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Algebraic Datatypes and Pattern Matching for Python

This project was inspired by Racket's define-type / type-case mechanism and David Beazley's Metaprogramming PyCon talk.

This README can be run as a doctest: python -m doctest

Python version 3.3 is required.

A List of Integers

A very simple example.

>>> from adt import ADT, Require
>>> class List(ADT):
...    pass

>>> class Nil(List):
...    pass

>>> class Cons(List):
...    car = Require(int)
...    cdr = Require(List)

>>> Cons(1, Nil())
Cons(car=1, cdr=Nil())

>>> Cons('foo', Nil())
Traceback (most recent call last):
TypeError: expected type <class 'int'>, got <class 'str'>

Constructors that take no arguments return singletons:

>>> Nil() is Nil()

>>> Cons(1, Nil()) is Cons(1, Nil())

>>> Cons(1, Nil()) == Cons(1, Nil())

A Heterogeneous List

>>> from adt import Anything
>>> class List(ADT):
...     pass

>>> class Nil(List):
...     pass

>>> class Cons(List):
...     car = Anything()
...     cdr = Require(List)

>>> Cons('foo', Cons('bar', Nil()))
Cons(car='foo', cdr=Cons(car='bar', cdr=Nil()))

>>> Cons(1, Cons('a', Nil()))
Cons(car=1, cdr=Cons(car='a', cdr=Nil()))

>>> Cons(1, 2)
Traceback (most recent call last):
TypeError: expected type <class '__main__.List'>, got <class 'int'>

Pattern Matching with Algebraic Types


Bindings are used to create patterns with named slots that are bound during matching.

>>> from adt import Binding
>>> Binding('a')

>>> Binding('3')
Traceback (most recent call last):
TypeError: not a valid Python identifier: '3'

A simple example of capturing values during matching:

>>> b = Binding # A short name, for convenience.
>>> pattern = Cons(b('a'), Cons(b('b'), b('c')))
>>> pattern
Cons(car=Binding('a'), cdr=Cons(car=Binding('b'), cdr=Binding('c')))

>>> lst = Cons(1, Cons(2, Nil()))

>>> from adt import match, MatchFailed
>>> match(pattern, lst)
CapturedValues(a=1, b=2, c=Nil())

>>> lst = Nil()
>>> match(Cons(b('car'), Nil()), lst)
Traceback (most recent call last):
adt.MatchFailed: expected Cons(car=Binding('car'), cdr=Nil()), got Nil()

ADT constructors can stand in for pattern elments and automatically bind their arguments.

>>> match(Cons, Cons(1, Nil()))
CapturedValues(car=1, cdr=Nil())

Pattern Matching with Regular Expressions

Patterns can include regular expressions. Named groups will bind the matched values to the respective names.

>>> import re
>>> tele_re = re.compile(r"(?P<area_code>\d{3})-"
...                      r"(?P<exchange>\d{3})-"
...                      r"(?P<subscriber>\d{4})")

>>> match(tele_re, "123-456-7890")
CapturedValues(area_code='123', exchange='456', subscriber='7890')

>>> pattern = Cons(tele_re, Cons(b('name'), Nil()))
>>> value = Cons('555-867-5309', Cons('Jenny', Nil()))
>>> match(pattern, value)
CapturedValues(area_code='555', exchange='867', subscriber='5309', name='Jenny')

Pattern Matching with Mapping Types

Patterns can include dictionaries or other mapping types to be matched against mapping types in the value.

If the pattern is an ordered dictionary, values will be captured in the order given. Otherwise the order will be determined by sorting the keys.

