell
import Person (Person(..), name, loadPerson)
```

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

### Derived Classes With Sorting

Source: https://fourmolu.github.io/config/sort-derived-classes

Example of a deriving clause where classes are sorted alphabetically. This behavior is enabled when `sort-derived-classes` is true. Note that comments within deriving clauses may be misplaced when this option is used.

```haskell
data A
    deriving stock (Eq, Generic, Ord, Show)

```

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

### Formatting with Configuration

Source: https://fourmolu.github.io/

Shows how to apply formatting using a specific configuration file. The config file allows for custom formatting rules.

```shell
fourmolu --config fourmolu.yaml src/MyModule.hs
```

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

### Run Query with Parameters

Source: https://fourmolu.github.io/

This snippet demonstrates how to execute a query with specific parameters. It shows the invocation of a `runQuery` function with a `queryText` and a list of arguments, likely for dynamic query generation or data filtering.

```haskell
runQuery queryText [arg1, arg2]
```

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

### Formatting Multiple Files

Source: https://fourmolu.github.io/

Illustrates formatting multiple Haskell files at once. This is useful for batch processing your codebase.

```shell
fourmolu src/
```

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

### Generate Default Configuration File

Source: https://fourmolu.github.io/config

Use this command to create a `fourmolu.yaml` file pre-filled with default configuration values. This file can then be edited to customize FourMolu's behavior.

```bash
$ fourmolu --print-defaults > fourmolu.yaml

```

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

### Ignoring Specific Files

Source: https://fourmolu.github.io/

Demonstrates how to exclude certain files or directories from the formatting process using a .fourmolu-ignore file.

```shell
fourmolu --ignore-config .fourmolu-ignore src/
```

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

### Minimal Imports with Internal Modules Grouped

Source: https://fourmolu.github.io/config/import-grouping

A concise import style focusing on essential modules, with internal modules grouped separately.

```haskell
import Data.Text (Text)
import qualified Data.Text

import Data.Maybe (maybe)
import Data.Text.IO (hGetLine)
import qualified System.IO as SIO
import Text.Printf (printf)

import Control.Monad (Monad (..))
import SomeInternal.Module1 (anotherDefinition, someDefinition)
import SomeInternal.Module1.SubModuleA

import qualified SomeInternal.Module2 as Mod2

import qualified Control.Monad.Error as Error
import Control.Monad.State.Lazy (MonadState (..))

```

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

### Standard Import Grouping

Source: https://fourmolu.github.io/config/import-grouping

Demonstrates a common import grouping strategy with standard library modules first, followed by internal and qualified imports.

```haskell
import Control.Monad (Monad (..))
import qualified Control.Monad.Error as Error
import Control.Monad.State.Lazy (MonadState (..))
import Data.Maybe (maybe)
import Data.Text (Text)
import qualified Data.Text
import Data.Text.IO (hGetLine)
import SomeInternal.Module1 (anotherDefinition, someDefinition)
import SomeInternal.Module1.SubModuleA
import qualified SomeInternal.Module2 as Mod2
import qualified System.IO as SIO
import Text.Printf (printf)

```

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

### Derived Classes Without Sorting

Source: https://fourmolu.github.io/config/sort-derived-classes

Example of a deriving clause where classes are not sorted alphabetically. This is the default behavior when `sort-derived-classes` is false.

```haskell
data A
    deriving stock (Show, Eq, Ord, Generic)

```

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

### Use OverloadedRecordDot for Getter Syntax

Source: https://fourmolu.github.io/changelog

Consider using the OverloadedRecordDot language extension as a replacement for the record-dot-preprocessor.

```haskell
OverloadedRecordDot
```

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

### Provide Fixity Overrides via Command Line

Source: https://fourmolu.github.io/changelog

Use the -f or --fixity command-line option to provide user-defined fixity declarations for Ormolu. Each declaration should be on a new line.

```bash
-f / --fixity
```

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

### Code Formatting with 40 Character Limit

Source: https://fourmolu.github.io/config/column-limit

This snippet demonstrates code formatting when a 40-character column limit is enforced. Lines are broken to stay within this limit.

```haskell
rootUsers :: [String]
rootUsers = ["root"]

adminUsers :: [String]
adminUsers =
    [ "hunter2"
    , "elon.musk"
    , "simon.peyton.jones"
    ]

regularUsers :: [String]
regularUsers =
    [ "alice"
    , "bob"
    , "charlie"
    , "david"
    , "eve"
    , "felicia"
    , "greg"
    , "hailey"
    , "isaac"
    ]

```

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

### Leading Import/Export Style

Source: https://fourmolu.github.io/config/import-export-style

Illustrates the 'leading' style for import and export lists. This style groups items on the same line when possible, with subsequent items on new lines.

