OCaml Style Guide

One important goal in this class is to teach you how to program elegantly. You have most likely spent many years in secondary school learning style with respect to the English language – programming should be no different. Every programming language demands a particular style of programming, and forcing one language’s style upon another can have disastrous results. Of course there are some elements of writing a computer program that are shared between all languages. You should be able to pick up these elements through experience.

Listed below are some style guidelines for OCaml. Egregious violation of these guidelines may result in loss of programming style points. Note that these guidelines cover many more OCaml features than we will be expecting you to use in this class.

Although the list below seems daunting, most of the suggestions are common sense. Also, you should note that these rules come no where near to the style mandates you will likely come across in industry. Many companies go so far as to dictate exactly where spaces can go.

File Submission Requirements:

  1. Code must compile
  2. 80 column limit
  3. No tab characters


  1. Comments go above the code they reference
  2. Avoid useless comments
  3. Avoid over-commenting
  4. Line breaks
  5. Proper multi-line commenting

Naming and Declarations:

  1. Use meaningful names
  2. Naming conventions
  3. Type annotations
  4. Avoid global mutable variables
  5. When to rename variables
  6. Order of declarations in a structure


  1. Indent two spaces at a time
  2. Indenting nested let expressions
  3. Indenting match expressions
  4. Indenting if expressions
  5. Indenting comments

Using Parentheses:

  1. Parenthesize to help indentation
  2. Wrap match expressions with parenthesis
  3. Over parenthesizing

Pattern Matching:

  1. No incomplete pattern matches
  2. Pattern match in the function arguments when possible
  3. Function arguments should not use values for patterns
  4. Avoid using too many projections
  5. Pattern match with as few match expressions as necessary
  6. Don't use List.hd, List.tl, or List.nth

Code Factoring:

  1. Don't let expressions take up multiple lines
  2. Breakup large functions into smaller functions
  3. Over-factoring code


  1. Don't rewrite existing code
  2. Misusing if expressions
  3. Misusing match expressions
  4. Other common misuses
  5. Don't rewrap functions
  6. Avoid computing values twice

File Submission Requirements

  1. Code Must Compile: Any code you submit must compile. If it does not compile, we won't grade the project and you will lose all the points for the project. You should treat any compiler warnings as errors.
  2. 80 Column Limit: No line of code should have more than 80 columns. Using more than 80 columns causes your code to wrap around to the next line which is devastating for readability. Ensuring that all your lines fall within the 80 column limit is not something you should do when you have finished programming.
  3. No Tab Characters: Do not use the tab character (0x09). Instead, use spaces to control indenting. Eclipse provides good tab stops by default. The Emacs package from the OCaml website avoids using tabs (with the exception of pasting text from the clipboard or kill ring). When in ml-mode, Emacs uses the TAB key to control indenting instead of inserting the tab character.


  1. Comments Go Above the Code They Reference: Consider the following:
    let sum = List.fold_left (+) 0
    (* Sums a list of integers. *)
    (* Sums a list of integers. *)
    let sum = List.fold_left (+) 0
    The latter is the better style, although you may find some source code that uses the first. We require that you use the latter.
  1. Avoid Useless Comments: Comments that merely repeat the code it references or state the obvious are a travesty to programmers. Comments should state the invariants, the non-obvious, or any references that have more information about the code.
  1. Avoid Over-commenting: Incredibly long comments are not very useful. Long comments should only appear at the top of a file -- here you should explain the overall design of the code and reference any sources that have more information about the algorithms or data structures. All other comments in the file should be as short as possible, after all brevity is the soul of wit. Most often the best place for any comment is just before a function declaration. Rarely should you need to comment within a function -- variable naming should be enough.
  1. Line Breaks: Obviously the best way to stay within the 80 character limit imposed by the rule above is pressing the enter key every once and a while. Empty lines should be included between value declarations within a struct block, especially between function declarations. Often it is not necessary to have empty lines between other declarations unless you are separating the different types of declarations (such as structures, types, exceptions and values). Unless function declarations within a let block are long, there should be no empty lines within a let block. There should never be an empty line within an expression.
  1. Proper Multi-line Commenting: When comments are printed on paper, the reader lacks the advantage of color highlighting performed by an editor such as Emacs. This makes it important for you to distinguish comments from code. When a comment extends beyond one line, it should be preceded with a * similar to the following:
    (* 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. *)
    let complicatedFunction () = ...

