Docs

These are all you need to import.

from pattern_matching import var, _, T, t, when, Match, overwrite

Type Matching

@when(var[T == int])
# T means the type would be capture.
def f(v, type_of_v):
    print(v, type_of_v)

f(1)
# => (1, int)

Remark: Using Match is similar to when/overwrite:

m = Match(1)
res = m.case(var[T == int])
if res:
    [a, b] = res.get
    assert [a, b] == [1, int]

If the pattern matched, Match.case returns a Result object.

class Result:
    __slots__ = 'get'

    def __init__(self, _):
        self.get = _

Otherwise the return is None.

Value Matching

@when(_ == 1)
def f():
    return 12

@when(_ == 2)
def f():
    return 0

@when(var)
def f(arg):
    return arg ** 3

f(1), f(2), f(3) # => 12, 0, 27

Wildcard for types

@when(var[t == float])
# the lowercase, "t", which indicates that the type just be matched without capture.
def f(v):
    print(v)
f(1.0)
# => 1.0

Wildcard for values

@when(_)
def f():
    return 1
f(1) == f("...") == f(1e-3)
# => True

Type Boundary

class MyList(list):
    pass
from collections import Iterable

@when(var[Iterable <= T <= MyList]
    .when(lambda x: 1 in x)
)
def f(x, T):
    return (x, T)

f([1, 2, 3])
# => ([1, 2, 3], list)

f({1, 2, 3})
# => UnsolvedCase: No entry for args<({1, 2, 3},)>, kwargs:<{}>

Overloading functions

Overloading functions are introduced through the following simple cases:

@overwrite(_ == [])
def summary():
    return 0

@when([var[int], *(_== [])])
def summary(head):
    return head

@when([var[int], *var[list]])
def summary(head, tail):
    return head + summary(tail)

summary([1, 2, 3])
# => 6

Note that above code is definitely useless for it doesn’t use tail call optimization.

Union Type

@when(var[(t == int) | (t == str)])
def disp(x):
    print(x)
disp(1) # => 1
disp('1') # => '1'

Intersection Type

class A:
    pass
class B:
    pass
class C(A, B):
    pass

@when(_[(T == A) | (T == B)])
def disp(ty):
    print(ty)
disp(C()) # => <class __main__.C>

Difference Type

class A:
    pass
class B:
    pass
class C(A, B):
    pass

@when(_[T != A])
def disp(ty):
    print(ty)
disp(C()) # => <class __main__.C>
disp(B()) # => <class __main__.B>

disp(A())
# => UnsolvedCase: No entry for args<(<__main__.A object at ...>,)>, kwargs:<{}>

Type Contracts

You can apply .when(predicate) methods on pattern_matching.T/t .

To avoid subclassing.

class A:
    pass
class B:
    pass
class C(A, B):
    pass

@overwrite(_[T.when(lambda _: not issubclass(_, A))])
def disp(ty):
    print(ty)
disp(C()) # => <class __main__.C>
# => UnsolvedCase: No entry for args<(<__main__.C object at ...>,)>, kwargs:<{}>

Match Argument Numbers

@when(var/2)
def f(g):
    return g(1, 2)

f(lambda a, b: a + b) # => 3
f(lambda a, b, c: a + b)
# => UnsolvedCase: No entry for args<(<function <lambda> at ...>,)>, kwargs:<{}>

class F:
    def apply(self, arg):
        return arg + 1

@when(var/1)
def f2(g):
    return g(1)


f2(lambda a, b: a + b)
# => UnsolvedCase: No entry for args<(<function <lambda> at ...>,)>, kwargs:<{}>
f2(F().apply) # => 2