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MGSloan Haskell Style

This is a short document describing my preferred Haskell style, which is a somewhat compactified version of the style outlined in tibbe's haskell-style-guide.


Line Length

Maximum line length is 80 characters.


Tabs are illegal. Use a minimal number spaces for indenting, while still having consistent indentation. This ends up being giving everything 2 space indents, with where indented by one.

sayHello :: IO ()
sayHello = do
  name <- getLine
  putStrLn $ greeting name
  greeting name = "Hello, " ++ name ++ "!"

filter :: (a -> Bool) -> [a] -> [a]
filter _ []   = []
filter p (x:xs)
  | p x       = x : filter p xs
  | otherwise = filter p xs

When definitions get longer than one line, and the root of the expression is not a short do / let / if / case introduction, place the function definition "=" symbol on the next line.

TODO: Example of this

Blank Lines

One blank line between top-level definitions. No blank lines between type signatures and function definitions. Add one blank line between functions in a type class instance declaration if the functions bodies are large. Use your judgement.


Surround binary operators with a single space on either side. Use your better judgement for the insertion of spaces around arithmetic operators but always be consistent about whitespace on either side of a binary operator. Don't insert a space after a lambda. Pad multiline parenthesis with a space.

Data Declarations

Align the constructors in a data type definition. Example:

data Tree a = Branch a (Tree a) (Tree a)
            | Leaf

For long type names the following formatting is also acceptable:

data HttpException
  = InvalidStatusCode Int
  | MissingContentHeader

Format records as follows:

data Person = Person
  { firstName :: String  -- ^ First name
  , lastName  :: String  -- ^ Last name
  , age       :: Int     -- ^ Age
  } deriving (Eq, Show)

Serial Declarations

Align the elements of lists, tuples, and curly brace/semicolon syntax. Example:

exceptions =
  [ InvalidStatusCode
  , MissingContentHeader
  , InternalServerError

Optionally, you can skip the first newline. Use your judgement.

directions = [ North
             , East
             , South
             , West

TODO: Extend this to boolean operators (particularly ($) and (.))


Put pragmas immediately following the function they apply to. Example:

id :: a -> a
id x = x
{-# INLINE id #-}

In the case of data type definitions you must put the pragma before the type it applies to. Example:

data Array e = Array
    {-# UNPACK #-} !Int

Hanging Lambdas

You may or may not indent the code following a "hanging" lambda. Use your judgement. Some examples:

bar :: IO ()
bar = forM_ [1, 2, 3] $ \n -> do
          putStrLn "Here comes a number!"
          print n

foo :: IO ()
foo = alloca 10 $ \a ->
      alloca 20 $ \b ->
      cFunction a b

Export Lists

Format export lists as follows:

module Data.Set
  -- * The @Set@ type
  , empty
  , singleton

  -- * Querying
  , member
  ) where


Imports should be grouped in the following order:

  1. standard library imports
  2. related third party imports
  3. local application/library specific imports

Put a blank line between each group of imports. The imports in each group should be sorted alphabetically, by module name.

Always use explicit import lists or qualified imports for standard and third party libraries. This makes the code more robust against changes in these libraries. Exception: The Prelude.



Write proper sentences; start with a capital letter and use proper punctuation.

Top-Level Definitions

Comment every top level function (particularly exported functions), and provide a type signature; use Haddock syntax in the comments. Comment every exported data type. Some examples:

-- | Send a message on a socket.  The socket must be in a connected
-- state.  Returns the number of bytes sent.  Applications are
-- responsible for ensuring that all data has been sent.
send :: Socket      -- ^ Connected socket
     -> ByteString  -- ^ Data to send
     -> IO Int      -- ^ Bytes sent

-- | Bla bla bla.
data Person = Person
    { age  :: Int     -- ^ Age
    , name :: String  -- ^ First name

For functions the documentation should give enough information to apply the function without looking at the function's definition.

End-of-Line Comments

Separate end-of-line comments from the code using 2 spaces. Align comments for data type definitions. Some examples:

data Parser = Parser
  Int         -- Current position
  ByteString  -- Remaining input

foo :: Int -> Int
foo n = salt * 32 + 9
  salt = 453645243  -- Magic hash salt.


Use in-line links economically. You are encouraged to add links for API names. It is not necessary to add links for all API names in a Haddock comment. We therefore recommend adding a link to an API name if:

  • The user might actually want to click on it for more information (in your judgment), and

  • Only for the first occurrence of each API name in the comment (don't bother repeating a link)


Use mixed-case when naming functions and camel-case when naming data types.

For readability reasons, don't capitalize all letters when using an abbreviation. For example, write HttpServer instead of HTTPServer. Exception: Two letter abbreviations, e.g. IO.


Use singular when naming modules e.g. use Data.Map and Data.ByteString.Internal instead of Data.Maps and Data.ByteString.Internals.



Code should be compilable with -Wall -Werror. There should be no warnings.

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