Inspect.Algebra

A set of functions for creating and manipulating algebra documents.

This module implements the functionality described in “Strictly Pretty” (2000) by Christian Lindig with small additions, like support for String nodes, and a custom rendering function that maximises horizontal space use.

iex> Inspect.Algebra.empty
:doc_nil

iex> "foo"
"foo"

With the functions in this module, we can concatenate different elements together and render them:

iex> doc = Inspect.Algebra.concat(Inspect.Algebra.empty, "foo")
iex> Inspect.Algebra.format(doc, 80)
["foo"]

The functions nest/2, space/2 and line/2 help you put the document together into a rigid structure. However, the document algebra gets interesting when using functions like break/2, which converts the given string into a line break depending on how much space there is to print. Let’s glue two docs together with a break and then render it:

iex> doc = Inspect.Algebra.glue("a", " ", "b")
iex> Inspect.Algebra.format(doc, 80)
["a", " ", "b"]

Notice the break was represented as is, because we haven’t reached a line limit. Once we do, it is replaced by a newline:

iex> doc = Inspect.Algebra.glue(String.duplicate("a", 20), " ", "b")
iex> Inspect.Algebra.format(doc, 10)
["aaaaaaaaaaaaaaaaaaaa", "\n", "b"]

Finally, this module also contains Elixir related functions, a bit tied to Elixir formatting, namely surround/3 and surround_many/5.

Implementation details

The original Haskell implementation of the algorithm by Wadler relies on lazy evaluation to unfold document groups on two alternatives: :flat (breaks as spaces) and :break (breaks as newlines). Implementing the same logic in a strict language such as Elixir leads to an exponential growth of possible documents, unless document groups are encoded explicitly as :flat or :break. Those groups are then reduced to a simple document, where the layout is already decided, per Lindig.

This implementation slightly changes the semantic of Lindig’s algorithm to allow elements that belong to the same group to be printed together in the same line, even if they do not fit the line fully. This was achieved by changing :break to mean a possible break and :flat to force a flat structure. Then deciding if a break works as a newline is just a matter of checking if we have enough space until the next break that is not inside a group (which is still flat).

Custom pretty printers can be implemented using the documents returned by this module and by providing their own rendering functions.

Summary

Types

t()

Functions

break()

break(s)

Document entity representing a break

concat(docs)

Concatenates a list of documents

concat(x, y)

Concatenates two document entities

empty()

Returns a document entity used to represent nothingness

fold_doc(list, fun)

Folds a list of document entities into a document entity using a function that is passed as the first argument

format(d, w)

The formatting function

glue(x, y)

Inserts a break between two docs. See break/1 for more info

glue(x, g, y)

Inserts a break, passed as the second argument, between two docs, the first and the third arguments

group(d)

Returns a group containing the specified document

line(x, y)

Inserts a mandatory linebreak between two document entities

nest(x, i)

Nests document entity x positions deep

space(x, y)

Inserts a mandatory single space between two document entities

surround(left, doc, right)

Surrounds a document with characters

surround_many(left, docs, right, opts, fun, separator \\ ",")

Maps and glues a collection of items

to_doc(map, opts)

Converts an Elixir structure to an algebra document according to the inspect protocol

Types

t()

t ::
  :doc_nil |
  :doc_line |
  doc_cons |
  doc_nest |
  doc_break |
  doc_group |
  binary

Functions

break()

break() :: doc_break

break(s)

break(binary) :: doc_break

Document entity representing a break.

This break can be rendered as a linebreak or as spaces, depending on the mode of the chosen layout or the provided separator.

Examples

Let’s glue two docs together with a break and then render it:

iex> doc = Inspect.Algebra.glue("a", " ", "b")
iex> Inspect.Algebra.format(doc, 80)
["a", " ", "b"]

Notice the break was represented as is, because we haven’t reached a line limit. Once we do, it is replaced by a newline:

iex> doc = Inspect.Algebra.glue(String.duplicate("a", 20), " ", "b")
iex> Inspect.Algebra.format(doc, 10)
["aaaaaaaaaaaaaaaaaaaa", "\n", "b"]

concat(docs)

concat([t]) :: doc_cons

Concatenates a list of documents.

concat(x, y)

concat(t, t) :: doc_cons

Concatenates two document entities.

Examples

iex> doc = Inspect.Algebra.concat "hello", "world"
iex> Inspect.Algebra.format(doc, 80)
["hello", "world"]

empty()

empty() :: :doc_nil

Returns a document entity used to represent nothingness.

