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@ -1,6 +1,4 @@
su4sml coding style
===================
# su4sml coding style
We here document coding-style guidelines that should be adhered to.
Much of the existing code is not yet following these guidelines,
@ -8,8 +6,7 @@ patches are welcome. This style-guide was inspired by the SML Style Guide
from Cornell university [1].
Chapter 1: Indentation and breaking long lines
==========
## Chapter 1: Indentation and breaking long lines
The limit on the length of lines is 80 columns and this is a hard
limit. Using more than 80 columns causes your code to wrap around to
@ -20,21 +17,21 @@ spaces to control indenting. Indent by two spaces.
Long expressions can be broken up and the parts aligned, as in the second
example. Either is acceptable.
```sml
val x = "Long line..."^
"Another long line."
val x = "Long line..."^
"Another long line."
```
Case expressions should be indented as follows:
```sml
case expr of
pat1 => ...
| pat2 => ...
```
If expressions should be indented according to one of the following schemes:
```sml
if exp1 then exp2 if exp1 then
else if exp3 then exp4 exp2
else if exp5 then exp6 else exp3
@ -42,13 +39,12 @@ else if exp5 then exp6 else exp3
if exp1 then exp2 else exp3 if exp1 then exp2
else exp3
```
Comments should be indented to the level of the line of code that follows the
comment.
Chapter 2: Factoring
==========
## Chapter 2: Factoring
Avoid breaking expressions over multiple lines. If a tuple consists of more
than two or three elements, you should consider using a record instead of a
@ -60,16 +56,17 @@ multiple lines to something that has good style is to factor the code using a
let expression. Consider the following:
Bad
```sml
fun euclid (m:int,n:int) : (int * int * int) =
if n=0
then (b 1, b 0, m)
else (#2 (euclid (n, m mod n)), u - (m div n) *
(euclid (n, m mod n)), #3 (euclid (n, m mod n)))
```
Better
```sml
fun euclid (m:int,n:int) : (int * int * int) =
if n=0
then (b 1, b 0, m)
@ -78,7 +75,7 @@ Better
u - (m div n) * (euclid (n, m mod n)),
#3 (euclid (n, m mod n)))
```
Best
fun euclid (m:int,n:int) : (int * int * int) =
@ -94,38 +91,36 @@ Best
Do not factor unnecessarily.
Bad
```sml
let
val x = TextIO.inputLine TextIO.stdIn
in
case x of
...
end
```
Good
```sml
case TextIO.inputLine TextIO.stdIn of
...
```
Bad (provided y is not a large expression):
```sml
let val x = y*y in x+z end
```
Good
```sml
y*y + z
```
Chapter 3: Comments
==========
## Chapter 3: Comments
Comments should be written with SMLDoc [2] in mind; a, nightly updated, API
documentation is available [3]. For example:
```sml
(**
* opens a file.
* @params {fileName, mode}
@ -133,21 +128,22 @@ documentation is available [3]. For example:
* @param mode mode flag
* @return file stream *)
val openFile : {fileName : string, mode : openMode} -> stream
```
We mainly restrict ourselves to the following SMLDoc tags:
```
@params gives names to formal parameters of functions and value constructors
@param a description of a formal parameter
@return a description about return value of the function
@see related items (specified text is not analyzed in the current version)
@throws same as the @exception tag
```
Signatures must be commented using SMLDoc.
Comments go above the code they reference, as in the following example:
```sml
(** Sums a list of integers. *)
val sum = foldl (op +) 0
```
Avoid Useless Comments. Avoid comments that merely repeat the code they
reference or state the obvious. Comments should state the invariants, the
non-obvious, or any references that have more information about the code.
@ -172,17 +168,16 @@ Multi-line Commenting. When comments are printed on paper, the reader lacks the
advantage of color highlighting performed by an editor such as Emacs.
Multiline comments can be distinguished from code by preceding each line of the
comment with a * similar to the following:
```sml
(**
* This is one of those rare but long comments
* that need to span multiple lines because
* the code is unusually complex and requires
* extra explanation. *)
fun complicatedFunction () = ...
```
Chapter 4: Parentheses
==========
## Chapter 4: Parentheses
Over Parenthesizing. Parentheses have many semantic purposes in ML, including
constructing tuples, grouping sequences of side-effect expressions, forcing a
@ -198,7 +193,7 @@ already wrapped by a let...in...end block, you can drop the parentheses.
Alternative Block Styles. Blocks of code such as let...in...end, struct...end,
and sig...end should be indented as follows. There are several alternative
styles to choose from.
