alba

Alba compiler
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Library alba.fmlib
Module Fmlib . Pretty_print

Overview

Example

The usage of the pretty printer is best explained by an example. Suppose we want to print the function application f a b (g c d) e where the function names and arguments might have different length. We create a document which represents the structure by

let doc =
    group (
        text "f" <+> space <+>
        indent
            2
            (stack_or_pack
                " "
                [text "a";
                 text "b";
                 group (
                     text "(g" <+> space <+>
                     indent
                        2
                        (stack_or_pack " " [text "c"; text "d"])
                     <+> text ")");
                 text "e"])
    )

where text "blabla" is a document with some unbreakable text, <+> concatenates two documents, space is a break hint whose alternative text is a blank, stack_or_pack atxt [...] stacks a list of documents separated by a break hint with the alternative text atxt.

The command

let stream = layout 5 doc 

creates a stream of characters which is nicely formatted using a desired line width of 5 characters. Since 5 characters are not enough to put any of the subterms completely on a line, the output is

123456789012345
f
  a
  b
  (g
    c
    d)
  d

i.e. each break hint is printed as a newline.

If we give the pretty printer a line width of 10, it could pack the application g c d on a line and print

123456789012345
f
  a
  b
  (g c d)
  d

If the pretty printer has enough line width e.g. a line width of 15, it can put the whole expression on a line.

123456789012345
f a b (g c d) d

By using stack_or_pack we instructed the pretty printer to either print all break hints as newlines or all break hints with their alternative texts. If we use pack instead of stack_or_pack, the pretty printer tries to pack as many arguments as possible on a line.

E.g. with a line width of 11 and using pack instead of stack_or_pack we get the output

123456789012345
f
  a b
  (g c d) d

With a line width of 10 and using pack we get

123456789012345
f
  a b
  (g c d)
  d

because the pretty printer cannot pack (g c d) and d on a single line.

Character Stream

The basic type t of the pretty printer is a lazy character stream. I.e. characters are only generated if needed. The pretty printer implements the interface Module_types.READABLE to represent a character stream. You can ask the stream has_more r whether there are more characters in the stream and peek r to get the next character. The instruction advance r returns the stream r advanced by one character position.

The pretty printer has a function string_of r to return a string representation of the character stream.

However you very rarely need a string representation of a character stream. All io function in Fmlib are able to handle character streams.

Generate Documents

Clearly, it is tedious to write documents by hand. Usually you have some tree like structure and you want to generate a document from the tree structure.

Let's assume you have a tree structure like

type tree =
    { name: string; children: tree list; }

let leaf (name: string): tree =
    {name; children = [] }

let tree (name: string) (children: tree list): tree =
    {name; children}

Write a function which converts the tree structure to a document.

let doc_of_tree (tree: tree): doc =
    let rec doc is_top tree =
        match tree.children with
        | [] ->
            text tree.name
        | _ ->
            let d =
                parent_child
                    " " 2
                    (text tree.name)
                    (children tree.children ())
            in
            if is_top then
                d
            else
                char '(' <+> d <+> char ')'
    and children lst () =
        match lst with
        | [last] ->
            doc false last
        | head :: tail ->
            doc false head <+> space
            >> children tail    (* Lazy concatenation!! *)
        | [] ->
            assert false (* 'lst' is never empty *)
    in
    doc true tree

Then the simple command

tree
    "f"
    [leaf "a";
     leaf "b";
     tree "g" [leaf "c"; leaf "d"];
     leaf "e"]
|> layout 10

generates the character stream

123456789012345
f
  a
  b
  (g c d)
  e

Note the usage of the lazy concatentation operator >> in the recursive part of the function handling the children. This makes sure that even if the tree structure is hugh, the iteration over it is done only on demand. I.e. recursive calls are made only if the corresponding characters are needed when processing the character stream.

API

type t

A readable character stream.

val has_more : t -> bool

has_more r Does the stream r have more characters to read? 

val peek : t -> char

peek r The next character in the stream r.

val advance : t -> t

advance r The character stream r advance by one position.

val string_of : t -> string

string_of r A string representation of the stream r.

type doc

A document.

val layout : int -> doc -> t

layout width doc Layout the document doc with a the line width.

val layout_with_ribbon : int -> int -> doc -> t

layout width ribbon doc Layout the document doc with a the line width and the ribbon width. Note: width is the complete line width and ribbon is the line width minus the indentation of the current line.

val empty : doc

An empty document.

val text : string -> doc

text str A document with the unbreakable string str.

val substring : string -> int -> int -> doc

substring str start length A document with the unbreakable string str starting at position start and having length.

val char : char -> doc

char c A document with the character c.

val fill : int -> char -> doc

fill n c A document with n repetitions of the character c.

val break : string -> doc

break str A break hint with the alternative text str.

val space : doc

space A break hint with a blank as alternative text.

val cut : doc

cut A break hint with an empty alternative text.

val group : doc -> doc

group doc

Treat all break hints belonging directly to doc consistently. Either print all as newlines or print all with their alternative text.

This is the basic operation to decide break hints.

If the whole group and all text which follows until the next break hint after the group fits on a line, then all break hints (directly or indirectly) in the group are flattened i.e. printed with their alternative texts.

If the whole group does not fit, then all break hints belonging directly to the group are printed as effective newlines. The break hints of inner groups are considered separately.

val nest : int -> doc -> doc

nest n doc

The document doc indented by n blanks.

This is the basic function to indicate a substructure to the pretty printer. The substructure is indented with respect to the parent document.

The indentation is valid after each effective line break. It is usually convenient to group the whole substructure and put a break hint before the group and group the parent structure and the substructure. This makes sure that either the parent and the child fit on a line or the child begins on a newline and is indented. See the function parent_child below which does exactly that.

val parent_child : string -> int -> doc -> doc -> doc

parent_child hint indent parent child

Put the parent and the child in a group and separate them by a break hint with the alternative text hint. Furthermore put the child in a separate group.

Equivalent to

parent
<+> break hint
<+> nest indent (group child)
|> group
val with_width : int -> doc -> doc

with_width n doc Format the document doc with line width.

val with_ribbon : int -> doc -> doc

with_ribbon n doc Format the document doc with ribbon width.

val (<+>) : doc -> doc -> doc

doc1 <+> doc2 Concatentate the documents doc1 and doc2.

val (>>) : doc -> ( unit -> doc ) -> doc

doc >> lazy_doc Concatenate the document doc with the lazy document lazy_doc.

val cat : doc list -> doc

cat list Concatenate all documents in the list of documents.

val separated_by : doc -> doc list -> doc

separated_by sep list Concatenate all documents in the list of documents separated by sep.

val pack : string -> doc list -> doc

pack str list Pack as much documents of the list of documents as possible into a line. I.e. separate all documents by a break hint with str as an alternative text.

val stack : string -> doc list -> doc

stack str list The same as separated_by (break str) list.

val stack_or_pack : string -> doc list -> doc

stack_or_pack str list Separate all documents of the list by a break hint with alternative text str and either print all break hints as newlines of with the alternative text str.

val wrap_words : string -> doc

wrap_words str Split the string str into words (words are substrings of str not containing blanks) and pack as many of them onto a line.