Examples

iex> Inspect.Algebra.empty
:doc_nil

fold_doc(list, fun)

fold_doc([t], (t, t -> t)) :: t

Folds a list of document entities into a document entity using a function that is passed as the first argument.

Examples

iex> doc = ["A", "B"]
iex> doc = Inspect.Algebra.fold_doc(doc, fn(x, y) ->
...>   Inspect.Algebra.concat [x, "!", y]
...> end)
iex> Inspect.Algebra.format(doc, 80)
["A", "!", "B"]

format(d, w)

format(t, non_neg_integer | :infinity) :: iodata

The formatting function.

Takes the maximum width and a document to print as its arguments and returns an IO data representation of the best layout for the document to fit in the given width.

glue(x, y)

glue(t, t) :: doc_cons

Inserts a break between two docs. See break/1 for more info.

glue(x, g, y)

glue(t, binary, t) :: doc_cons

Inserts a break, passed as the second argument, between two docs, the first and the third arguments.

group(d)

group(t) :: doc_group

Returns a group containing the specified document.

Examples

iex> doc = Inspect.Algebra.group(
...>   Inspect.Algebra.concat(
...>     Inspect.Algebra.group(
...>       Inspect.Algebra.concat(
...>         "Hello,",
...>         Inspect.Algebra.concat(
...>           Inspect.Algebra.break,
...>           "A"
...>         )
...>       )
...>     ),
...>     Inspect.Algebra.concat(
...>       Inspect.Algebra.break,
...>       "B"
...>     )
...> ))
iex> Inspect.Algebra.format(doc, 80)
["Hello,", " ", "A", " ", "B"]
iex> Inspect.Algebra.format(doc, 6)
["Hello,", "\n", "A", " ", "B"]

line(x, y)

line(t, t) :: doc_cons

Inserts a mandatory linebreak between two document entities.

Examples

iex> doc = Inspect.Algebra.line "Hughes", "Wadler"
iex> Inspect.Algebra.format(doc, 80)
["Hughes", "\n", "Wadler"]

nest(x, i)

nest(t, non_neg_integer) :: doc_nest

Nests document entity x positions deep.

Nesting will be appended to the line breaks.

Examples

iex> doc = Inspect.Algebra.nest(Inspect.Algebra.glue("hello", "world"), 5)
iex> Inspect.Algebra.format(doc, 5)
["hello", "\n     ", "world"]

space(x, y)

space(t, t) :: doc_cons

Inserts a mandatory single space between two document entities.

Examples

iex> doc = Inspect.Algebra.space "Hughes", "Wadler"
iex> Inspect.Algebra.format(doc, 80)
["Hughes", " ", "Wadler"]

surround(left, doc, right)

surround(binary, t, binary) :: t

Surrounds a document with characters.

Puts the document between left and right enclosing and nesting it. The document is marked as a group, to show the maximum as possible concisely together.

Examples

iex> doc = Inspect.Algebra.surround "[", Inspect.Algebra.glue("a", "b"), "]"
iex> Inspect.Algebra.format(doc, 3)
["[", "a", "\n ", "b", "]"]

surround_many(left, docs, right, opts, fun, separator \\ ",")

surround_many(binary, [any], binary, Inspect.Opts.t, (term, Inspect.Opts.t -> t), binary) :: t

Maps and glues a collection of items.

It uses the given left and right as surrounding and a separator for each item. A limit can be passed which, once reached, stops gluing and outputs “…” instead.

Examples

iex> doc = Inspect.Algebra.surround_many("[", Enum.to_list(1..5), "]",
...>         %Inspect.Opts{limit: :infinity}, fn i, _opts -> to_string(i) end)
iex> Inspect.Algebra.format(doc, 5) |> IO.iodata_to_binary
"[1,\n 2,\n 3,\n 4,\n 5]"

iex> doc = Inspect.Algebra.surround_many("[", Enum.to_list(1..5), "]",
...>         %Inspect.Opts{limit: 3}, fn i, _opts -> to_string(i) end)
iex> Inspect.Algebra.format(doc, 20) |> IO.iodata_to_binary
"[1, 2, 3, ...]"

iex> doc = Inspect.Algebra.surround_many("[", Enum.to_list(1..5), "]",
...>         %Inspect.Opts{limit: 3}, fn i, _opts -> to_string(i) end, "!")
iex> Inspect.Algebra.format(doc, 20) |> IO.iodata_to_binary
"[1! 2! 3! ...]"

to_doc(map, opts)

to_doc(any, Inspect.Opts.t) :: t

Converts an Elixir structure to an algebra document according to the inspect protocol.