```Haskell
module MyModule
    ( User (..)
    , createUser
    , deleteUser
    ) where

import MyModule.Internal
    ( User
    , createUser
    , deleteUser
    )


```

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

### Format Backpack Signature Files with Ormolu

Source: https://fourmolu.github.io/changelog

Ormolu supports formatting Backpack signature files (.hsig). The source type can be overridden with the -t or --source-type command line option.

```bash
-t / --source-type
```

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

### Default Code Formatting (No Limit)

Source: https://fourmolu.github.io/config/column-limit

This snippet shows the default formatting when no column limit is applied. Lines are kept as short as possible.

```haskell
rootUsers :: [String]
rootUsers = ["root"]

adminUsers :: [String]
adminUsers = ["hunter2", "elon.musk", "simon.peyton.jones"]

regularUsers :: [String]
regularUsers = ["alice", "bob", "charlie", "david", "eve", "felicia", "greg", "hailey", "isaac"]

```

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

### Diff-Friendly Import/Export Style

Source: https://fourmolu.github.io/config/import-export-style

Demonstrates the 'diff-friendly' style for import and export lists, which is the default. This style places each item on a new line, indented.

```Haskell
module MyModule (
    User (..),
    createUser,
    deleteUser,
) where

import MyModule.Internal (
    User,
    createUser,
    deleteUser,
 )

```

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

### Load Person Function Definition

Source: https://fourmolu.github.io/

Defines a function `loadPerson` which is likely used for loading person data. The snippet shows the start of a function binding and pattern matching.

```Haskell
(L
 (EpAnn
  (EpaSpan { <interactive>:(27,1)-(35,72) })
  (AnnListItem
   [])
  (EpaComments
   []))
 (ValD
  (NoExtField)
  (FunBind
   (NoExtField)
   (L
    (EpAnn
     (EpaSpan { <interactive>:27:1-10 })
     (NameAnnTrailing
      [])
     (EpaComments
      []))
    (Unqual
     {OccName: loadPerson}))
   (MG
    (FromSource)
    (L
     (EpAnn
      (EpaSpan { <interactive>:(27,1)-(35,72) })
      (AnnList
       (Nothing)
       (ListNone)
       []
       (NoEpTok)
       [])
      (EpaComments
       []))
     [(L
       (EpAnn
        (EpaSpan { <interactive>:(27,1)-(35,72) })
        (AnnListItem
         [])
        (EpaComments
         []))
       (Match
        (NoExtField)
        (FunRhs
         (L
          (EpAnn

```

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

### Set local modules via CLI

Source: https://fourmolu.github.io/config/local-modules

Use this option to manually define local modules for import grouping when running Ormolu from the command line. This is useful when a Cabal file is not available or needs to be supplemented.

```shell
fourmolu Main.hs --import-grouping by-scope --local-modules SomeInternal.Module1 --local-modules SomeModule

```

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

### Manually Specify Ormolu Packages

Source: https://fourmolu.github.io/changelog

Use the -p or --package option to manually specify packages for Ormolu to infer fixity and precedence information from. This option can be repeated.

```bash
-p / --package
```

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

### Diffing Formatted Code

Source: https://fourmolu.github.io/

Compares the current code with its formatted version without applying changes. This is helpful for code reviews.

```shell
fourmolu --diff src/MyModule.hs
```

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

### Standard Imports with Internal Modules Grouped

Source: https://fourmolu.github.io/config/import-grouping

Presents a style where standard library imports are listed first, followed by internal modules, and then qualified imports.

```haskell
import Control.Monad (Monad (..))
import Control.Monad.State.Lazy (MonadState (..))
import Data.Maybe (maybe)
import Data.Text (Text)
import Data.Text.IO (hGetLine)
import Text.Printf (printf)

import qualified Control.Monad.Error as Error
import qualified Data.Text
import qualified System.IO as SIO

import SomeInternal.Module1 (anotherDefinition, someDefinition)
import SomeInternal.Module1.SubModuleA

import qualified SomeInternal.Module2 as Mod2

```

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

### Function Signature with Leading Haddock Comments

Source: https://fourmolu.github.io/config/haddock-location-signature

Demonstrates placing Haddock comments before each argument and the return type when using leading function arrows.

```Haskell
func
    -- | Arg 1
    :: Int
    -- | Arg 2
    -> String
    -- | Return value
    -> Int
```

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

### Mixed Grouping with Internal Imports Later

Source: https://fourmolu.github.io/config/import-grouping

Illustrates a grouping where standard library modules are imported first, followed by internal modules, and then qualified imports.

