pymatched

pymatched⇒

What is pymatched?

pymatched is a library which provides functional pattern matching.

Installation

pip install pymatched

Syntax

result = match(<'func'>) >> Mapping[Hashable, Any]

Match order

1. exact match
2. oneof match
3. placeholder match (if target is immutable iterable)
4. type match with guard (Contravariant match)
5. type match (Invariant match)
6. type match (Contravariant match)
7. handling default if exists

Usage

Note: How to match mutable value?

as you know, mutable things cannot be key of dict so we can not match easly.

this is the example of list.

Case A: use type guards

x = match([1, 2, 3]) >> {
    list                            : "list",
    oneof([1], [1, 2], [1, 2, 3])   : "[1] | [1, 2] | [1, 2, 3]",
    (list, lambda x: x == [1, 2, 3]): "(list, f(list) -> bool)",
    # [1, 2, 3]: "[1, 2, 3]",  --> list is unhashable so not working
}

Case B: use nested match

x = match([1, 2, 3]) >> {
    list: match(...) >> {
        (list, lambda v: v == [1, _, 3]): "pattern is (1, * ,3)",
        ...                             : "default"
    } 
}

Value match

from pymatched import match

match(1) >> {
    1: "It's 1",
    5: "It's 5",
}

Handling default case

use elipsis ... or typing.Any

if nothing catched but default handler not defined, RuntimeError will be raised.

from typing import Any
from pymatched import match

match(None) >> {
    ...: "default",
    # Any: "also default",
}

Type match

from pymatched import match

match(42) >> {
    int: "int caught",
    ...: lambda v: f"{type(v)} caught"
}

Type match with guard

If tuple's first element is type and second element is lambda, this case will be considered as type match with guard.

from pymatched import match

match(42) >> {
    (int, lambda: v: v == 42): "42 caught",
    int                      : "int except 42",
}

type match with guard can use typing.Any.

from typing import Any
from pymatched import match

match(42) >> {
    (Any, lambda: v: v in (42, "42")): "42 caught",
    int                              : "int except 42",
}

Exception match in type match

pymatched.do wraps executing function. when wrapped function raises error, do catch it and return it as normal return.

from pymatched import match, do

def fx(v):
    raise Exception("Ooops!")

match(do(fx, None)) >> {
    Exception: "exception caught",
    ...      : lambda v: f"{v} caught",
}

Oneof match

from pymatched import oneof, match

fx = lambda x: x

match(fx(5)) >> {
    oneof(1, 2, 3): "one of 1, 2, 3",
    oneof(4, 5, 6): "one of 4, 5, 6",
}

Placeholder match

from pymatched import oneof, match

match((1, 2, 3, 4)) >> {  # change (1, 2, 3, 4) into (100, 2, 3, 4) or (1, 9, 3, 9)
    (1, _, 3, _): "pattern (1, *, 3, *)",
    (_, 2, _, 4): "pattern (*, 2, *, 4)",
}

Nested match

If match with pymatchied._ (PlaceholderTyoe) or ... (Ellipsis), this match will be considered as nested match.

from pymatched import match, _

match(5) >> {
    int: match(_) >> {
        5: "It's 5",
        ...: "default"
    },
}

Mixed match

cases could be mixed, but resolved by designated match order.

from pymatched import oneof, match

v = (1, 2, 3)

x = match(v) >> {
    tuple                         : "Tuple caught",
    (tuple, lambda v: v[-1] == 3) : "last item of tuple is 3",
    (1, _, 3)                     : "pattern is (1, *, 3)".
    oneof((1,), (1, 2), (1, 2, 3)): "one of (1,) | (1, 2) | (1, 2, 3)",
    (1, 2, 3)                     : "(1, 2, 3)"
}