Naming and Declarations

  1. Use Meaningful Names: Variable names should describe what they are for. Distinguishing what a variable references is best done by following a particular naming convention (see suggestion below). Variable names should be words or combinations of words. Cases where variable names can be one letter are in a short let blocks. Often it is the case that a function used in a fold, filter, or map is bound to the name f. Here is an example for short variable names:
     let d = Unix.localtime (Unix.time ()) in
    let m = d.Unix.tm_min in
    let s = d.Unix.tm_min in
    let f n = (n mod 3) = 0 in
    List.filter f [m;s]
  1. Naming Conventions: The following are the naming guidelines that are followed by the OCaml library; try to follow similar conventions:
    Token Convention Example
    Variables and functions Symbolic or initial lower case. Use underscores for multiword names: get_item
    Constructors Initial upper case. Use embedded caps for multiword names. Historic exceptions are true, and false. Rarely are symbolic names like :: used. Node
    Types All lower case. Use underscores for multiword names. priority_queue
    Module Types Initial upper case. Use embedded caps for multiword names. PriorityQueue
    Modules Same as module type convention. PriorityQueue
    Functors Same as module type convention. PriorityQueue
    These conventions are not enforced by the compiler, though violations of the variable/constructor conventions ought to cause warning messages because of the danger of a constructor turning into a variable when it is misspelled.
  1. Type Annotations: Complex or potentially ambiguous top-level functions and values should be declared with types to aid the reader. Consider the following:
     let get_bit bitidx n =
       let shb = Int32.shift_left 1l bitidx in
       Int32.logand shb n = shb
     let get_bit (bitidx:int) (n:int32):bool =
       let shb = Int32.shift_left 1l bitidx in
       Int32.logand shb n = shb
    The latter is considered better. Such type annotations can also help significantly when debugging typechecking problems.
  1. Avoid Global Mutable Variables: Mutable values should be local to closures and almost never declared as a structure's value. Making a mutable value global causes many problems. First, running code that mutates the value cannot be ensured that the value is consistent with the algorithm, as it might be modified outside the function or by a previous execution of the algorithm. Second, and more importantly, having global mutable values makes it more likely that your code is nonreentrant. Without proper knowledge of the ramifications, declaring global mutable values can extend beyond bad style to incorrect code.
  1. When to Rename Variables: You should rarely need to rename values, in fact this is a sure way to obfuscate code. Renaming a value should be backed up with a very good reason. One instance where renaming a variable is common and encouraged is aliasing structures. In these cases, other structures used 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:
      module H = Hashtbl
      module L = List
      module A = Array
  1. Order of Declarations in a Structure: When declaring elements in a file (or nested module) you 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:
      module L = List
      type foo = unit
      exception InternalError
      let first list = L.nth list 0
    Note that every declaration within the structure should be indented the same amount.


  1. Indent Two Spaces at a Time: Most lines that indent code should only indent by two spaces more than the previous line of code.
  2. Indenting nested let expressions: Blocks of code that have nested let expressions should not be indented.
       let x = exp1 in
          let y = exp2 in
            x + y

       let x = exp1 in
       let y = exp2 in
         x + y
  3. Indenting match Expressions: Indent similar to the following.
    match expr with
    | pat1 -> ...
    | pat2 -> ...
  4. Indenting if Expressions: Indent similar to the following.
    if exp1 then exp2              if exp1 then
    else if exp3 then exp4           exp2
    else if exp5 then exp6         else exp3
         else exp8
    if exp1 then exp2 else exp3
    if exp1 then exp2
    else exp3
  5. Indenting Comments: Comments should be indented to the level of the line of code that follows the comment.