>>> from collections import OrderedDict
>>> pattern = OrderedDict([('a', 1), ('list', Cons), ('foo', b('foo_value'))])
>>> match(pattern, {'a': 1, 'list': Cons(1, Nil()), 'foo': 'bar', 'b': 2})
CapturedValues(car=1, cdr=Nil(), foo_value='bar')

>>> pattern = {'a': 1, 'list': Cons, 'foo': b('foo_value')}
>>> match(pattern, {'a': 1, 'list': Cons(1, Nil()), 'foo': 'bar', 'b': 2})
CapturedValues(foo_value='bar', car=1, cdr=Nil())

>>> match(pattern, {'a': 1, 'foo': 2, 'b': 3})
Traceback (most recent call last):
adt.MatchFailed: pattern has key 'list' which is not in value

Pattern Matching with Sequence Types

>>> pattern = [1, b('second'), 3, b('fourth')]
>>> match(pattern, (1, 2, 3, 4))
CapturedValues(second=2, fourth=4)

>>> match(pattern, (1,2,3))
Traceback (most recent call last):
adt.MatchFailed: pattern and value had different lengths

>>> match(('a', 'b', 'c', 'd'), "abcd")

Rest Bindings

When matching a sequence, rest bindings can be used to capture all remaining elements of a sequence as an iterable.

>>> from adt import BindingRest
>>> pattern = [0,1,2,BindingRest('rest')]
>>> result = match(pattern, range(6))
>>> result # doctest: +ELLIPSIS
CapturedValues(rest=<generator object rest at ...>)

>>> list(
[3, 4, 5]

>>> from itertools import islice
>>> result = match(pattern, range(10**100))
>>> list(islice(, 10))
[3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

Ignored Bindings

Constructing bindings from an empty string (the only non-Python-identifier accepted) results in a slot that does not capture any value. This is useful for "don't care" values.

>>> match(Binding('foo'), 'bar')

>>> match(Binding(''), 'baz')

Making an ignored rest binding allows remaining elements of a sequence to be ignored.

>>> pattern = (0, Binding('a'), BindingRest(''))
>>> match(pattern, range(10))

>>> def loop():
...    while True:
...       yield 0
>>> match(pattern, loop())

Match Cases

Using metaclass magic and function annotations admits a construct similar in flavor to Racket's type-case. Subclasses of MatchCases analyze their contained functions to collect a series of patterns to match. The patterns are given by the annotation on the first argument of each function. The patterns are tried in the order the functions are defined in the class. When a pattern is matched the corresponding function will be invoked with the value that matched as the first argument and the resulting CapturedValues namedtuple as the second. After a match is found, no further patterns are examined. If no pattern matches, a CasesExhausted exception is raised.

>>> from adt import MatchCases
>>> class ListIter(MatchCases):
...    def nil(match: Nil, bindings):
...        yield from ()
...    def cons(match: Cons, bindings):
...        yield
...        yield from ListIter(bindings.cdr)

>>> lst = Cons(1, Cons(2, Cons(3, Nil())))
>>> tuple(ListIter(lst))
(1, 2, 3)

>>> class MissingCases(MatchCases):
...    def only_nil(match: Nil(), bindings):
...        pass

>>> MissingCases(lst) # doctest: +ELLIPSIS
Traceback (most recent call last):
adt.CasesExhausted: no case for Cons(...) in <class '__main__.MissingCases'>

Functions in a MatchCases class can exclude the second, CapturedValues, argument. This triggers special behavior whereby the function is replaced by a dynamically generated version with arguments inserted for each captured value. This eliminates the need to repeat the names of the bindings from the pattern.

>>> class ListSum(MatchCases):
...    def nil(match: Nil()):
...        return 0
...    def cons(match: Cons(b('head'), b('tail'))):
...        return head + ListSum(tail)

>>> ListSum(lst)

Here's another example, with a binary tree ADT this time:

>>> class Tree(ADT): pass

>>> class Leaf(Tree):
...   value = Require(int)

>>> class Node(Tree):
...    left = Require(Tree)
...    right = Require(Tree)

>>> tree = Node(Leaf(1), Node(Node(Leaf(2), Leaf(3)), Leaf(4)))

>>> class TreeSum(MatchCases):
...    def leaf(match: Leaf):
...        return value
...    def node(match: Node):
...        return TreeSum(left) + TreeSum(right)

>>> TreeSum(tree)