Programming guidelines shall help to make the code of a project better

readable and maintainable by the varying number of contributors.

It takes some programming experience to develop something like a

personal "coding style" and guidelines only serve as rough shape

for code. Guidelines should be followed by all members working on the

project even if they prefer (or are already used to) different

guidelines.

In the following I will describe documentation, file format, naming

conventions and good programming practice (adapted form Matt's C/C++

Programming Guidelines and the Linux kernel coding style).

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Documentation:

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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 punctation (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" (see "Good Programming Practice")

Put a haddock comment on top of every exported function and data type!

Make sure haddock accepts these comments by "make doc" after full

compilation with "make all".

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File Format:

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

{- |

Module : $Header$

Copyright : (c) <You> and Uni Bremen 2005

License : similar to LGPL, see HetCATS/LICENSE.txt or LIZENZ.txt

Maintainer : maeder@tzi.de

Stability : provisional

Portability : portable

<Description>

-}

A possible compiler pragma (like {-# OPTIONS -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 (Haskell98) code. If you (indirectly) import

Logic/Logic.hs the code becomes "non-portable".

The Dollar-Header-Dollar entry is automatically expanded by cvs (and will wrap

around). All other lines should not be longer than 80 (preferably 75)

characters to avoid wrapped lines (for casual readers)!

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.

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

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Naming Conventions:

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In Haskell types start with capital and functions with lowercase

letters, so only avoid infix identifiers! (i.e. "\\" causes preprocessor

problems and DrIFT has some limitation with infix constructors)

Names (especially global ones) should be descriptive and if you need

long names write them with underlines and lowercase letters or as

mixed case words. (but "tmp" is to be preferred over

"thisVariableIsATemporaryCounter")

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Good Programming Practice:

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"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."

It's not fixed how deep you indent (4 or 8 chars). You can break the

line after "do", "let", "where", and "case .. of". Make sure that

renamings don't destroy your layout. (If you get to 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 using

"error" 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!

Case expressions

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Prefer case expressions over pattern binding declarations.

Not always nice:

longFunctionName (Foo: _ : _) = e1

longFunctionName (Bar: _) = e2

Better (I think):

longFunctionName arg = case arg of

Foo : _ : _ -> e1

Bar : _ -> e2

_ -> error "Programming-Guidelines.longFunctionName"

For partial functions 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 of "head" should be used with

care or avoided by a case statement.

Usually a case statement (and the import of isJust and formJust from

Data.Maybe) can be avoided by using the "maybe" function:

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

Do avoid mixing "let" and "where". (I prefer "let" and have auxiliary

function on the top-level that are not exported.) Export lists also

support the detection of unused functions.

If you notice that you're doing the same task again, try to generalize

your first one in order to avoid duplicate code. It is

frustrating to change the same error in several places.

Records

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For (large) records avoid to use of the constructor directly and consider

that the order and number of fields may change.

IO

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Try to strictly separate IO, Monad and pure (without do) function

programming (possibly via different modules).

Bad:

x <- return y

...

Good:

let x = y

...

List Comprehensions

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Use these only when "short and sweet". (I prefer map, filter, and foldr.)

Trace

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Tracing is for debugging purposes only and should not be used as

feedback for the user. Clean code is not cluttered by trace calls.

Application notation

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Many parentheses can be avoid using the infix application operator "$"

with lowest priority. Try at least to avoid unnecessary parentheses in

standard infix expression.

f x : g x ++ h x

a == 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)" becomes "f $ g x" and consecutive applications

"f (g (h x))" can be written as "f $ g $ h x" or "f . g $ h x".

A function definition like

"f x = g $ h x" can be abbreviated to "f = g . h".

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

map id $ c : l

filter (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 save to use "$" on right hand sides.

Ok:

do y <- f $ l

++

do y <- g $ l

Last warning: always leave spaces around "$" (and other mixfix

operators) since a clash with template haskell is possible.

(Also write "\ t" instead of "\t" in lambda expressions)

Types

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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.

Imports

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Standard library modules like Char. List, Maybe, Monad, etc should be

imported by their hierarchical module name (so that haddock finds them):

import Data.List

import Control.Monad

import System.Environment

The libraries for Set, Map and Rel are to be imported qualified:

import qualified Common.Lib.Set as Set

import qualified Common.Lib.Map as Map

import qualified Common.Lib.Rel as Rel

Glasgow extensions and Classes

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Stay away form 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, undecidable and possibly incoherent

instances and then you are "in the wild" (SPJ).

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Final remarks:

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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:

* programmers can go into any code and figure out what's going on

* new people can get up to speed quickly

* people new to Haskell are spared the need to develop a personal

style and defend it to the death

* people new to Haskell are spared making the same mistakes over

and over again

* people make fewer mistakes in consistent environments

* programmers have a common enemy :-)

Bad Points:

* the standard is usually stupid because it was made by someone

who doesn't understand Haskell

* the standard is usually stupid because it's not what I do

* standards reduce creativity

* standards are unnecessary as long as people are consistent

* standards enforce too much structure

* people ignore standards anyway

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