F# Coding Guidelines

Formatting Your Code

Using #light

Code should be either

"#light" code (without in/;; etc.); or

"neutral" code that doesn't give warnings when "#light" is added

Whitespace:

Recommendation: Use 4-space indentation. If using Visual Studio set the "Tab" setting for F# code editing to 4-space and "replace tabs by spaces"

let myFunction x y =

let g z =

let q = z * z

x + y + z + q

let w = x + y + g 3

(x,y,w)

Recommendation: Consider aligning constructs vertically, especially when polishing code, but not when rapid-prototyping:

Formatting lists, tuples, arrays, records

Recommendation: use trailing closing token for records, lists and arrays normally on end of line of last construct

let rainbow =

let rainbow =

Not everyone likes this style, and variation is OK. For large constructs (> 6 lines) the closing token can be on a fresh line:

let rainbow =

Note: you can optionally include a trailing ";" for the last entry

Recommendation: Tuples that split across multiple lines follow a similar rule:

let tree1 =

    Binary_node

        (Binary_node(Binary_value , Binary_value ),

         Binary_node(Binary_value , Binary_value ))

However the following is also acceptable:

let tree1 =

       Binary_node(

      Binary_node(Binary_value , Binary_value ),

      Binary_node(Binary_value , Binary_value )

Recommendation: Arrays that split across multiple lines follow a similar rule:

let pascals_triangle =

    [| [| |];

; |];

; ; |];

; ; ; |];

; ; ; ; |];

; ; ; ; ; |];

; ; ; ; ; ; |];

; ; ; ; ; ; ; |];

; ; ; ; ; ; ; ; |];

Recommendation: Always use at least one space between two distinct parantherical operators (e.g. leave a space after a "[" and between a "[" and a "{")

Not:

Formatting begin/end

Recommendation: Note these are essentially never needed in #light code. However, if a begin/end is included then either dangle or indent

e.g.

    if not file.Exists then

        begin

            Idioms.using

        (file.CreateText())

                (fun stream ->

                    stream.WriteLine('hello world'))

        end;

or

    if not file.Exists then begin

        Idioms.using

            (file.CreateText())

            (fun stream ->

                stream.WriteLine('hello world'))

     end;

or use #light:

    #light

if not file.Exists then

        Idioms.using (file.CreateText()) (fun stream ->

                stream.WriteLine 'hello world')

but not

    if not file.Exists then

    begin

        Idioms.using

            (file.CreateText())

            (fun stream ->

                stream.WriteLine('hello world'))

    end;

or

    if not file.Exists then

        begin

        Idioms.using

            (file.CreateText())

            (fun stream ->

                stream.WriteLine('hello world'))

        end;

Formatting object expressions

Recommendation: Within object expressions, with/and line up underneath "new"

Recommendation: Strongly consider the "member" syntax for object expression members. This is a relatively new addition to the language, so many samples don't use it.

Recommendation: if interfaces supported then align these and format "with"/"and" in the same way

let the_players =

Another example:

let comparer =

not

let comparer =

Formatting match and try/with

Recommendation: Rules of a pattern match should be aligned with "match". Rules of a "with" in a "try"/"with" can be optionally 4-space indented:

e.g.

    try

        if System.DateTime.Now.Second % 3 = 0 then

            raise (new System.Exception())

        else

            raise (new System.ApplicationException())

    with

    | :? System.ApplicationException ->

        print_endline 'A second that was not a multiple of 3'   

    | _ ->

        print_endline 'A second that was a multiple of 3'

but this is also OK:

    try

        if System.DateTime.Now.Second % 3 = 0 then

            raise (new System.Exception())

        else

            raise (new System.ApplicationException())

    with

        | :? System.ApplicationException ->

            print_endline 'A second that was not a multiple of 3'   

        | _ ->

           print_endline 'A second that was a multiple of 3'

Recommendation: Use a "|" for each clause of a match (strictly speaking it is optional for the first), except when the match is all on one line.