```haskell
import Control.Monad (Monad (..))
import qualified Control.Monad.Error as Error
import Control.Monad.State.Lazy (MonadState (..))
import Data.Maybe (maybe)
import Data.Text (Text)
import qualified Data.Text
import qualified System.IO as SIO
import Text.Printf (printf)

import SomeInternal.Module1 (anotherDefinition, someDefinition)
import SomeInternal.Module1.SubModuleA
import qualified SomeInternal.Module2 as Mod2

```

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

### Type Signature with Leading Arguments Arrow Style

Source: https://fourmolu.github.io/config/function-arrows

Shows the type signature with the `leading-args` arrow style, which places the arrow at the beginning of the line, similar to `leading`, but with specific handling for arguments.

```haskell
show2WithLabel ::
    forall a b.
    (Show a, Show b) =>
    String
    -> (a, b)
    -> String
```

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

### Grouped Qualified Imports Last

Source: https://fourmolu.github.io/config/import-grouping

Shows an import style where standard and specific imports come first, followed by qualified imports grouped at the end.

```haskell
import Control.Monad (Monad (..))
import Control.Monad.State.Lazy (MonadState (..))
import Data.Maybe (maybe)
import Data.Text (Text)
import Data.Text.IO (hGetLine)
import SomeInternal.Module1 (anotherDefinition, someDefinition)
import SomeInternal.Module1.SubModuleA
import Text.Printf (printf)

import qualified Control.Monad.Error as Error
import qualified Data.Text
import qualified SomeInternal.Module2 as Mod2
import qualified System.IO as SIO

```

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

### Code Formatting with 80 Character Limit

Source: https://fourmolu.github.io/config/column-limit

This snippet illustrates code formatting with an 80-character column limit. Lines are broken to adhere to this limit, showing a different breaking strategy than a tighter limit.

```haskell
rootUsers :: [String]
rootUsers = ["root"]

adminUsers :: [String]
adminUsers = ["hunter2", "elon.musk", "simon.peyton.jones"]

regularUsers :: [String]
regularUsers =
    [ "alice"
    , "bob"
    , "charlie"
    , "david"
    , "eve"
    , "felicia"
    , "greg"
    , "hailey"
    , "isaac"
    ]

```

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

### Type Signature with Leading Arrow Style

Source: https://fourmolu.github.io/config/function-arrows

Illustrates the type signature using the `leading` arrow style, where the arrow is placed at the beginning of the line.

```haskell
show2WithLabel
    :: forall a b
     . (Show a, Show b)
    => String
    -> (a, b)
    -> String
```

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

### Configure Operator Fixities in fourmolu.yaml

Source: https://fourmolu.github.io/changelog

Define custom operator fixities and precedences in the fourmolu.yaml configuration file. This is an alternative to using a .ormolu file.

```yaml
fixities:
  - 'infixr 8 .='
  - 'infixr 5 #'

```

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

### Align 'in' Keyword Left with 'let'

Source: https://fourmolu.github.io/config/in-style

Illustrates the `left-align` style for the 'in' keyword. This option aligns the 'in' keyword directly under the 'let' keyword.

```haskell
testUser =
    let name = "Alice"
        age = 30
    in  User name age

```

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

### Without Indented Wheres

Source: https://fourmolu.github.io/config/indent-wheres

Demonstrates the default behavior where 'where' bindings are half-indented relative to the preceding code body.

```Haskell
setPassword :: String -> String -> IO ()
setPassword username password = do
    user <- getUser username >>= maybe userNotFound pure
    hash <- hashPassword password
    saveUser user{userPasswordHash = hash}
  where
    userNotFound = error $ "User not found: " ++ username

```

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

### No Space Between 'let' and 'in'

Source: https://fourmolu.github.io/config/in-style

Shows the `no-space` style for the 'in' keyword. This option removes any extra space between the 'let' binding and the 'in' keyword.

```haskell
testUser =
    let name = "Alice"
        age = 30
    in User name age

```

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

### Haskell Abstract Syntax Tree (AST) Representation

Source: https://fourmolu.github.io/

This snippet represents a fragment of a Haskell Abstract Syntax Tree (AST). It details the structure of a module, including its name, imports, and exported entities.

