This document contains a description of the preferred style for writing Curry programs. The described style is oriented to the Haskell Style Guide von Johan Tibell, but it has been adapted in various points.

In principle, this style guide is a guideline from which one can deviate if there are good reasons for it.

1 General Formatting

1.1 Line Length

The maximum line length is 80 characters.

1.2 Indentation

Tabulator stops are not allowed in source programs, since the actual indentation space of a tabulator stop might influence the semantics of the program. Instead, use blanks for indentation. Code blocks should be intended with 2 spaces. Most text editors can be configured so that tabs are automatically replaced by (at most) two spaces.

1.3 Local Definitions

The keyword where is indented by one space and the subsequent local definitions by a further space. Hence, the local definitions are intended, as other code blocks, by 2 spaces. If there is only one short local definition, it can be written directly after the keyword where. Several local value definitions should be aligned at the equality symbol if they have a comparable number of parameters. Local definitions should be aligned like top-level definitions (see below):

main :: IO ()
main = do
  line <- getLine
  putStrLn $ answer line
  answer s = "Your input: " ++ s

f y = x + y
 where x = 1

g x = x + val1 + secondVal
  val1      = 1
  secondVal = if isZero x then 1 else 2

  -- local operation
  isZero x | x == 0    = True
           | otherwise = False

1.4 Blank Lines

There should be one blank line between two top-level definitions. There are no blank lines between type signature and the defining rules. Similarly, comments of top-level definitions are not separated from the actual definition with a blank line.

-- f increments the argument by one
f :: Int -> Int
f x = x + 1

-- fInv decrements the argument by one
fInv x = x - 1

1.5 Spaces

Binary operators, like (++), are surrounded with a single space on either side.

-- bad

-- good
"Very" ++ "good"

It is allowed to omit the surrounding spaces from simple infix operators, like (+), or the infix list constructor (:). However, this is not required.

f n = n * (n+1) / 2

head (x:_) = x

Similarly to written texts, a space follows a comma, but there is no space in front of the comma:

aList  = [1, 2, 3]
aTuple = (True, "True", 1)

One can put a space after a lambda, in particular, if there is non-variable pattern after a lambda. If there is a variable after a lambda, one can omit the space:

map (\ (_:_) -> True)
map (\ x -> x + 1)
map (\x -> x + 1)

2 Formatting of Specific Language Constructs

2.1 Module Header

If the export list fits into one line, it can be written as follows:

module Set (Set, empty) where

Longer export lists should be aligned as follows:

module Data.Set
  ( Set
  , empty
  , singleton
  , member
  ) where

Optional, one can also put several names into line:

module Data.Set
  ( Set, empty, singleton
  , member, union, intersect
  ) where

If there are types exported with constructors, one should separate the type name and the constructors with a blank:

module Tree (Tree (..), BinTree (Leaf, Branch)) where

2.2 Import Declarations

The list of imported modules should be ordered in the following three categories:

  1. Standard library modules, e.g., List, IO
  2. Third party libraries (often not used)
  3. Other local modules (developed with the current application)

The list of imports in each category should be sorted alphabetically. With the exception of the prelude, all used entities from imported modules should be explicitly or qualified imported. If the list of imported entities is long, one can omit this general rule.

import           List        (isInfixOf)
import qualified Set  as Set

import SecondParty.Module1 (fun)
import ThirdParty.Module1  (($$$))

import MyUtilsModule -- import everything

2.3 data Declarations

The constructors of a data declaration should be vertically aligned:

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

If there are only a few constructors that fits into one line, one can write the data declaration as follows:

data Bit = Zero | One

Records (data declarations with named selectors) should be vertically aligned as follows:

data Person = Person
    { firstName :: String
    , lastName  :: String
    , age       :: Int

2.4 Type Signatures

In type signatures one should put a blank before and after the function arrow ->.

map :: (a -> b) -> [a] -> [b]

The type signature should be written in one line if it fits. If the type signature is long or if one want to put a comment after the individual types, one should align the function arrows:

uncurry10 :: (a -> b -> c -> d -> e -> f -> g -> h -> i -> j -> k)
          -> (a, b, c, d, e, f, g, h, i, j)
          -> k

area :: Int -- width
     -> Int -- height
     -> Int -- area

2.5 Argument Patterns

If an operation is defined by several rules, one should vertically align the parameters with the same position:

and True True = True
and _    _    = False

The equal signs = should also be aligned.

