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Programming guidelines

From HaskellWiki

Programming guidelines shall help to make the code of a project betterreadable and maintainable by the varying number of contributors.

It takes some programming experience to develop something like apersonal "coding style" and guidelines only serve as rough shape forcode. Guidelines should be followed by all members working on theproject even if they prefer (or are already used to) differentguidelines.

These guidelines have been originally set up for thehets-project and arenow put on theHaskellWiki graduallyintegrating parts of the old hawikientriesThingsToAvoid andHaskellStyle (hopefully nothurting someone's copyrights). The other related entryTipsAndTricks treats morespecific points that are left out here,

Surely some style choices are a bit arbitrary (or "religious") andtoo restrictive with respect to language extensions. Nevertheless I hopeto keep up these guidelines (at least as a basis) for our projectin order to avoid maintaining diverging guidelines. Of course I wantto supply - partly tool-dependent - reasons for certain decisions andalso show alternatives by possibly bad examples. At the time ofwriting I use ghc-6.4.1, haddock-0.7 and (GNU-) emacs with the latesthaskell mode.

The following quote and links are taken fromHaskellStyle:

We all have our own ideas about good Haskell style. There's More Than One Way To Do It. But some ways are better than others.

Some comments from the GHC team about their internal codingstandards can be found athttp://hackage.haskell.org/trac/ghc/wiki/WorkingConventions

Alsohttps://simon.peytonjones.org/publications-2000/#wearing-the-hair-shirt-a-retrospective-on-haskell-2003 contains some brief comments on syntax and style.

What now follows are descriptions of program documentation, fileformat, naming conventions and good programming practice (adapted formMatt's C/C++ Programming Guidelines and the Linux kernel codingstyle).

Documentation

Comments are to be written in application terms (i.e. user's point of view). Don't use technical terms - that's what the code is for!

Comments should be written using correct spelling and grammar in complete sentences with punctuation (in English only):

Generally, you want your comments to tell WHAT your code does, not HOW. Also, try to avoid putting comments inside a function body: if the function is so complex that you need to separately comment parts of it, you should probably [... decompose it.]

Put ahaddock comment on top of every exported function and data type! Make sure haddock accepts these comments.

File Format

All Haskell source files start with a haddock header of the form:

{- |Module      :  <File name or $Header$ to be replaced automatically>Description :  <optional short text displayed on contents page>Copyright   :  (c) <Authors or Affiliations>License     :  <license>Maintainer  :  <email>Stability   :  unstable | experimental | provisional | stable | frozenPortability :  portable | non-portable (<reason>)<module description starting at first column>-}

(the\$Header\$ entry will be automatically expanded.)

A possible compiler pragma (like{-# LANGUAGE CPP #-}) may precede this header. The following hierarchical module name must, of course, match the file name.

Make sure that the description is changed to meet the module (if the header was copied from elsewhere). Insert your email address as maintainer.

Try to write portable (Haskell 2010) code. If you use e.g.multi-parameter type classes (MTPC) andfunctional dependencies (FD) the code becomes"non-portable (MPTC with FD)".

Lines should not be longer than 80 (preferably 75) characters. Code with short lines reads casually and easier to understand. If the expression is longer than 80 lines, try to structure & rewrite the code in a more expressive way. 80 character lines is considered good practice across IT industry and supported in all cases.

Don't leave trailing white space in your code in every line.

Expand all your tabs to spaces to avoid the danger of wrongly expanding them (or a different display of tabs versus eight spaces). Possibly put something like the following in your~/.emacs file:

(custom-set-variables '(indent-tabs-mode nil))

The last character in your file should be a newline! Under Solaris you'll get a warning if this is not the case and sometimes last lines without newlines are ignored (i.e.#endif without newline). Emacs usually asks for a final newline.

You may usehttp://hackage.haskell.org/package/scan to check your file format.

The whole module should not be too long (about 400 lines)

Please have a look at theHaddock module header documentation.