```haskell
(HsModule
 (XModulePs
  (EpAnn
   (EpaSpan { <interactive>:1:1 })
   (AnnsModule
    (NoEpTok)
    (EpTok
     (EpaSpan { <interactive>:6:1-6 }))
    (EpTok
     (EpaSpan { <interactive>:12:3-7 }))
    []
    []
    (Just
     ((,)
      { <interactive>:36:1 }
      { <interactive>:35:72 })))))
 (EpaCommentsBalanced
  [(L
    (EpaSpan
     { <interactive>:1:1-30 })
    (EpaComment
     (EpaBlockComment
      "{-# LANGUAGE UnicodeSyntax #-}")
     { <interactive>:1:1 }))
  ,(L
    (EpaSpan
     { <interactive>:(3,1)-(5,2) })
    (EpaComment
     (EpaDocComment
      (NestedDocString
       (HsDocStringNext)
       (L
        { <interactive>:(3,5)-(5,2) }
        (HsDocStringChunk
         " This is the\nperson module.\n"))))
     { <interactive>:1:1-30 }))]
  []))
 (EpVirtualBraces
  (1))
 (Nothing)
 (Just
  (L
   { <interactive>:(3,1)-(5,2) }
   (WithHsDocIdentifiers
    (NestedDocString
     (HsDocStringNext)
     (L
      { <interactive>:(3,5)-(5,2) }
      (HsDocStringChunk
       " This is the\nperson module.\n"))) 
    []))))
 (Just
  (L
   (EpAnn
    (EpaSpan { <interactive>:6:8-13 })
    (AnnListItem
     [])
    (EpaComments
     []))
   {ModuleName: Person}))
 (Just
  (L
   (EpAnn
    (EpaSpan { <interactive>:(6,15)-(12,1) })
    (AnnList
     (Nothing)
     (ListParens
      (EpTok
       (EpaSpan { <interactive>:6:15 }))
      (EpTok
       (EpaSpan { <interactive>:12:1 }))) 
     []
     ((,)
      (NoEpTok)
      [])
     [])
    (EpaComments
     [])
   [(L
     (EpAnn
      (EpaSpan { <interactive>:(7,5)-(10,5) })
      (AnnListItem
       [(AddCommaAnn
         (EpTok
          (EpaSpan { <interactive>:10:6 })))])
      (EpaComments
       [])
     (IEThingWith
      ((,)
       (Nothing)
       ((,,,)
        (EpTok
         (EpaSpan { <interactive>:7:12 }))
        (NoEpTok)
        (NoEpTok)
        (EpTok
         (EpaSpan { <interactive>:10:5 }))))) 
      (L
       (EpAnn
        (EpaSpan { <interactive>:7:5-10 })
        (AnnListItem
         [])
        (EpaComments
         [])
       (IEName
        (NoExtField)
        (L
         (EpAnn
          (EpaSpan { <interactive>:7:5-10 })
          (NameAnnTrailing
           [])
          (EpaComments
           [])
         )
         (Unqual
          {OccName: Person}))))
      (NoIEWildcard)
      [(L
        (EpAnn
         (EpaSpan { <interactive>:8:9-14 })
         (AnnListItem
          [(AddCommaAnn
            (EpTok
             (EpaSpan { <interactive>:8:15 })))])
         (EpaComments
          [])
        )
        (IEName
         (NoExtField)
         (L
          (EpAnn
           (EpaSpan { <interactive>:8:9-14 })
           (NameAnnTrailing
            [])
           (EpaComments
            [])
          )
          (Unqual
           {OccName: Person}))))
      ,(L
        (EpAnn
         (EpaSpan { <interactive>:9:9-12 })
         (AnnListItem
          [])
         (EpaComments
          [])
        )
        (IEName
         (NoExtField)
         (L
          (EpAnn
           (EpaSpan { <interactive>:9:9-12 })
           (NameAnnTrailing
            [])
           (EpaComments
            [])
          )
          (Unqual
           {OccName: name}))))]
      (Nothing)))
   ,(L
     (EpAnn
      (EpaSpan { <interactive>:11:5-14 })
      (AnnListItem
       [(AddCommaAnn
         (EpTok
          (EpaSpan { <interactive>:11:15 })))])
      (EpaComments
       [])
     )
     (IEVar
      (Nothing)
      (L
       (EpAnn
        (EpaSpan { <interactive>:11:5-14 })
        (AnnListItem
         [])
        (EpaComments
         [])
       )
       (IEName
        (NoExtField)
        (L
         (EpAnn
          (EpaSpan { <interactive>:11:5-14 })
          (NameAnnTrailing
           [])
          (EpaComments
           [])
         )
         (Unqual
          {OccName: loadPerson}))))
      (Nothing)))]))
 [(L
   (EpAnn
    (EpaSpan { <interactive>:(14,1)-(18,2) })
    (AnnListItem
     [])
    (EpaComments
     [])
   )
   (ImportDecl
    (XImportDeclPass
     (EpAnn
      (EpaSpan { <interactive>:(14,1)-(18,2) })
      (EpAnnImportDecl
       (EpTok
        (EpaSpan { <interactive>:14:1-6 }))
       (Nothing)

```

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

### Basic Import Grouping Rule Definition

Source: https://fourmolu.github.io/config/import-grouping

A simple rule definition for an import group, specifying a name and a glob pattern for matching modules.

