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Code formatting

This page demonstrates how to use FSharp.Formatting.CodeFormat to tokenize F# source code, obtain information about the source code (mainly tooltips from the type-checker) and how to turn the code into a nicely formatted HTML.

First, we need to load the assembly and open necessary namespaces:

open FSharp.Formatting.CodeFormat
open System.Reflection

If you want to process multiple snippets, it is a good idea to keep the formatting agent around if possible. The agent needs to load the F# compiler (which needs to load various files itself) and so this takes a long time.

Processing F# source

The formatting agent provides a CodeFormatAgent.ParseAndCheckSource method (together with an asynchronous version for use from F# and also a version that returns a .NET Task for C#). To call the method, we define a simple F# code as a string:

let source =
    let hello () =
      printfn "Hello world"

let snippets, diagnostics =
    CodeFormatter.ParseAndCheckSource("C:\\snippet.fsx", source, None, None, ignore)

When calling the method, you need to specify a file name and the actual content of the script file. The file does not have to physically exist. It is used by the F# compiler to resolve relative references (e.g. #r) and to automatically name the module including all code in the file.

You can also specify additional parameters, such as *.dll references, by passing a third argument with compiler options (e.g. "-r:Foo.dll -r:Bar.dll").

This operation might take some time, so it is a good idea to use an asynchronous variant of the method. It returns two arrays - the first contains F# snippets in the source code and the second contains any errors reported by the compiler. A single source file can include multiple snippets using the same formatting tags as those used on fssnip.net as documented in the about page.

Working with returned tokens

Each returned snippet is essentially just a collection of lines and each line consists of a sequence of tokens. The following snippet prints basic information about the tokens of our sample snippet:

// Get the first snippet and obtain list of lines
let (Snippet (title, lines)) = snippets |> Seq.head

// Iterate over all lines and all tokens on each line
for (Line (_, tokens)) in lines do
    for token in tokens do
        match token with
        | TokenSpan.Token (kind, code, tip) ->
            printf "%s" code

            |> Option.iter (fun spans -> printfn "%A" spans)
        | TokenSpan.Omitted _
        | TokenSpan.Output _
        | TokenSpan.Error _ -> ()

    printfn ""

The TokenSpan.Token is the most important kind of token. It consists of a kind (identifier, keyword, etc.), the original F# code and tool tip information. The tool tip is further formatted using a simple document format, but we simply print the value using the F# pretty printing, so the result looks as follows:

let hello[Literal "val hello : unit -> unit"; ...] () =
  printfn[Literal "val printfn : TextWriterFormat<'T> -> 'T"; ...] "Hello world"

The Omitted token is generated if you use the special (*[omit:...]*) command. The Output token is generated if you use the // [fsi:...] command to format output returned by F# interactive. The Error command wraps code that should be underlined with a red squiggle if the code contains an error.

Generating HTML output

Finally, the CodeFormat type also includes a method CodeFormat.FormatHtml that can be used to generate nice HTML output from an F# snippet. This is used, for example, on F# Snippets. The following example shows how to call it:

let prefix = "fst"
let html = CodeFormat.FormatHtml(snippets, prefix)

// Print all snippets, in case there is more of them
for snip in html.Snippets do
    printfn "%s" snip.Content

// Print HTML code that is generated for ToolTips
printfn "%s" html.ToolTip

If the input contains multiple snippets separated using the //[snippet:...] comment, e.g.:

// [snippet: First sample]
printf "The answer is: %A" 42
// [/snippet]
// [snippet: Second sample]
printf "Hello world!"
// [/snippet]

then the formatter returns multiple HTML blocks. However, the generated tool tips are shared by all snippets (to save space) and so they are returned separately.

