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Usage Guide

Installation

Use the Julia package manager.

julia> using Pkg
julia> Pkg.add("PrettyPrinting")

Using PrettyPrinting

First, import the module.

using PrettyPrinting

Use the function pprint() to print composite data structures formed of nested tuples, vectors, and dictionaries. The data will be formatted to fit the screen size.

To demonstrate how to use pprint(), we take a small dataset of city departments with associated employees.

data = [(name = "POLICE",
         employees = [(name = "JEFFERY A", position = "SERGEANT", salary = 101442, rate = missing),
                      (name = "NANCY A", position = "POLICE OFFICER", salary = 80016, rate = missing)]),
        (name = "OEMC",
         employees = [(name = "LAKENYA A", position = "CROSSING GUARD", salary = missing, rate = 17.68),
                      (name = "DORIS A", position = "CROSSING GUARD", salary = missing, rate = 19.38)])]

The built-in print() function prints this data on a single line, making the output unreadable.

print(data)
#-> NamedTuple … [(name = "POLICE", employees = … [(name = "JEFFERY A", position = "SERGEANT", salary = 101442, rate = missing), … ]) … ]

By contrast, pprint() formats the data to fit the screen size.

pprint(data)
#=>
[(name = "POLICE",
  employees = [(name = "JEFFERY A",
                position = "SERGEANT",
                salary = 101442,
                rate = missing),
               (name = "NANCY A",
                position = "POLICE OFFICER",
                salary = 80016,
                rate = missing)]),
 (name = "OEMC",
  employees = [(name = "LAKENYA A",
                position = "CROSSING GUARD",
                salary = missing,
                rate = 17.68),
               (name = "DORIS A",
                position = "CROSSING GUARD",
                salary = missing,
                rate = 19.38)])]
=#

The width of the output is controlled by the displaysize property of the output stream.

pprint(IOContext(stdout, :displaysize => (24, 100)), data)
#=>
[(name = "POLICE",
  employees = [(name = "JEFFERY A", position = "SERGEANT", salary = 101442, rate = missing),
               (name = "NANCY A", position = "POLICE OFFICER", salary = 80016, rate = missing)]),
 (name = "OEMC",
  employees = [(name = "LAKENYA A", position = "CROSSING GUARD", salary = missing, rate = 17.68),
               (name = "DORIS A", position = "CROSSING GUARD", salary = missing, rate = 19.38)])]
=#

To add a line break after the output, use the function pprintln().

pprintln(data[1])
pprintln(data[2])
#=>
(name = "POLICE",
 employees = [(name = "JEFFERY A",
               position = "SERGEANT",
               salary = 101442,
               rate = missing),
              (name = "NANCY A",
               position = "POLICE OFFICER",
               salary = 80016,
               rate = missing)])
(name = "OEMC",
 employees = [(name = "LAKENYA A",
               position = "CROSSING GUARD",
               salary = missing,
               rate = 17.68),
              (name = "DORIS A",
               position = "CROSSING GUARD",
               salary = missing,
               rate = 19.38)])
=#

Formatting Julia Code

PrettyPrinting can format Julia code represented as an Expr object. It supports a fair subset of Julia syntax including top-level declarations, statements, and expressions.

ex = quote
    fib(n::Number) = n > 1 ? fib(n-1) + fib(n-2) : n
    @show fib(10)
end

pprint(ex)
#=>
quote
    fib(n::Number) = n > 1 ? fib(n - 1) + fib(n - 2) : n
    @show fib(10)
end
=#

Extending PrettyPrinting

It is customary to display a Julia object as a valid Julia expression that constructs the object. The ability of pprint() to format Julia code makes it easy to implement this functionality for user-defined types.

