Elixir v1.3.0-rc.0 Inspect.Algebra View Source
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.
Link to this section Summary
Functions
Document entity representing a break
Concatenates a list of documents
Concatenates two document entities
Returns a document entity used to represent nothingness
Folds a list of document entities into a document entity using a function that is passed as the first argument
The formatting function
Inserts a break between two docs. See break/1 for more info
Inserts a break, passed as the second argument, between two docs, the first and the third arguments
Returns a group containing the specified document
Inserts a mandatory linebreak between two document entities
Nests document entity x positions deep
Inserts a mandatory single space between two document entities
Surrounds a document with characters
Maps and glues a collection of items
Converts an Elixir structure to an algebra document according to the inspect protocol
Link to this section Types
t :: :doc_nil | :doc_line | doc_cons | doc_nest | doc_break | doc_group | binary
Link to this section Functions
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"]Concatenates a list of documents.
Concatenates two document entities.
Examples
iex> doc = Inspect.Algebra.concat "hello", "world"
iex> Inspect.Algebra.format(doc, 80)
["hello", "world"]Returns a document entity used to represent nothingness.
Examples
iex> Inspect.Algebra.empty
:doc_nilFolds 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"]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.
Inserts a break between two docs. See break/1 for more info.
Inserts a break, passed as the second argument, between two docs, the first and the third arguments.
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"]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"]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"]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"]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(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! ...]"Converts an Elixir structure to an algebra document according to the inspect protocol.