```yaml
name: "Text modules"
rules:
  - glob: Data.Text

```

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

### KNR Record Style

Source: https://fourmolu.github.io/config/record-style

Illustrates the 'knr' (Kernighan and Ritchie) style for record declarations and default values. Braces are placed on the same line as the record definition.

```Haskell
data User = User {
    userName :: String,
    userAge :: Int,
    userActive :: Bool
}

defaultUser =
    User {
        userName = "",
        userAge = -1,
        userActive = false
    }

```

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

### Trailing Import/Export Style

Source: https://fourmolu.github.io/config/import-export-style

Shows the 'trailing' style for import and export lists. This style places each item on a new line, with a trailing comma on the last item.

```Haskell
module MyModule
    ( User (..),
      createUser,
      deleteUser,
    ) where

import MyModule.Internal
    ( User,
      createUser,
      deleteUser,
    )


```

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

### Haskell Module Declaration with Language Extension

Source: https://fourmolu.github.io/

This snippet shows a basic Haskell module declaration with the UnicodeSyntax language extension enabled. It includes documentation comments for the module.

```haskell
{-# LANGUAGE UnicodeSyntax #-}

-- | This is the
-- person module.
module Person where
```

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

### Function Signature with Trailing Haddock Comments (Haddock '^')

Source: https://fourmolu.github.io/config/haddock-location-signature

Shows how to use the '^' syntax for Haddock comments placed after the type they describe, with leading function arrows.

```Haskell
func
    :: Int
    -- ^ Arg 1
    -> String
    -- ^ Arg 2
    -> Int
    -- ^ Return value
```

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

### Type Signature with Trailing Arrow Style

Source: https://fourmolu.github.io/config/function-arrows

Demonstrates the type signature with the default `trailing` arrow style, where the arrow is placed at the end of the line.

```haskell
show2WithLabel ::
    forall a b.
    (Show a, Show b) =>
    String ->
    (a, b) ->
    String
```

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

### Two Newlines Between Declarations

Source: https://fourmolu.github.io/config/newlines-between-decls

This snippet demonstrates how to configure two newlines between top-level declarations.

```haskell
newtype Username = Username String


newtype Password = Password String

```

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

### Haskell AST Representation

Source: https://fourmolu.github.io/

This snippet shows a Haskell Abstract Syntax Tree (AST) representation, likely for parsing or code analysis. It details annotations, spans, and variable occurrences.

```haskell
(AnnListItem [])
                                                          (EpaComments [])
                                                          (HsVar (NoExtField)
                                                          (L (EpAnn (EpaSpan { <interactive>:21:47-50 }) (NameAnnTrailing []) (EpaComments [])) (Unqual {OccName: show})))))
                                                        (L (EpAnn (EpaSpan { <interactive>:21:52-55 }) (AnnListItem []) (EpaComments [])) (HsVar (NoExtField) (L (EpAnn (EpaSpan { <interactive>:21:52-55 }) (NameAnnTrailing []) (EpaComments [])) (Unqual {OccName: rows})))))))]
                                                 (EmptyLocalBinds (NoExtField)))))]))))]
                                        []))
                                      (L (EpAnn (EpaSpan { <interactive>:21:60-71 }) (AnnListItem []) (EpaComments [])) (HsApp (NoExtField) (L (EpAnn (EpaSpan { <interactive>:21:60-69 }) (AnnListItem []) (EpaComments [])) (HsVar (NoExtField) (L (EpAnn (EpaSpan { <interactive>:21:60-69 }) (NameAnnTrailing []) (EpaComments [])) (Unqual {OccName: logAndFail})))) (L (EpAnn (EpaSpan { <interactive>:21:71 }) (AnnListItem []) (EpaComments [])) (HsVar (NoExtField) (L (EpAnn (EpaSpan { <interactive>:21:71 }) (NameAnnTrailing []) (EpaComments [])))))))))]
```

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

### Load Person Function

Source: https://fourmolu.github.io/

This snippet defines a function to load a person, likely used for data retrieval or initialization. It involves pattern matching and potentially a query execution mechanism.

```haskell
loadPerson :: Person
loadPerson = id
```