For example, consider the following hierarchical data type.

struct Node
    name::Symbol
    arms::Vector{Node}
end

Node(name) = Node(name, [])

Let us create a nested tree of this type.

tree =
    Node(:a, [Node(:an, [Node(:anchor, [Node(:anchorage),
                                        Node(:anchorite)]),
                         Node(:anchovy),
                         Node(:antic, [Node(:anticipation)])]),
              Node(:arc, [Node(:arch, [Node(:archduke),
                                       Node(:archer)])]),
              Node(:awl)])
#-> Node(:a, …)

To make pprint() format this tree, we need to implement the function quoteof(::Node), which should return an Expr object.

import PrettyPrinting: quoteof

quoteof(n::Node) =
    if isempty(n.arms)
        :(Node($(quoteof(n.name))))
    else
        :(Node($(quoteof(n.name)), $(quoteof(n.arms))))
    end

That's it! Now pprint() displays a nicely formatted Julia expression that represents the tree.

pprint(tree)
#=>
Node(:a,
     [Node(:an,
           [Node(:anchor, [Node(:anchorage), Node(:anchorite)]),
            Node(:anchovy),
            Node(:antic, [Node(:anticipation)])]),
      Node(:arc, [Node(:arch, [Node(:archduke), Node(:archer)])]),
      Node(:awl)])
=#

We can even override show() to make it display this representation.

Base.show(io::IO, ::MIME"text/plain", n::Node) =
    pprint(io, n)

display(tree)
#=>
Node(:a,
     [Node(:an,
           [Node(:anchor, [Node(:anchorage), Node(:anchorite)]),
            Node(:anchovy),
            Node(:antic, [Node(:anticipation)])]),
      Node(:arc, [Node(:arch, [Node(:archduke), Node(:archer)])]),
      Node(:awl)])
=#

Layout Expressions

Internally, PrettyPrinting represents all potential layouts of a data structure in the form of a layout expression.

We will use the following definitions.

using PrettyPrinting: best_fit, indent, list_layout, literal, pair_layout

A fixed single-line layout is created with literal().

ll = literal("salary")
#-> literal("salary")

Layouts could be combined using horizontal (*) and vertical (/) composition operators.

lhz = literal("salary") * literal(" = ") * literal("101442")
#-> literal("salary") * literal(" = ") * literal("101442")

lvt = literal("salary") * literal(" =") /
      indent(4) * literal("101442")
#-> literal("salary") * literal(" =") / indent(4) * literal("101442")

Here, indent(4) is equivalent to literal(" "^4).

Function pprint() serializes the layout.

pprint(ll)
#-> salary

pprint(lhz)
#-> salary = 101442

pprint(lvt)
#=>
salary =
    101442
=#

To indicate that we can choose between several different layouts, we use the choice (|) operator.

l = lhz | lvt
#=>
literal("salary") * literal(" = ") * literal("101442") |
literal("salary") * literal(" =") / indent(4) * literal("101442")
=#

The pretty-printing engine can search through all potential layouts to find the best fit, which is expressed as a layout expression without the choice operator.

best_fit(l)
#-> literal("salary") * (literal(" = ") * literal("101442"))

In addition to the primitive operations, PrettyPrinting can generate some common layouts. A delimiter-separated pair can be generated with pair_layout().

pair_layout(literal("salary"), literal("101442"), sep=" = ")
#=>
(literal("salary") * literal(" = ") |
 literal("salary") * literal(" =") / indent(4)) *
literal("101442")
=#

A delimiter-separated list of items can be generated with list_layout().

list_layout([literal("salary = 101442"), literal("rate = missing")])
#=>
(literal("(") | literal("(") / indent(4)) *
(literal("salary = 101442") * literal(",") / literal("rate = missing")) *
literal(")") |
literal("(") *
(literal("salary = 101442") * literal(", ") * literal("rate = missing")) *
literal(")")
=#

Custom Layouts

We can customize how pprint() formats objects of a user-defined type by implementing function tile(), which should map an object to the corresponding layout expression.

Continuing with the type Node defined in section Extending PrettyPrinting, let us give it a custom layout generated with list_layout().

import PrettyPrinting: tile

tile(n::Node) =
    if isempty(n.arms)
        literal(n.name)
    else
        literal(n.name) *
        literal(" -> ") *
        list_layout(tile.(n.arms))
    end

Now pprint() will render a new representation of the tree.

pprint(stdout, tree)
#=>
a -> (an -> (anchor -> (anchorage, anchorite),
             anchovy,
             antic -> (anticipation)),
      arc -> (arch -> (archduke, archer)),
      awl)
=#

In summary, there are two ways to customize pprint() for a user-defined type T.

  1. Define PrettyPrinting.quoteof(::T), which should return an Expr object.
  2. Define PrettyPrinting.tile(::T), which should return a layout expression.