fsharp-code-formatting.instructions.md

description	applyTo
F# code formatting and indentation conventions	**/*.fs,**/*.fsx

FSharp Code Formatting Instructions

Block indentation should follow a general standard of 4 spaces per indentation level. Also, whatever is delimiting the scope (like [ or { or () should be aligned with the indentation level of the block when it is closed. if then else blocks, match with blocks, let bindings that span multiple lines, and function bodies should all follow this convention.

// Good: indentation by 4 spaces
for i in 1..10 do
    printfn $"{i}"


// unformatted 
let myList =
    [
        "first"
        "second"
        "third"
    ]


// Fantomas formatted
// cannot easily copy past additional 
// elements to the list
let myList = [ "first"; "second"; "third" ]

// Good: indentation by 4 spaces
// and aligned delimiters
let myList =
    [
        "first"
        "second"
        "third"
        "first"
        "second"
        "third"
        "first"
        "second"
        "third"
    ]


// Fantomas formatted
// cannot easily use the default indentation 
// setting of the editor, als moving the first 
// or last element to reorder is awkward
let myList =
    [ "first"
      "second"
      "third"
      "first"
      "second"
      "third"
      "first"
      "second"
      "third" ]


// Good: indentation by 4 spaces
// and aligned delimiters
type myRecord =
    {
        Name: string
        Age: float
        BirthDay: DateTime
    }


// Fantomas formatted
// Same problem, just quicly moving record 
// fields arround is not very easy
type myRecord =
    { Name: string
      Age: float
      BirthDay: DateTime }


// Good : indentation by 4 spaces
// and aligned delimiters
myList
|> List.append
    [
        "first"
        "second"
        "third"
        "first"
        "second"
        "third"
        "first"
        "second"
        "third"        
    ]


// Fantomas formatted
myList
|> List.append
    [ "first"
      "second"
      "third"
      "first"
      "second"
      "third"
      "first"
      "second"
      "third" ]


// Good : indentation by 4 spaces
// and aligned delimiters
vars
|> List.sortBy(fun (_, xs) ->
    // some other code
    xs |> List.iter (printfn "%A")

    xs
    |> List.tail
    |> List.sumBy Variable.count
)


// Fantomas formatted
// again it is more difficult to see what is
// the indented block container (in this case ")")
// also separation betheen different subsections
// of the function body is lost
vars
|> List.sortBy (fun (_, xs) ->
    // some other code
    xs |> List.iter (printfn "%A")
    xs |> List.tail |> List.sumBy Variable.count)

Also, try to put the simple cases on the then case on the same line, and use indentation for more complex cases in the else case.

// Good: simple then case on same line
// more complex else case indented
if b = 0 then []
else
    // start a scope block
    [ 
        for i in 0..b do
            let result = calculate b

            if result.IsNone then None
            else
                result 
                |> Option.map (fun x -> x / 2)
    ]


// Fantomas formatted
// used indentation on a very simple scope block
if b = 0 then
    []
else
    // start a scope block
    [ for i in 0..b do
          let result = calculate b

          if result.IsNone then
              None
          else
              result |> Option.map (fun x -> x / 2) ]

Try to put pipeline operators at the beginning of the line when breaking long pipelines.

// Good: pipeline indentation
result
|> Result.map (fun x -> x + 1)
|> Result.toOption

// Fantomas formatted
result |> Result.map (fun x -> x + 1) |> Result.toOption