Naming Conventions

In Haskell types start with capital and functions with lowercase letters, so only avoid infix identifiers! Defining symbolic infix identifiers should be left to library writers only.

(The infix identifier"\\" at the end of a line causes CPP preprocessor problems.)

Names (especially global ones) should be descriptive. If you need long names write them inlowerCamelCase. Laconic names are preferred.

Similarly, type, type class, and constructor names are written usingUpperCamelCase.

In the standard libraries, some parts of Haskell code usesnake_case for long identifiers to better reflect names given with hyphens in the required documentation. If used in outer code - such names should be transliterated to camlCase identifiers possibly adding a (consistent) suffix or prefix to avoid conflicts with keywords. And instead of a recurring prefix or suffix, you may consider using qualified imports and names.

Good Programming Practice

Functions should be short and sweet, and do just one thing. They should fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24, as we all know), and do one thing and do that well.

Most Haskell functions should be at most a few lines, only case expressions over large data types (that should be avoided, too) may need corresponding space.

For lambda expressions, write\ t -> … instead of\t -> ….

The code should be succinct (though not obfuscated), readable and easy to maintain (after unforeseeable changes). Don't exploit exotic language features without good reason.

It's not fixed how deep you indent (4 or 8 chars). You can break the line afterdo,let,where, andcase … of …. Make sure that renamings don't destroy your layout. (If you get too far to the right, the code is unreadable anyway and needs to be decomposed.)

Bad:

case foo of Foo -> "Foo"            Bar -> "Bar"

Good:

case <longer expression> of    Foo -> "Foo"    Bar -> "Bar"

Avoid the notation with braces and semicolons since the layout rule forces you to properly align your alternatives.

Respect compiler warnings. Supply type signatures, avoid shadowing and unused variables. Particularly avoid non-exhaustive and overlapping patterns. Missing unreachable cases can be filled in usingerror with a fixed string"<ModuleName>.<function>" to indicate the error position (in case the impossible should happen). Don't invest time to "show" the offending value, only do this temporarily when debugging the code.

Don't leave unused or commented-out code in your files! Readers don't know what to think of it.

Partial functions

For partial functions do document their preconditions (if not obvious) and make sure that partial functions are only called when preconditions are obviously fulfilled (i.e. by a case statement or a previous test). Particularly the call ofhead should be used with care or (even better) be made obsolete by a case statement.

Usually a case-expression (and the import ofisJust andfromJust fromData.Maybe) can be avoided by using themaybe function:

maybe (error "<ModuleName>.<function>") id $ Map.lookup key map

Generally we require you to be more explicit about failure cases. Surely a missing (or an irrefutable) pattern would precisely report the position of a runtime error, but these are not so obvious when reading the code.

Let or where expressions

Do avoid mixing and nestinglet andwhere. (I prefer the expression-stylisticlet.) Use auxiliary top-level functions that you do not export. Export lists also support the detection of unused functions.

Code reuse

If you notice that you're doing the same task again, try to generalize it in order to avoid duplicate code. It is frustrating to change the same error in several places.

Application notation

Many parentheses can be eliminated using the infix application operator$ with lowest priority. Try at least to avoid unnecessary parentheses in standard infix expression.

f x : g x ++ h xa == 1 && b == 1 || a == 0 && b == 0

Rather than putting a large final argument in parentheses (with a distant closing one) consider using$ instead.

f (g x) becomesf $ g x and consecutive applications
f (g (h x)) can be written asf $ g $ h x orf . g $ h x.

A function definition likef x = g $ h x can be abbreviated tof = g . h.

Note that the final argument may even be an infix or case-expression:

map id $ c : lfilter (const True) . map id $ case l of …

However, be aware that$-terms cannot be composed further in infix expressions.

Probably wrong:

f $ x ++ g $ x

But the scope of an expression is also limited by the layout rule, so it is usually safe to use$ on right hand sides:

Ok:

do f $ l++do g $ l

Of course$ can not be used in types. GHC has also some primitive functions involving the kind# that cannot be applied using$.

