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Haskell Style Guide

This document describes coding and comment style for the Snap projects. Currently we're more interested in building a working web framework than being code nazis and enforcing this style guide. However, it will be easier on everyone in the long run if contributors follow these guidelines. When something isn't covered by this guide you should stay consistent with the style used in our existing code.

Table of Contents

  1. Formatting
  2. Imports
  4. Naming
  5. Misc

1. Formatting

Line Length

Maximum line length is 80 characters.


Tabs are illegal. Use spaces for indenting. Indent your code blocks with 4 spaces. Indent the where keyword two spaces to set it apart from the rest of the code and indent the definitions in a where clause 2 spaces. Some examples:

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

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 judgment 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.

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)


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

2. Imports

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.


You should strive to write self-documenting code. Comments should be more about the why than the what. They should make the reader aware of higher-level concerns, ideas, constraints, pitfalls that are not obvious from the code at hand.

Line Length

Maximum line length is 70 characters. This increases readability as the eye has to travel back to the start of the next line.


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


Comments should be written from the perspective of a narrator addressing the user talking about the code. The comment for a function called send should be "Sends a message...", not "Send a message...". It is understood that the subject of the sentence is "The send function", so those words should be omitted.

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:

-- | Sends 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.

4. Naming

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.

5. Misc


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

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