3 Other Languages

3.1 Haskell

For Haskell we recommend Johan Tibell’s style guide. This style guide is similar to the designs we choose for implementing python code and is a good fit for projects.

Projects are managed using stack. However, the cabal tool is also suitable. So long as the project can be passed on and compiles then whichever tool you use to sandbox your work is fine.

It is recommended that you keep your GHC up to date and document which version your program is to be compiled with.

Key points from the above style guide are included below.

Line Length

Maximum line length is 80 characters.

Indentation

Tabs are illegal. Use spaces for indenting. Indent your code blocks either 2 spaces or 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. Other spacing rules should follow Haskell guidelines.

Pragmas

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
    !ByteArray

Export Lists

Format export lists as follows:

module Data.Set
    (
      -- * The @Set@ type
      Set
    , 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 should be consistent with the 4-spaces indent rule, and the then and the else keyword should be aligned.

Examples:

foo = do
    someCode
    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

A case expression can be indented using the two following style:

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

Align the -> arrows when it helps readability.

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.

Comments

Punctuation

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.
send :: 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
  where
    salt = 453645243  -- Magic hash salt.

Links

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)

Naming

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.

Modules

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

Point-free style

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

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

Warnings

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

Exception handling

The safe-exceptions library should be used. The library contains guidance on its use.

3.2 Scala

Scala comes with style recommendations here. These are recommended for Scala development.