```sml
fun foo bar = fun foo bar = fun foo bar = let
let let val p = 4 val p = 4
val p = 4 val q = 38 val q = 38
@ -206,11 +201,10 @@ fun foo bar = fun foo bar = fun foo bar = let
in bar * (p + q) bar * (p + q)
bar * (p + q) end end
end
```
Chapter 5: Pattern matching
==========
## Chapter 5: Pattern matching
No Incomplete Pattern Matches. Incomplete pattern matches are flagged with
compiler warnings, which should be treated as errors.
@ -219,7 +213,7 @@ Pattern Match in the Function Arguments When Possible. Tuples, records and
datatypes can be deconstructed using pattern matching. If you simply
deconstruct the function argument before you do anything useful, it is better
to pattern match in the function argument. Consider these examples:
```sml
Bad Good
fun f arg1 arg2 = let fun f (x,y) (z,_) = ...
val x = #1 arg1
@ -237,7 +231,7 @@ fun f arg1 = let fun f {foo=x, bar=y, baz} = ...
in
...
end
```
Avoid Unnecessary Projections. Prefer pattern matching to projections with
@ -246,6 +240,7 @@ as it is infrequent and the meaning is clearly understood from the context.
The above rule shows how to pattern-match in the function arguments. Here is
an example for pattern matching with value declarations.
```sml
Bad Good
let let
val v = someFunction() val (x,y) = someFunction()
@ -254,7 +249,7 @@ let let
in end
x+y
end
```
Combine nested case Expressions. Rather than nest case expressions, you can
@ -262,7 +257,7 @@ combine them by pattern matching against a tuple, provided the tests in the
case expressions are independent. Here is an example:
Bad
```sml
let
val d = Date.fromTimeLocal(Time.now())
in
@ -277,11 +272,11 @@ Bad
10 => print "Happy Metric Day"
| _ => ())
end
```
Good
```sml
let
val d = Date.fromTimeLocal(Time.now())
in
@ -291,7 +286,7 @@ Good
| (Date.Oct, 10) => print "Happy Metric Day"
| _ => ()
end
```
Avoid the use valOf, hd, or tl. The functions valOf, hd, and tl are used to
deconstruct option types and list types. However, they raise exceptions on
certain inputs. You should avoid these functions altogether. It is usually
@ -299,8 +294,7 @@ easy to achieve the same effect with pattern matching. If you cannot manage to
avoid them, you should handle any exceptions that they might raise.
Chapter 6: Naming and declarations
==========
## Chapter 6: Naming and declarations
Naming Conventions. The best way to tell at a glance something about the type
of a variable is to use the standard SML naming conventions. The following are
@ -309,20 +303,20 @@ SML/NJ libraries:
Token SML Naming Convention
Variables Symbolic or initial lower case. Use embedded caps for multiword names.
Example: getItem
Example: `getItem`
Functions Initial lower case. Use embedded caps for multiword names.
Example: nameOf
Example: `nameOf`
Constructors Initial upper case. Use embedded caps for multiword names. Historic
exceptions are nil, true, and false. Rarely are symbolic names like :: used.
Example: Node, EmptyQueue
Example: `Node`, `EmptyQueue`
Types All lower case. Use underscores for multiword names.
Example: priority_queue
Example: `priority_queue`
Signatures All upper case. Use underscores for multiword names.
Example: PRIORITY_QUEUE
Example: `PRIORITY_QUEUE`
Structures Initial upper case. Use embedded caps for multiword names.
Example: PriorityQueue
Example: `PriorityQueue`
Functors Same as structure convention, except Fn completes the name.
Example: PriorityQueueFn
Example: `PriorityQueueFn`
These conventions are not enforced by the compiler, though violations of the
variable/constructor conventions ought to cause warning messages because of the
@ -333,7 +327,7 @@ variable names that reflect their intended use. Choose words or combinations
of words describing the value. Variable names may be one letter in short let
blocks. Functions used in a fold, filter, or map are often bound to the name
f. Here is an example for short variable names:
```sml
let
val d = Date.fromTimeLocal(Time.now())
val m = Date.minute d
@ -342,7 +336,7 @@ let
in
List.filter f [m,s]
end
```
Avoid Global Mutable Variables. Mutable values should be local to closures and
almost never declared as a structure's value. Global mutable values cause many
@ -362,26 +356,26 @@ by functions within the current structure are aliased to one or two letter
variables at the top of the struct block. This serves two purposes: it shortens
the name of the structure and it documents the structures you use. Here is an
example:
```sml
struct
structure H = HashTable
structure T = TextIO
structure A = Array
...
end
```
Order of Declarations in a Structure. When declaring elements in a structure,
you should first alias the structures you intend to use, followed by the types,
followed by exceptions, and lastly list all the value declarations for the
structure. Here is an example:
```sml
struct
structure L = List
type foo = unit
exception InternalError
fun first list = L.nth(list,0)
end
```
Every declaration within the structure should be indented the same amount.
