OCaml Style Guide

Acknowledgement: Most of this style guide is taken from CIS 500 at UPenn, which in fine functional programming style were recursively stolen from CS 312 at Cornell University.

Mandatory Rules:

  1. 80 column limit
  2. No tab characters
  3. Code must compile
  4. Comments go above the code they reference
  5. Avoid useless comments
  6. Avoid over-commenting
  7. Line breaks
  8. Proper multi-line commenting
  9. Use meaningful names
  10. Type annotations
  11. Avoid global mutable variables
  12. When to rename variables
  13. Order of declarations in a structure
  14. Indent two spaces at a time
  15. Over parenthesizing
  16. Indenting match expressions
  17. Indenting if expressions
  18. Indenting comments
  19. No incomplete pattern matches
  20. Pattern match in the function arguments when possible
  21. Function arguments should not use values for patterns
  22. Avoid using too many projections
  23. Pattern match with as few match expressions as necessary
  24. Don't use valOf, hd, or tl
  25. Don't let expressions take up multiple lines
  26. Breakup large functions into smaller functions
  27. Over-factoring code
  28. Don't rewrite existing code
  29. Misusing if expressions
  30. Misusing match expressions
  31. Other common misuses
  32. Don't rewrap functions
  33. Avoid computing values twice


File Submission

  1. 80 Column Limit: No line of code can have more than 80 columns. Using more than 80 columns causes your code to wrap around to the next line which is devastating to the readability of your code. This is so important that we will not allow you to submit code that has any line with more than 80 columns. Ensuring that all your lines fall within the 80 column limit is not something you should do when you have finished programming. The course staff reserves the right to refuse assistance with code that violates this rule.
  1. No Tab Characters: (Perhaps not an issue in VS Code?) Do not use the tab character (0x09). Instead, use spaces to control indenting. The Emacs package from this website avoids using tabs (with the exception of pasting text from the clipboard or kill ring). Instead when in ml-mode, Emacs uses the TAB key to control indenting instead of inserting the tab character.
  1. Code Must Compile: Any code you submit must compile. If it does not compile, we won't grade the problem and you will lose all the points for the problem. There is no excuse for it to not compile. You should treat any compiler warnings as an error.


  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. Including empty lines should only be done 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 absolutely 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 () = ...
    An alternative form is to place the closing paren below the last comment line to create separation:
    (* 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. Type Annotations: Top-level functions and values should always be declared with types. Consider the following:
    let foo x = x+1
    let foo(x:int):int = x+1
    The latter is considered better.
  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.
  • Naming Conventions: Although we don't expect you to follow this convention, the following are the rules that are followed by the OCaml library:
    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
    Obviously, 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. Indent Two Spaces at a Time: Most lines that indent code should only indent by two spaces more than the previous line of code.
    • 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.
    • Wrap match Expressions with Parenthesis: This avoids a common (and confusing) error that you get when you have a nested match expression.
  1. 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 never 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.
  1. Indenting match Expressions: Indent similar to the following.
    match expr with
      pat1 -> ...
    | pat2 -> ...
  1. 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
  1. Indenting Comments: Comments should be indented to the level of the line of code that follows the comment.

Pattern Matching

  1. No Incomplete Pattern Matches: Incomplete pattern matches are flagged with compiler warnings. We do not allow any compiler warnings when grading; thus, if there is a compiler warning, the problem will get no points.
  1. 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 = in
      let y = in
      let baz = arg1.baz in
    let f {foo=x, bar=y, baz} = ...
  1. 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.
  1. 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
  1. 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 d with
                            1 -> print "Happy New Year"
                          | _ -> ())
           | Date.Jul -> (match d with
                            4 -> print "Happy Independence Day"
                          | _ -> ())
           | Date.Oct -> (match d with
                            10 -> print "Happy Metric Day"
                          | _ -> ())
         let  d = Date.fromTimeLocal(Unix.time()) in
           match (Date.month d, d) of
             (Date.Jan, 1) -> print "Happy New Year"
           | (Date.Jul, 4) -> print "Happy Independence Day"
           | (Date.Oct, 10) -> print "Happy Metric Day"
           | _ -> ()
  1. Don't use List.hd, or The functions hd and tl are used to deconstruct list types; however, they raise exceptions on certain inputs. You should never 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.


  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)
           else let
             val q = m div n
             val r = m mod n
             val (u,v,g) = euclid (n,r)
             (v, u-(q*v), g)
  1. 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.
  1. Over-factoring code: In some situations, it's not necessary to bind the results of an expression to a variable. Consider the following:
         let 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,, and similar functions. A more subtle situation for recoding is all the fold functions. Writing a function that recursively walks down the list should make vigorous use of List.fold_left or List.fold_right. Other data structures often have a folding function; use them whenever they are available.
  1. 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
  1. 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 often preferred, unless you believe the list of constants matched on will expand from c to c1, c2, c3, etc. The other misuse is using match when pattern matching with a let declaration is enough. Consider the following:
    let x = match expr with (y,z) -> y
    let x,_ = expr
    The latter is considered better.
  1. Other Common Misuses: Here is a bunch of other common mistakes to watch out for:
    Bad Good
    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) x y = 0 x=y x y < 0 x<y x y > 0 x>y
  1. 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: (fun x -> sqrt x) [1.0; 4.0; 9.0; 16.0] 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.
  1. 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.