-- |The 'square' function squares an integer.
square :: Int -> Int
square x = x * x

square :: Int -> Int
-- ^The 'square' function squares an integer.
square x = x * x

-- |The 'square' function squares an integer.
-- It takes one argument, of type 'Int'.
square :: Int -> Int
square x = x * x

{-|
  The 'square' function squares an integer.
  It takes one argument, of type 'Int'.
-}
square :: Int -> Int
square x = x * x

class C a where
   -- | This is the documentation for the 'f' method
   f :: a -> Int
   -- | This is the documentation for the 'g' method
   g :: Int -> a

data T a b
  -- | This is the documentation for the 'C1' constructor
  = C1 a b
  -- | This is the documentation for the 'C2' constructor
  | C2 a b

data T a b
  = C1 a b  -- ^ This is the documentation for the 'C1' constructor
  | C2 a b  -- ^ This is the documentation for the 'C2' constructor

data R a b =
  C { -- | This is the documentation for the 'a' field
      a :: a,
      -- | This is the documentation for the 'b' field
      b :: b
    }

data R a b =
  C { a :: a  -- ^ This is the documentation for the 'a' field
    , b :: b  -- ^ This is the documentation for the 'b' field
    }

data T a = A { someField :: a -- ^ Doc for someField of A
             }
         | B { someField :: a -- ^ Doc for someField of B
             }

f  :: Int      -- ^ The 'Int' argument
   -> Float    -- ^ The 'Float' argument
   -> IO ()    -- ^ The return value

{-|
Module      : W
Description : Short description
Copyright   : (c) Some Guy, 2013
                  Someone Else, 2014
License     : GPL-3
Maintainer  : sample@email.com
Stability   : experimental
Portability : POSIX

Here is a longer description of this module, containing some
commentary with @some markup@.
-}
module W where
...

To Haddock the export list has even more significance than just specifying the entities to be included in the documentation. It also specifies the order that entities will be listed in the generated documentation. This leaves the programmer free to implement functions in any order he/she pleases, and indeed in any module he/she pleases, but still specify the order that the functions should be documented in the export list. Indeed, many programmers already do this: the export list is often used as a kind of ad-hoc interface documentation, with headings, groups of functions, type signatures and declarations in comments.

module Foo (
  -- * Classes
  C(..),
  -- * Types
  -- ** A data type
  T,
  -- ** A record
  R,
  -- * Some functions
  f, g
  ) where

module Foo (
  -- * Classes
  , C(..)
  -- * Types
  -- ** A data type
  , T
  -- ** A record
  , R
  -- * Some functions
  , f
  , g
  ) where

If modules are imported wholly and without any hiding qualifiers, then the documentation will just contain a cross-reference to the documentation for B and C.

module A (
  module B,
  module C
 ) where

If the modules are not completely re-exported, then Haddock behaves as if the set of entities re-exported from B and C had been listed explicitly in the export list.

module A (
  module B,
  module C
 ) where

import B hiding (f)
import C (a, b)

If there is no export list in the module, then every entity will be mentioned as defined at the top level in the module. The generated documentation will retain the order in which entities are defined in the module.

module Foo where

And the module body may also include section headings.

module Foo where

-- * This heading will now appear before foo.

-- | Documentation for 'foo'.
foo :: Integer
foo = 5

module Foo (
   -- * A section heading

   -- | Some documentation not attached to a particular Haskell entity
   ...
 ) where

module Foo (
   -- * A section heading

   -- $doc
   ...
 ) where

-- $doc
-- Here is a large chunk of documentation which may be referred to by
-- the name $doc.

This cheat sheet used the achievements from

• Haddock User Guide - Chapter 3. Documentation and Markup
• Haddock User Guide - Table of content
• License

The following license covers this documentation, and the Haddock source code, except where otherwise indicated.