Multiple items
namespace FSharp

namespace Microsoft.FSharp
namespace FSharp.Formatting
namespace FSharp.Formatting.CodeFormat
namespace System
namespace System.Reflection
val source: string
val snippets: Snippet array
val diagnostics: SourceError array
module CodeFormatter from FSharp.Formatting.CodeFormat
<summary> Uses agent to handle formatting requests </summary>
val ParseAndCheckSource: file: string * source: string * options: string option * defines: string option * onError: (string -> unit) -> Snippet array * SourceError array
<summary> Parse, check and annotate the source code specified by 'source', assuming that it is located in a specified 'file'. Optional arguments can be used to give compiler command line options and preprocessor definitions </summary>
union case Option.None: Option<'T>
val ignore: value: 'T -> unit
Multiple items
union case Snippet.Snippet: title: string * lines: Line list -> Snippet

type Snippet = | Snippet of title: string * lines: Line list
<summary> An F# snippet consists of a snippet title and a list of lines </summary>
val title: string
val lines: Line list
module Seq from Microsoft.FSharp.Collections
val head: source: 'T seq -> 'T
Multiple items
union case Line.Line: originalLine: string * tokenSpans: TokenSpans -> Line

type Line = | Line of originalLine: string * tokenSpans: TokenSpans
<summary> Represents a line of source code as a list of TokenSpan values. This is a single case discriminated union with Line constructor. </summary>
val tokens: TokenSpans
val token: TokenSpan
type TokenSpan = | Token of kind: TokenKind * body: string * tip: ToolTipSpans option | Error of kind: ErrorKind * message: string * body: TokenSpans | Omitted of body: string * hidden: string | Output of string
<summary> A token in a parsed F# code snippet. Aside from standard tokens reported from the compiler (Token), this also includes Error (wrapping the underlined tokens), Omitted for the special [omit:...] tags and Output for the special [output:...] tag </summary>
union case TokenSpan.Token: kind: TokenKind * body: string * tip: ToolTipSpans option -> TokenSpan
val kind: TokenKind
val code: string
val tip: ToolTipSpans option
val printf: format: Printf.TextWriterFormat<'T> -> 'T
module Option from Microsoft.FSharp.Core
val iter: action: ('T -> unit) -> option: 'T option -> unit
val spans: ToolTipSpans
val printfn: format: Printf.TextWriterFormat<'T> -> 'T
union case TokenSpan.Omitted: body: string * hidden: string -> TokenSpan
union case TokenSpan.Output: string -> TokenSpan
union case TokenSpan.Error: kind: ErrorKind * message: string * body: TokenSpans -> TokenSpan
Multiple items
type LiteralAttribute = inherit Attribute new: unit -> LiteralAttribute

new: unit -> LiteralAttribute
val prefix: string
val html: FormattedContent
type CodeFormat = static member FormatFsx: snippets: Snippet seq -> FormattedContent static member FormatHtml: snippets: Snippet array * prefix: string * ?openTag: string * ?closeTag: string * ?lineNumbers: bool * ?openLinesTag: string * ?closeLinesTag: string * ?addErrors: bool * ?tokenKindToCss: (TokenKind -> string) -> FormattedContent static member FormatLatex: snippets: Snippet array * ?openTag: string * ?closeTag: string * ?lineNumbers: bool -> FormattedContent
<summary> Exposes functionality of the F# code formatter with a nice interface </summary>
static member CodeFormat.FormatHtml: snippets: Snippet array * prefix: string * ?openTag: string * ?closeTag: string * ?lineNumbers: bool * ?openLinesTag: string * ?closeLinesTag: string * ?addErrors: bool * ?tokenKindToCss: (TokenKind -> string) -> FormattedContent
val snip: FormattedSnippet
property FormattedContent.Snippets: FormattedSnippet array with get
<summary> Returns the processed snippets as an array </summary>
property FormattedSnippet.Content: string with get
<summary> Returns the formatted content code for the snipet </summary>
property FormattedContent.ToolTip: string with get
<summary> Returns string with ToolTip elements for all the snippets </summary>

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