Patty - A pattern matching library

Patty is a library to perform pattern matching in Nim. The patterns have to be variant objects, which in Nim are encoded with a field (usually called kind) which varies in an enum, and a different object layout based on the value of this tag. An example would be

type
  ShapeKind = enum
    Circle, Rectangle
  Shape = object
    case kind: ShapeKind
    of Circle:
      r: float
    of Rectangle:
      w, h: float

If you have such an algebraic data type, you can do the following with Patty:

import patty

proc makeRect(w, h: float): Shape = Shape(kind: Rectangle, w: w, h: h)

match makeRect(3, 4):
  Circle(r: radius):
    echo "it is a circle of radius ", radius
  Rectangle(w: width, h: height):
    echo "it is a rectangle of height ", height

This will be translated by the match macro into the following form

let :tmp = makeRect(3, 4)
case :tmp.kind
of Circle:
  let radius = :tmp.r
  echo "it is a circle of radius ", radius
of Rectangle:
  let
    width = :tmp.w
    height = :tmp.h
  echo "it is a rectangle of height ", height

Matching by position is also valid, like this:

match makeRect(3, 4):
  Circle(radius):
    echo "it is a circle of radius ", radius
  Rectangle(width, height):
    echo "it is a rectangle of height ", height

One can also use _ for a variable, in which case it will not be bound. That is, the following

import patty

proc makeRect(w, h: float): Shape = Shape(kind: Rectangle, w: w, h: h)

match makeRect(3, 4):
  Circle(r: radius):
    echo "it is a circle of radius ", radius
  Rectangle(w: _, h: height):
    echo "it is a rectangle of height ", height

becomes

let :tmp = makeRect(3, 4)
case :tmp.kind
of Circle:
  let radius = :tmp.r
  echo "it is a circle of radius ", radius
of Rectangle:
  let height = :tmp.h
  echo "it is a rectangle of height ", height

Notice that in the examples, the field you dispatch on is called kind, but any other name would do. Also, checks are exhaustive: if you miss a case, the compiler will complain.

One can instead pattern-match on non-variant objects, which essentially amounts to deconstructing fields:

type Person = object
  name: string
  age: int
let p = Person(name: "John Doe", age: 37)
match p:
  Person(name: n, age: a):
    echo n, "is ", a, " years old"

Again, this is the same as

match p:
  Person(n, a):
    echo n, "is ", a, " years old"

Constructing variant objects

Patty also provides another macro to create algebraic data types. It looks like

variant Shape:
  Circle(r: float)
  Rectangle(w: float, h: float)
  UnitCircle

and expands to

type
  ShapeKind {.pure.} = enum
    Circle, Rectangle, UnitCircle
  Shape = object
    case kind: ShapeKind
    of ShapeKind.Circle:
      r: float
    of ShapeKind.Rectangle:
      w: float
      h: float
    of ShapeKind.UnitCircle:
      nil

proc `==`(a: Shape; b: Shape): bool =
  if a.kind == b.kind:
    case a.kind
    of ShapeKind.Circle:
      return a.r == b.r
    of ShapeKind.Rectangle:
      return a.w == b.w and a.h == b.h
    of ShapeKind.UnitCircle:
      return true
  else:
    return false

proc Circle(r: float; x: float; y: float): Shape =
  Shape(kind: ShapeKind.Circle, r: r)

proc Rectangle(w: float; h: float): Shape =
  Shape(kind: ShapeKind.Rectangle, w: w, h: h)

proc UnitCircle(side: int): Shape =
  Shape(kind: ShapeKind.UnitCircle)

Notice that the macro also generates three convenient constructors (Circle ,Rectangle and UnitCircle), and in fact the enum is pure to avoid a name conflict. Also, a proper definition of equality based on the actual contents of the record is generated.

By default the generated ADT is private to the module. If you want to generate a public ADT use the variantp macro, which has the same syntax as variant but makes the types, fields, equality definition and generated constructors public.

A couple of limitations fo the variant macro:

• field names must be unique across branches (that is, different variants cannot have two fields with the same name). This is actually a limitation of Nim.
• the shortcut that groups field names by type does not seem to work, that is, in the above example one could not write Rectangle(w, h: float).

In the future, Patty may also add copy constructors. Also, some work needs to be done to make it easier to use the generated contructors with ref types, in particular for the important case of recursive algebraic data types.

Things that do not work (yet)

One would expect many forms of pattern matching but, at least for now, the support in Patty is very limited. Things that would be nice to support but do not work yet include:

• matching a constant

match c:
  "hello":
    echo "the string was hello"

• matching an existing variable

let x = 5
match c:
  x:
    echo "c == 5"

• nested pattern matching

match c:
  Circle(Point(x: x, y: y), r: r):
    echo "the abscissa of the center is ", x

• matching without binding

match c:
  Circle:
    echo "it is a circle!"

• binding subpatterns

match getMeACircle():
  c@Circle(x, y, r):
    echo "there you have ", c

• pattern matching as an expression

let coord = match c:
  Circle(x: x, y: y, r: r):
    x
  Rectangle(w: w, h: h):
    h

• unification

match r:
  Rectangle(w: x, h: x):
    echo "it is a square"

• guards

match c:
  Circle(x: x, y: y, r: r) if r < 0:
    echo "the circle has negative length"

• variable-length pattern matching, such as with arrays

match c:
  [a, b, c]:
    echo "the length is 3 and the first elements is ", a

• custom pattern matchers, such as in regexes

let Email = r"(\w+)@(\w+).(\w+)"
match c:
  Email(name, domain, tld):
    echo "hello ", name

• combining patterns with or

match c:
  Circle or Rectangle:
    echo "it is a shape"