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Elevence Haskell Style Guide

This is a short document describing the preferred coding style for Elevence Haskell code. It is based on Johan Tibell's haskell-style-guide.

Guiding Principles

  • When in doubt optimize for readability and ease of reasoning about code.

  • Change-aware coding: we acknowledge that all code is going to be changed in the future. We simplify reasoning about changes and enlist the compiler where possible to guide us through changes.

  • We prefer code layout that scales to large numbers of functions and modules.


Line Length

Maximum line length is 80 characters.


Tabs are illegal. Use spaces for indenting. We use 4 spaces by default for indenting and make use half-indents of 2 spaces for punctuation and cases of case distinctions. We also indent the where keyword with 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

greetingLines :: Maybe String -> [String]
greetingLines mbName =
    -- Here we use half-indents to save indentation space. The idea is that
    -- prefixed punctuation characters are light-weight enough to serve as
    -- almost an indentation space.
    case x of
      Nothing -> ["Hello John Doe."]
      Just name ->
        [ "Hello " <> name
        , "It is a pleasure to get to know you."
        , "We hope you enjoy your stay at hotel Transylvania."

Try to avoid using indentation whose depth depends on the length of an identifier, i.e., use

    :: Foo a
    => a -> IO ()
foobar x = do
    a <- alloca x
    b <- alloca 20
    cFunction a b

instead of

foobar :: Foo a
       => IO ()
foobar = do a <- alloca 10
            b <- alloca 20
            cFunction a b

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 judgment.


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 on the following line. Example:

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

Format records as follows:

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

List Declarations

Align the elements in the list. Example:

exceptions =
    [ InvalidStatusCode
    , MissingContentHeader
    , InternalServerError


Put pragmas immediately before the function/constructor/field they apply to. Example:

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

Hanging Lambdas

You may or may not indent the code following a "hanging" lambda. Use your judgment. 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

If-then-else clauses

Generally, guards and pattern matches should be preferred over if-then-else clauses, where possible. Short cases should usually be put on a single line (when line length allows it).

When writing non-monadic code (i.e. when not using do) and guards and pattern matches can't be used, you can align if-then-else clauses like you would normal expressions:

foo =
    if ...
    then ...
    else ...

Otherwise, you are free to choose between 2 and 4 spaces to indent, but make sure that the then and the else keywords are aligned. Examples:

foo = do
    if condition
        then someMoreCode
        else someAlternativeCode
foo = bar $ \qux ->
    if predicate qux
      then doSomethingSilly
      else someOtherCode

The same rule applies to nested do blocks:

foo = do
    instruction <- decodeInstruction
    skip <- load Memory.skip
    if skip == 0x0000
        then do
            execute instruction
            addCycles $ instructionCycles instruction
        else do
            store Memory.skip 0x0000
            addCycles 1

Case expressions

The alternatives in a case expression can be indented using either of the two following styles:

foobar = case something of
    Just j  -> foo
    Nothing -> bar

or as

foobar = case something of
           Just j  -> foo
           Nothing -> bar

Align the -> arrows when it helps readability.


Imports should be sorted alphabetically and grouped by top-level module-hierarchy name. Align common keywords per import group and break explicit import lists as follows.

import           Control.Lens
                 ( preview, ix, at, traverseOf, toListOf
                 , view, use
import qualified Control.Monad.Catch as Catch

Always prefer explicit import lists or qualified imports. This makes the code more robust against changes in the imported modules.

For qualified imports you should either use the full or abbreviated name of the last name(s) in the module hierarchy. Here are a few examples.

import qualified Control.Monad.Catch  as Catch

import qualified Data.ByteString      as B
import qualified Data.ByteString.Lazy as BL
import qualified Data.Map             as M
import qualified Data.HashMap         as HM
import qualified Data.Set             as S
import qualified Data.Text            as T
import qualified Data.Text.Encoding   as TE

There are two exceptions to the qualified import rule. First, our custom prelude Elevence.Prelude can always be imported unqualified. Second, qualified imports of .Extended versions of modules from third-party libraries (see Section "Common Patterns") can be abbreviated without mentioning the .Extended in the abbreviated name.



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. Function example:

-- | 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.
    :: Socket      -- ^ Connected socket
    -> ByteString  -- ^ Data to send
    -> IO Int      -- ^ Bytes sent

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

Record example:

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

For fields that require longer comments format them like so:

data Record = Record
    { -- | This is a very very very long comment that is split over
      -- multiple lines.
      field1 :: !Text

      -- | This is a second very very very long comment that is split
      -- over multiple lines.
    , field2 :: !Int

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 camel case (e.g. functionName) when naming functions and upper camel case (e.g. DataType) 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.


Avoid unprincipled abbreviations; in particular when naming top-level functions. If you must abbreviate an identifier, then use the sequence of its initial letter and all following capital-case letters as the abbreviation.

Records and Constructors

Prefix record fields either with the full name of the type or with the abbreviated name of the type. For example,

data EmailAddress = EmailAddress
    { eaName   :: !Text
    , eaDomain :: !Text


data EmailAddress = EmailAddress
    { emailAddressName   :: !Text
    , emailAddressDomain :: !Text

If you need to disambiguate record constructors, then do this by post-fixing either the full or abbreviated name of the type. For example,

data ValidationError
    = ReferenceVE !Reference
    | CharacterVE !Char


data ValidationError
    = ReferenceValidationError !Reference
    | CharacterValidationError !Char


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

Name spaces

Avoid repeating a module's name in the name of the types and values it is defining. In particular avoid abbreviating the actual interesting part of the name in favor of repeating the module name. Modules form name spaces that should be made use of. For example,

-- Bad
module Foo.Bar where

data BarS = A | B
-- Good
module Foo.Bar

data State = A | B

Dealing with laziness

By default, use strict data types and lazy functions.

Data types

Constructor fields should be strict, unless there's an explicit reason to make them lazy. This avoids many common pitfalls caused by too much laziness and reduces the number of brain cycles the programmer has to spend thinking about evaluation order.

-- Good
data Point = Point
    { pointX :: !Double  -- ^ X coordinate
    , pointY :: !Double  -- ^ Y coordinate
-- Bad
data Point = Point
    { pointX :: Double  -- ^ X coordinate
    , pointY :: Double  -- ^ Y coordinate

Additionally, unpacking simple fields often improves performance and reduces memory usage:

data Point = Point
    { pointX :: {-# UNPACK #-} !Double  -- ^ X coordinate
    , pointY :: {-# UNPACK #-} !Double  -- ^ Y coordinate

As an alternative to the UNPACK pragma, you can put

{-# OPTIONS_GHC -funbox-strict-fields #-}

at the top of the file. Including this flag in the file itself instead of e.g. in the Cabal file is preferable as the optimization will be applied even if someone compiles the file using other means (i.e. the optimization is attached to the source code it belongs to).

Note that -funbox-strict-fields applies to all strict fields, not just small fields (e.g. Double or Int). If you're using GHC 7.4 or later you can use NOUNPACK to selectively opt-out for the unpacking enabled by -funbox-strict-fields.


Have function arguments be lazy unless you explicitly need them to be strict.

The most common case when you need strict function arguments is in recursion with an accumulator:

mysum :: [Int] -> Int
mysum = go 0
    go !acc []    = acc
    go acc (x:xs) = go (acc + x) xs


Point-free style

Avoid over-using point-free style. For example, this is hard to read:

-- Bad:
f = (g .) . h


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


  • We use FIXME (author-name): what-to-fix to mark pieces of code that must be fixed before being merged into master.
  • We use TODO (author-name): what-to-do to mark explain future extensions/changes to the code that might be worthwhile.
  • We use NOTE (author-name): note to annotate a piece of code with explanations that help understanding this code.

We add the author-name to each of these elements, as we always want to make sure that one knows whom to ask in case further clarification is needed. If a FIXME addresses a very specific person that has to fix it then we state this be referencing this person in the explanation of what has to be fixed.

Design Patterns

Orphan Instances

All our orphan instances are grouped per library in Orphans.Lib_<library-name> modules, which in turn are all imported in Elevence.Prelude. This makes sure that we do not have any duplicate orphan instance definitions. This ensures that we stay away from the orphan-instance hell with conflicting, hidden orphan instances.

.Extended modules

We adapt third-party libraries using the .Extended modules pattern.

The service pattern

We structure our IO layer using the service pattern.

Modules with batteries included

We prefer working with modules that come with batteries included, i.e., where a single import brings all useful library functions into scope. This reduces import clutter.

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