2.6 Guards

Guards should immediately follow the patterns or they should be indented in the following line. In any case, they should be vertically aligned:

f x y z | x == y    = z
        | otherwise = z + 1

g x y z
  | x == y && not z = 1
  | otherwise       = 0

If the conditions are very long, the equal signs can start in the next line below the condition:

f x y z
  | thisIsAVeryLongConditionWhichNeedsAllTheSpaceAvailableInTheLine x y z
  = 42
  | otherwise
  = 0

If an operation is defined by rules with and without guards, the guards | and equal signs = should be vertically aligned:

f :: [a] -> Int -> [a]
f []     _ = []
f (x:xs) n | n<=0      = []
           | otherwise = x : f xs (n-1)

2.7 if-then-else

As a general rule, one should try to use patterns and guards instead of if-then-else expressions. Short if-then-else expressions can be written in one line (if this fits into the maximal line length):

f x = g (if x then 0 else 1) 42

Otherwise, one should indent the then and else branch, which are vertically aligned:

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

If the condition is short, one can also write the if and then part in one line, but align the then and else parts:

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

2.8 case Expressions

The alternatives in a case expression should be aligned in the following form:

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


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

The arrows -> should be aligned to improve readability.

2.9 let Expressions

Short let expressions with a single local declaration can be written in one line (if the line is not too long):

squareSum x y = let z = x + y in z * z

grandfather g c | let f free in father g f && father f c = True

Long let expressions or let expressions with more then one local declaration should be aligned so that the keywords let and in are in the same column:

qsort (x:xs) = let (l,r) = split x xs
               in qsort l ++ (x:qsort r)

doubleSquareSum x y =
  let z  = x + y
      sq = z * z
  in sq + sq

The equal signs of all local declarations should be vertically aligned. The same rule is used for let expressions in do blocks (where there is no in expression).

2.10 do Blocks

The statements in a do blocks should start immediately after the keyword do or they are indented in the next line:

echo = do name <- getLine
          putStrLn name

greet = do
  putStr "How is your name? "
  name <- getLine
  putStrLn ("Hello " ++ name ++ "!")

2.11 List/Tuple Definitions

The element of long lists should be aligned as follows:

exceptions =
  [ InvalidStatusCode
  , MissingContentHeader
  , InternalServerError

One can also omit the first line break:

directions = [ North
             , East
             , South
             , West

Short lists can be written in one line:

short = [1, 2, 3]

The same rules apply to tuple definitions:

t       = (1, True)
ignored = ( InvalidStatusCode
          , MissingContentHeader

2.12 Brackets

Superfluous brackets should be avoided:

seven = 1 + 2 * 3

f x = if cond x then 0 else 1

instead of

seven = (1 + (2 * 3))

f x = if (cond x) then 0 else 1

In case of specific infix operators (e.g., not defined in the prelude) or if one is not sure about the precedence or want to document the predence, one can write brackets.


3.1 Language

Comments should be written in correct English. The identifiers used in a program should be also meaningful in English.

3.2 Top-Level Declarations

All top-level operations should have a comment and a type signature. This is a must for exported operations. One should use the syntax of CurryDoc for the comments of exported operations so that the program documentation can easily be generated.

--- Splits the list argument into a list of lists of related adjacent
--- elements.
--- @param eq - the relation to classify adjacent elements
--- @param xs - the list of elements
--- @return the list of lists of related adjacent elements
groupBy :: (a -> a -> Bool) -> [a] -> [[a]]

The comment of an operation should contain enough information so that one can use the operation without studying its implementation.

Data types should be commented in a similar way. Exported constructors should have individual comments:

--- The data type for representing XML expressions.
--- @cons XText - a text string (PCDATA)
--- @cons XElem - an XML element with tag field, attributes, and a list
---               of XML elements as contents
data XmlExp = XText String
            | XElem String [(String,String)] [XmlExp]

For records with explicit selectors (labels), one should add a comment for each selector:

--- A natural person
--- @field firstName - First name, may contain several ones
--- @field lastName  - Last name
--- @field height    - Height in centimeters
data Person =
      { firstName :: String
      , lastName  :: String
      , height    :: Int

3.3 End-of-Line Comments

Comments at the end of a line should be separated from the code by at least one space:

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

4 Naming

Use camel case for names that consists of several logical units (words):

data BankAccount = ...

thisIsTheAnswer = 42

To improve readability, one should not capitalize all letters in abbreviations. For instance, one should write showXmlDoc instead of showXMLDoc. An exception are abbreviations with only two letters, like IO.

Parameter names and names of local declarations are usually short, but one has to take the following rule into account:

Entity with larger visibility regions have longer names.

The name x can be used in a one-line operation but never as a top-level declaration.

One should also take into account the following conventions for short names:

5 Compiler Warnings

Code should be compilable with the parsing option

:set parser -Wall

without producing any warnings. If one uses a more logic-oriented programming style, operations are often defined with overlapping rules or incomplete pattern matching. In this case, one can explicitly omit the warnings by the parsing option

{-# OPTIONS_CYMAKE -Wno-incomplete-patterns -Wno-overlapping #-}

in the beginning module header. However, this should be done only in a program that depends on these features of Curry.