OK:

       match l with

       | :: tail -> x

       | _ :: tail -> find_david tail

       | [] -> failwith 'Couldn't find David'

Not:

       match l with

           | :: tail -> x

           | _ :: tail -> find_david tail

           | [] -> failwith 'Couldn't find David'

OK:

       match l with [] -> false | _ :: _ -> true

Formatting applications

Recommendation: arguments always indented from functions

OK:

     (Printf.sprintf 't%s - %inr'

         x.ingredient_name x.quantity) )

OK:

     (Printf.sprintf

         't%s - %inr'

         x.ingredient_name x.quantity))

OK:

let print_volumes x =

       Printf.printf 'Volume in liters = %f, in us pints = %f, in imperial = %f'

        (convert_volume_to_liter x)

        (convert_volume_us_pint x)

        (convert_volume_imperial_pint x)

Not:

let print_volumes x =

       Printf.printf 'Volume in liters = %f, in us pints = %f, in imperial = %f'

       (convert_volume_to_liter x)

       (convert_volume_us_pint x)

       (convert_volume_imperial_pint x)

Not:

             (Printf.sprintf 't%s - %inr'

             x.ingredient_name x.quantity) )

Recommendation: infix expressions are ok to lineup on same column

OK:

             acc +

             (Printf.sprintf 't%s - %inr'

                x.ingredient_name x.quantity) )

Recommendation: function arguments (e.g. to Add, Iterate, ForEach etc.) can be either on next line or with a dangling "fun" on the argument line:

OK:

    toggle.Click.Add(fun _ ->

        if !eventAdded then

            event.Click.RemoveHandler(stuffHandler)

        else

            event.Click.AddHandler(stuffHandler);

eventAdded := not !eventAdded;

        setText eventAdded);

Tolerable:

    toggle.Click.Add(

        fun _ ->

            if !eventAdded then

        event.Click.RemoveHandler(stuffHandler)

            else

        event.Click.AddHandler(stuffHandler);

eventAdded := not !eventAdded;

            set_text eventAdded);

Recommendation: In general no space is used after a data constructor if arguments are parenthesized:

OK: BinaryNode

Not: BinaryNode

Formatting derived control constructs

OK:

        Idioms.using (file.CreateText()) (fun stream ->

            stream.WriteLine('hello world'))

OK:

        Idioms.using

           (file.CreateText

       "arg1"

       "arg2")

           (fun stream ->

               stream.WriteLine('hello world'))

Formatting declarations

Recommendation: Multi-line definitions where a token is on the same line as an "=" should either be fully indented past the point of the "="

let couples = [ ;

        ;

        ;

        ]

But not:

let couples = [ ;

       ;

       ;

       ]

And not:

let couples =

     [ ;

       ;

       ;

       ]

Formatting pipelines

Recommendation: Pipeline |> should go at the start of a line immediately under the expression being operated on:

OK:

let methods2 = System.AppDomain.CurrentDomain.GetAssemblies()

       |> List.of_array

       |> List.map ( fun assm -> assm.GetTypes() )

               |> Array.concat

               |> List.of_array

               |> List.map ( fun t -> t.GetMethods() )

               |> Array.concat

OK:

let methods2 =

    System.AppDomain.CurrentDomain.GetAssemblies

    |> List.of_array

    |> List.map ( fun assm -> assm.GetTypes() )

    |> Array.concat

    |> List.of_array

    |> List.map ( fun t -> t.GetMethods() )

    |> Array.concat

Not:

let methods2 = System.AppDomain.CurrentDomain.GetAssemblies()

            |> List.of_array

            |> List.map ( fun assm -> assm.GetTypes() )

            |> Array.concat

            |> List.of_array

            |> List.map ( fun t -> t.GetMethods() )

            |> Array.concat

Formatting comprehensions

TBD

Formatting type definitions

Recommendation: Indent "|" in type definition by 4 spaces:

OK:

type volume =

    | Liter of float

    | USPint of float

    | ImperialPint of float

Not:

type volume =

| Liter of float

| USPpint of float

| ImperialPint of float

Common Recommended Coding Idioms

The following operators are defined in the F# standard library and should be used wherever possible instead of defining equivalents. This is spelled out explicitly because OCaml doesn't support all of these operators, and thus F# users who have first learnt OCaml are often not aware of this.

f >> g -- forward composition

g << f -- reverse composition

x |> f -- forward pipeline

f <| x -- reverse pipeline

x |> ignore -- throwing away a value

x + y -- overloaded addition (including string concatenation)

x - y -- overloaded subtraction

x * y -- overloaded multiplication

x / y -- overloaded division

x % y -- overloaded modulus

x <<< y -- bitwise left shift

x >>> y -- bitwise right shift

x ||| y -- bitwise left shift

x &&& y -- bitwise right shift

x ^^^ y -- bitwise left shift

x && y -- lazy/short-cut 'and'

x || y -- lazy/short-cut 'or'

Use comprehensions, but be aware of their limitations (e.g. you can't do a 'fold').