Last warning: always leave spaces around$ (and other mixfix operators) since a clash with template haskell is possible.

List Comprehensions

Use these only when "short and sweet". Prefermap,filter, andfoldr!

Instead of:

[toUpper c | c <- s]

write:

map toUpper s

Consider:

[toUpper c | s <- strings, c <- s]

Here it takes some time for the reader to find out which value depends on what other value and it is not so clear how many times the interim valuess andc are used.

In contrast to that the following can't be clearer:

map toUpper (concat strings)

When using higher-order functions you can switch easier to data structures different from list. Compare:

map (1+) list

and:

Set.map (1+) set

Records

For (large) records avoid the use of the constructor directly and remember that the order and number of fields may change.

Take care with (the rare case of) depend polymorphic fields:

data Fields a = VariantWithTwo    { field1 :: a    , field2 :: a }

The type of a valuev can not be changed by only settingfield1:

v { field1 = f }

Better construct a new value:

VariantWithTwo { field1 = f }  -- leaving field2 undefined

Or use a polymorphic element that is instantiated by updating:

empty = VariantWithTwo { field1 = [], field2 = [] }empty { field1 = [f] }

Several variants with identical fields may avoid some code duplication when selecting and updating, though possibly not in a few depended polymorphic cases.

However, I doubt if the following is a really good alternative to the above dataMode with dataBoxOrDiamond.

data Mode f p =      Box     { formula :: f,  positions :: p }    | Diamond { formula :: f,  positions :: p }

Types

Prefer proper data types over type synonyms or tuples even if you have to do more constructing and unpacking. This will make it easier to supply class instances later on. Don't put class constraints on a data type, constraints belong only to the functions that manipulate the data.

Using type synonyms consistently is difficult over a longer time, because this is not checked by the compiler. (The types shown by the compiler may be unpredictable: i.e.FilePath,String or[Char].)

Take care if your data type has many variants (unless it is an enumeration type). Don't repeat common parts in every variant since this will cause code duplication.

Bad (to handle arguments in sync):

data Mode f p = Box f p | Diamond f p

Good (to handle arguments only once):

data BoxOrDiamond = Box | Diamonddata Mode f p = Mode BoxOrDiamond f p

Consider (bad):

data Tuple a b = Tuple a b | Undefined

versus (better):

data Tuple a b = Tuple a b

and using:

Maybe (Tuple a b)

(or another monad) whenever an undefined result needs to be propagated

I/O

Try to strictly separate monadic I/O and pure (withoutdo) function programming (possibly via separate modules).

Bad:

    x <- return y    ...

Good:

    let x = y    ...

Don't usePrelude.interact and make sure your program does not depend on the (not always obvious) order of evaluation e.g. don't read and write to the same file!

This will fail:

do s <- readFile f    writeFile f $ 'a' : s

because of lazy I/O! (Writing is starting before reading is finished).

Imports

Standard library modules likeChar,List,Maybe,Monad, etc should be imported by their hierarchical module name, i.e. the base package (so that haddock finds them):

import Data.Listimport Control.Monadimport System.Environment

The libraries forSet andMap are to be imported qualified:

import qualified Data.Set as Setimport qualified Data.Map as Map

Implementation-specific extensions and classes

Stay away from extensions as long as possible. Also use classes with care because soon the desire for overlapping instances (like for lists and strings) may arise. Then you may want MPTC (multi-parameter type classes), functional dependencies (FD), undecidable and possibly incoherent instances and then you are "in the wild" (according to SPJ).

Trace

Tracing is for debugging purposes only and should not be used as feedback for the user. Clean code is not cluttered by trace calls.

Style in other languages

OCaml style

Final remarks

Despite guidelines, writing "correct code" (without formal proof support yet) still remains the major challenge. As motivation to follow these guidelines consider the points that are from the "C++ Coding Standard", where I replaced "C++" with "Haskell".

Good Points: