[Types-sig] Type annotations

[Paul Prescod]

Okay, I see four different approaches to the syntax short-term type
annotation. Personally, I do not think that it is too early to talk
about syntax because we need to communicate these ideas. Here are my
metrics:

Python 1.5 compatibility: Sill the Python 1.5 compiler accept it? Will
it have reasonable semantics?
Logical separation: Will users be able to understand that runtime
objects are not available?
Convenience: How easy is it to edit?
Syntactic Cleanliness: How "obvious" is it what the declaration means?
 
1. separate file:
  Python 1.5 compatibility: high
  Logical separation: high
  Convenience: low
  Syntactic Cleanliness: high

2. labelled string expressions: (like 3, but in strings)
  Python 1.5 compatibility: high
  Logical separation: high
  Convenience: medium
  Syntactic Cleanliness: medium

3. in separate decl statements: (Incompatible with Python 1.5, but
easily converted)

  Python 1.5 compatibility: low
  Logical separation: high
  Convenience: medium
  Syntactic Cleanliness: high

4. in-line in "other" declarations

  Python 1.5 compatibility: low
  Logical separation: low
  Convenience: high
  Syntactic Cleanliness: high

5. in dictionaries, lists, and other basic Python objects, "overloaded"
with special meaning

  Python 1.5 compatibility: high
  Logical separation: medium
  Convenience: high
  Syntactic Cleanliness: low

Of course if we use a backwards-incompatible expression syntax then
backwards-incompatibility is not an issue. That is one reason I propose
check_type( expr, type ) which can be interpreted in old Python as a
function call.

My preference is to allow three different syntaxes according to a
schedule:

	January: separate files (we need this anyhow to define types for the
builtin modules)
	February: in-module string declarations and build separate files
	Python 2.0: either 3 or 4

I admit that I don't find the compatibility benefits of 5 to be worth
the obfuscation. Parsing is not THAT hard.

-- 
 Paul Prescod  - ISOGEN Consulting Engineer speaking for himself
Three things never trust in: That's the vendor's final bill
The promises your boss makes, and the customer's good will 
http://www.geezjan.org/humor/computers/threes.html