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Inquisitive types and multiple dispatch, now for Python!

Project description implements multimethods (also known as multiple dispatch) for Python. However, instead of basing the types used for dispatch on the standard Python class system, uses a method called inquisitive types.

Diving In

Start off by importing the typed module; you don’t need anything besides that.

from typed import *

Some of the names in the typed module may conflict with names you yourself use: typed, typedef, restrict, fn, from_pytype, and eq. If so, take special care to eliminate shadowing.

Now then, let’s write a function:

@typed(String, String)
def combine(a, b):
    return a + b

This function will require both arguments to be strings:

>>> combine("adsf", "adsf")
>>> combine(1, 2)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "typed/", line 108, in __call__
    return self.match(args)[1](*args)
File "typed/", line 103, in match
    raise TypedError("No matches", self, [x[0] for x in self.sigs])
typed.specialize.TypedError: No matches for function <typed function combine at 0xb733a4ec>. Alternatives:
        (<Type String>, <Type String>)

Scary looking error, huh? Note that it tells you the alternative signatures.

Anyway, let’s define another variant to this function:

@typed(String, Integer)
def combine(a, n):
    return a + str(n)

And, we can use it normally:

>>> combine("asdf", 12)
>>> combine("asdf", "asdf")

Yay, both signatures work!

The builtin types include:

  • Object (anything at all, not just things that inherit from object)
  • Complex (complex, float, or and integral type)
  • Real (float or an integral type)
  • Integer (int or long; an integral type)
  • Boolean (bool)
  • String (guess!)
  • Type (An inquisitive type)
  • PyType (A python class or type)
  • Tuple (A tuple of any length or type)
  • List (A list of any type) and List(T) (A list of type T elements)
  • Dict (A dict of any type to any type), Dict(T) (A list where all keys have type T), and Dict(K, V) (A dict with keys of type K and values of type V)
  • OneOf(*x) (Type of things equal to one of the elements of x)

What are Inquisitive Types?

An inquisitive type is defined by two properties: is and sub. I’ll skip the second for now, but is (which in Python is the method is_ of a type) just takes an object and tells you if that object is an instance of the given type. For example, the is method for Object is simply:

def is(self, x):
    return True

Because, after all, everything is an object. Not that a priori, objects have no type; one can only ask a type if an object is an instance. This is why they are called inquisitive: you always have to ask if you want to know if an object is an instance of something.

Now, what about that sub method I mentioned? This is the analog of issubclass for inquisitive types: it takes another type, and tells you if the given type is a subclass of it. For example, the one for Object is:

def sub(self, X):
    if X == self:
        return True
        return False

These subclass relations are used to determine which method to call if a function has, for example, two implementations with signatures Integer, Integer and Real, Real: clearly you want the one with Integers to be called if possible; and it is, because Integer.sub(Real) returns True.

Now, the nice thing is that this allows you to construct more complex types easily, using type functions. Don’t worry, the concept is easy: it’s a function that takes a type and returns a type. For example, consider the function restrict. restrict(T, f, name) takes a type T and returns a type that only says things are instances if they are instances of that type, and satisfy the function f. The name just names the resulting type. So, for example:

>>> EvenInt = restrict(Integer, lambda x: x % 2 == 0, "EvenInt")
<type EvenInt>
>>> isinstance(4, EvenInt)
>>> isinstance(3, EvenInt)

Some other type functions are built in to fn(f) is another way of saying restrict(Object, f). typedef(T, name) copies the type T and renames it (sometimes useful if you use type functions to construct types). eq(x) creates a new type whose only instance is x; it’s equivalent to OneOf(x). Finally, from_pytype is a compatibility feature, taking any Python type and returning a new inquisitive type.

Furthermore, a few useful type functions have nice operator syntax. For example:

>>> isinstance((3, 4), Integer * Integer)
>>> isinstance(("foo", "bar", "baz"), String ** 3)
>>> isinstance(5, Integer < 3)
>>> isinstance(7, Integer > -2)
>>> isinstance("a", eq("a") | eq("b"))
>>> map(lambda x: isinstance(x, Integer | String), [1, "a", 17.3, "17.3", True])
[True, True, False, True, False]
>>> isinstance(12, (3 < Integer) < 17)

Note that in the last example, you must place parentheses around one of the two comparisons. It doesn’t matter which one; this is simply a fault of Python’s syntax.

Use in APIs

One can use for simple type checking:

def sqrt(x):

Or, you can use it to dispatch based on type:

def sqrt(x):
    import math
    return math.sqrt(x)

def sqrt(x):
    import cmath
    return cmath.sqrt(x)

You can eliminate boring conditional checks:

@typed(Integer < 10000)
def is_prime(x):
    return do_bruteforce(x)

def is_prime(x):
    return elliptic_curve_primality_test(x)

You can also dispatch, because these are inquisitive types, on specifics of the arguments:

@typed(restrict(String, lambda x: x.startswith("http")))
def download(x):
    import urllib
    return urllib.urlopen(x).read()

@typed(restrict(String, lambda x: x.startswith("ftp")))
def download(x):
    import ftplib
    f = ftplib.FTP(dir)
    return f.getwelcome()

Finally, you can combine these to make those strings you ask people to pass to identify things to do type-safe:

Color = typedef(OneOf("green", "yellow", "red"), "Color")

@typed(Banana, Color)
def eat(b, color):
    if color == "green":
        print "Hold on"
    elif color == "yellow":
        print "Go ahead"
    elif color == "red":
        print "Where the **** did you get that banana?" # -- Mitch Hedburg

Admittedly, in that last example, you’d probably write three methods and use eq to create types corresponding to each color.

Lastly, you can now create singletons that are easy to use: just use eq constructors on a string.

Type Types and so on

>>> isinstance(Integer, Type)

See, types can have types (it’s like metatypes, only it makes sense!). In fact, it actually gets a bit spookier, because:

>>> isinstance(Type, Type)

It’s Types all the way down!

>>> isinstance(Object, Object)

Until you hit the Objects, that is.

A Short Note on Covariance

>>> a = [1, 2, 3]
>>> isinstance(a, List(Integer))
>>> isinstance(a, List(Complex))
>>> a[1] = "asdf"
>>> isinstance(a, List(Integer))

Lists (and Dictionaries) covary with their contained types. This raises the usual endless troubles. Cry me a river. Or, better yet, deal with it.

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