Using Parentheses:

  1. Parenthesize to Help Indentation: Indentation algorithms are often assisted by added parenthesization. Consider the following:
    let x = "Long line..."^
      "Another long line."
    let x = ("Long line..."^
             "Another long line.")
    The latter is considered better style.
  2. Wrap match Expressions with Parenthesis: This avoids a common (and confusing) error that you get when you have a nested match expression.
  3. Over Parenthesizing: Parenthesis have many semantic purposes in ML, including constructing tuples, grouping sequences of side-effect expressions, forcing higher-precedence on an expression for parsing, and grouping structures for functor arguments. Clearly, the parenthesis must be used with care. You may only use parentheses when necessary or when it improves readability. Consider the following two function applications:
    let x = function1 (arg1) (arg2) (function2 (arg3)) (arg4)
    let x = function1 arg1 arg2 (function2 arg3) arg4
    The latter is considered better style. Parentheses should usually not appear on a line by themselves, nor should they be the first graphical character -- parentheses do not serve the same purpose as brackets do in C or Java.

Pattern Matching

  1. No Incomplete Pattern Matches: Incomplete pattern matches are flagged with compiler warnings. We strongly discourage compiler warnings when grading; thus, if there is a compiler warning, the project will get reduced style points.
  2. 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:
    Bad Good
    let f arg1 arg2 =
      let x = fst arg1 in
      let y = snd arg1 in
      let z = fst arg2 in
    let f (x,y) (z,_) = ...
    let f arg1 =
      let x = arg1.foo in
      let y = arg1.bar in
      let baz = arg1.baz in
    let f {foo=x, bar=y, baz} = ...
  3. Function Arguments Should Not Use Values for Patterns: You should only deconstruct values with variable names and/or wildcards in function arguments. If you want to pattern match against a specific value, use a match expression or an if expression. We include this rule because there are too many errors that can occur when you don't do this exactly right. Consider the following:
    let fact 0 = 1
      | fact n = n * fact(n-1)
    let fact n =
      if n=0 then 1
      else n * fact(n-1)
    The latter is considered better style.
  4. Avoid Using Too Many Projections: Frequently projecting a value from a record or tuple causes your code to become unreadable. This is especially a problem with tuple projection because the value is not documented by a variable name. To prevent projections, you should use pattern matching with a function argument or a value declaration. Of course, using projections is okay as long 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.
    Bad Good
    let v = someFunction() in
    let x = fst v in
    let y = snd v in
    let  x,y = someFunction() in
  5. Pattern Match with as Few match Expressions as Necessary: Rather than nest match expressions, you can combine them by pattern matching against a tuple. Of course, this doesn't work if one of the nested match expressions matches against a value obtained from a branch in another match expression. Nevertheless, if all the values are independent of each other you should combine the values in a tuple and match against that. Here is an example:
         let d = Date.fromTimeLocal(Unix.time()) in
         match Date.month d with
           | Date.Jan -> (match Date.day d with
                          | 1 -> print "Happy New Year"
                          | _ -> ())
           | Date.Jul -> (match Date.day d with
                          | 4 -> print "Happy Independence Day"
                          | _ -> ())
           | Date.Oct -> (match Date.day d with
                          | 10 -> print "Happy Metric Day"
                          | _ -> ())
         let  d = Date.fromTimeLocal(Unix.time()) in
           match (Date.month d, Date.day d) of
           | (Date.Jan, 1) -> print "Happy New Year"
           | (Date.Jul, 4) -> print "Happy Independence Day"
           | (Date.Oct, 10) -> print "Happy Metric Day"
           | _ -> ()
  6. Don't use List.hd, List.tl, or List.nth: The functions hd, tl, and nth are used to deconstruct list types; however, they raise exceptions on certain inputs. You should rarely use these functions. In the case that you find it absolutely necessary to use these (something that probably won't ever happen), you should handle any exceptions that can be raised by these functions.