@ -389,14 +383,14 @@ Moreover, every top-level structure should be restricted by a (documented)
signature.
Functions should declared in their their curried form, e.g.,
```sml
fun f x y = ... instead of fun f(x,y) = ...
```
Datatypes should be preferred to type synonyms in particular for
record types
Chapter 7: Verbosity
==========
## Chapter 7: Verbosity
Don't Rewrite Library Functions. The basis library and the SML/NJ library have
a great number of functions and data structures -- use them! Often students
@ -412,6 +406,7 @@ case that the type is bool, you should not be using if at all. Consider the
following:
```sml
Bad Good
if e then true else false e
if e then false else true not e
@ -421,40 +416,40 @@ if x then true else y x orelse y
if x then y else false x andalso y
if x then false else y not x andalso y
if x then y else true not x orelse y
```
Misusing case Expressions. The case expression is misused in two common
situations. First, case should never be used in place of an if expression
(that's why if exists). Note the following:
```sml
case e of
true => x
| false => y
if e then x else y
```
The latter is much better. Another situation where if expressions are
preferred over case expressions is as follows:
```sml
case e of
c => x (* c is a constant value *)
| _ => y
if e=c then x else y
```
The latter is definitely better. The other misuse is using case when pattern
matching with a val declaration is enough. Consider the following:
```sml
val x = case expr of (y,z) => y
val (x,_) = expr
```
The latter is better.
Other Common Misuses. Here are some other common mistakes to watch out for:
```sml
Bad Good
l::nil [l]
l::[] [l]
length + 0 length
@ -471,30 +466,30 @@ Int.compare(x,y)=GREATER x>y
Int.sign(x)=~1 x<0
Int.sign(x)=0 x=0
Int.sign(x)=1 x>0
```
Don't Re-wrap Functions. When passing a function as an argument to another
Do not re-wrap Functions. When passing a function as an argument to another
function, don't re-wrap the function unnecessarily. Here's an example:
```sml
List.map (fn x => Math.sqrt x) [1.0, 4.0, 9.0, 16.0]
List.map Math.sqrt [1.0, 4.0, 9.0, 16.0]
```
The latter is better. Another case for rewrapping a function is often
associated with infix binary operators. To prevent rewrapping the binary
operator, use the op keyword as in the following example:
```sml
foldl (fn (x,y) => x + y) 0
foldl (op +) 0
```
The latter is better.
Don't Needlessly Nest let Expressions. Multiple declarations may occur in the
first block of a let...in...end expression. The bindings are performed
sequentially, so you may use a name bound earlier in the same block. Consider
the following:
```sml
let
val x = 42
in
@ -511,7 +506,7 @@ let
in
x + y
end
```
The latter is better.
@ -522,8 +517,7 @@ This has the added benefit of letting you document the purpose of the value
with a name.
Chapter 9: File names and encoding
==========
## Chapter 8: File names and encoding
In general, a source file should only contain one signature or structure. In more
detail:
@ -537,8 +531,7 @@ and separated by underscore, e.g., ocl_term.sig
Source files should use the Unix line ending convention and be either encoding
using ASCII (preferred) or UTF-8.
Chapter 9: Compatibility
==========
## Chapter 9: Compatibility
Any code developed must be portable among the supported SML systems
(currently: sml/NJ, mlton, polyml 5.x). Moreover, the code should
@ -547,25 +540,23 @@ warnings as errors. Keep in mind that polyml does only provide a subset
of the SML standard library.
Appendix I: Machine-support
===========
## Appendix
### Appendix I: Machine-support
The following elisp-snippet provides marginal support for this coding-style for
the sml-mode [4] of Emacs [5]:
```emacs
(setq sml-indent-level 2)
(setq sml-pipe-indent -2)
(setq sml-case-indent t)
(setq sml-nested-if-indent t)
(setq sml-type-of-indent nil)
(setq sml-electric-semi-mode nil)
```
Appendix II: References
============
[1] http://www.cs.cornell.edu/Courses/cs312/2007fa/handouts/style.htm
[2] SMLDoc. http://www.pllab.riec.tohoku.ac.jp/smlsharp/?SMLDoc
[3] http://projects.brucker.ch/su4sml/smldoc/
[4] http://www.smlnj.org/doc/Emacs/sml-mode.html
[5] http://www.gnu.org/software/emacs/
--
Last updated on $Date:$
### Appendix II: References
1. http://www.cs.cornell.edu/Courses/cs312/2007fa/handouts/style.htm
2. SMLDoc. http://www.pllab.riec.tohoku.ac.jp/smlsharp/?SMLDoc
3. http://projects.brucker.ch/su4sml/smldoc/
4. http://www.smlnj.org/doc/Emacs/sml-mode.html
5. http://www.gnu.org/software/emacs/