## Algebraic ABCs - A DSL for Types in Python

From: andrew cooke <andrew@...>

Date: Fri, 29 Apr 2011 22:28:21 -0300

This is a draft, and the pytyp library it describes is not the one available
for download (although it is in hg on Google code), but I wanted to put it out
there in case anyone had any comments (which would be MUCH appreciated)

I think it's pretty cool.  It's a DSL for Python that integrates with ABCs to
extend Python's types.  Some examples:

>>> isinstance(5, Or(int, str))
True
>>> isinstance("five", Or(int, str))
True

>>> @checked
... def int_list_len(s:[int]) -> int:
...     return len(s)
>>> int_list_len([1,2,3])
3
>>> int_list_len('abc')
Traceback (most recent call last):
...
TypeError: Type Seq(int) inconsistent with 'abc'.

>>> class Foo:
...     def __init__(self, x):
... 	self.x = x
>>> ifoo = Foo(1)
>>> sfoo = Foo('one')
>>> assert isinstance(ifoo, Foo)
>>> assert isinstance(sfoo, Cls(Foo))
>>> assert isinstance(ifoo, And(Foo, Atr(x=int)))
>>> assert not isinstance(sfoo, And(Foo, Atr(x=int)))

Anyway, the paper is at http://www.acooke.org/pytyp.pdf

Andrew

PS If you are reading this post in the future (woot!  flying cars!) then I may
have replaced the link with a more up-to-date version, and updated the
library, of course.

### Updated Python Types Draft

From: andrew cooke <andrew@...>

Date: Fri, 6 May 2011 21:49:18 -0300

I've just posted an updated draft of the Python Types paper to the same link
http://www.acooke.org/pytyp.pdf

This has a bunch of (I hope) improvements:

- It tries to avoid confusion by avoiding terms that sounds like
previously-existing terms for ideas that are only vaguely related.

- It is clearer about what it doesn't do (but still has some way to go).

- I'm starting to add multimethods.

I'm hoping that the sections before (and excluding) Conclusions are now pretty
much final (except that I need to implement and document more control over
exceptions for back-tracking).

Andrew

### Maybe not Multimethods

From: andrew cooke <andrew@...>

Date: Fri, 6 May 2011 21:52:59 -0300

Ironically, as I read what I just posted, I notice that what I am calling
mutlimethods are not really *methods* of the types used in dispatch.  So I
will need to fix that too...

Sorry,
Andrew