Code Factoring

  1. Don't Let Expressions Take Up Multiple Lines: If a tuple consists of more than two or three elements, you should consider using a record instead of a tuple. Records have the advantage of placing each name on a separate line and still looking good. Constructing a tuple over multiple lines makes your code look hideous -- the expressions within the tuple construction should be extraordinarily simple. Other expressions that take up multiple lines should be done with a lot of thought. The best way to transform code that constructs expressions over multiple lines to something that has good style is to factor the code using a let expression. Consider the following:
         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)))
         fun euclid (m:int,n:int) : (int * int * int) =
           if n=0
             then (b 1, b 0, m)
             let q = m div n in
             let r = n mod n in
             let (u,v,g) = euclid (n,r) in
               (v, u-(q*v), g)
  2. Breakup Large Functions into Smaller Functions: One of the greatest advantages of functional programming is that it encourages writing smaller functions and combining them to solve bigger problems. Just how and when to break up functions is something that comes with experience.
  3. Over-factoring code: In some situations, it's not necessary to bind the results of an expression to a variable. Consider the following:
         letl x = TextIO.inputLine TextIO.stdIn in
           match x with
         match TextIO.inputLine TextIO.stdIn with
    Here is another example of over-factoring (provided y is not a large expression):
    let  x = y*y in x+z
    y*y + z
    The latter is considered better.


  1. Don't Rewrite Existing Code: The OCaml standard libraries have a great number of functions and data structures -- use them! Often students will recode List.filter, List.map, and similar functions. Another common way in which one can avoid recoding is to use the fold functions. Writing a function that recursively walks down a list can almost always make use of List.fold_left or List.fold_right. Other data structures often have similar folding functions; use them whenever they are available.
  2. Misusing if Expressions: Remember that the type of the condition in an if expression is bool. In general, the type of an if expression is 'a, but in the case that the type is bool, you should not be using if at all. Consider the following:
    Bad Good
    if e then true else false e
    if e then false else true not e
    if beta then beta else false beta
    if not e then x else y if e then y else x
    if x then true else y x || y
    if x then y else false x && y
    if x then false else y not x && y
    if x then y else true not x || y
  3. Misusing match Expressions: The match expression is misused in two common situations. First, match should never be used in place of an if expression (that's why if exists). Note the following:
    match e with
    | true -> x
    | false -> y
    if e then x else y
    The latter expression is much better. Another situation where if expressions are preferred over match expressions is as follows:
    match e with
    | c -> x   (* c is a constant value *)
    | _ -> y
    if e=c then x else y
    The latter expression is definitely better. The other misuse is using match when pattern matching with a val declaration is enough. Consider the following:
    letl x = match expr with (y,z) -> y
    let x,_ = expr
    The latter is considered better.
  4. Other Common Misuses: Here is a bunch of other common mistakes to watch out for:
    Bad Good
    l::nil [l]
    l::[] [l]
    length + 0 length
    length * 1 length
    big exp * same big exp let x = big exp in x*x
    if x then f a b c1
    else f a b c2
    f a b (if x then c1 else c2)
  5. Don't Rewrap Functions: When passing a function around as an argument to another function, don't rewrap the function if it already does what you want it to. Here's an example:
    List.map (fun x -> sqrt x) [1.0; 4.0; 9.0; 16.0]
    List.map 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. Consider this example:
    fold_left (fun  x y -> x + y) 0
    fold_left (+) 0
    The latter is considered better style.
  6. Avoid Computing Values Twice: When computing values twice you're wasting the CPU time and making your program ugly. The best way to avoid computing things twice is to create a let expression and bind the computed value to a variable name. This has the added benefit of letting you document the purpose of the value with a variable name -- which means less commenting.