This discussion should move to the types-sig, types-sig@[...].org,
http://www.python.org/sigs/types-sig/.  I am cross posting to the
types sig.  Replies to this message should be posted to
types-sig@[...].org and *not* to comp.lang.python.

Gordon McMillan wrote:
>  
>  I don't object, but I worry. For example, the "sequence protocol" is
>  a very fuzzy thing.

Yes it is.  It should be factored.
 
>  If you start with the abstract sequence protocol (as defined by the C
>  API), only List is an implementation of which B. Meyer would approve,
>  (SetItem is in the abstract interface, for example).

Yup.

>  OK, so lets pretend that's cleaned up. Now we're getting methods on
>  List so it can be treated like a Stack. Stack is certainly not
>  another name for List, nor for Sequence (whatever that means). It's
>  another protocol, right?

Yes.

>  But it's one that is automatically
>  implemented if you implement the List protocol.

Right.  If you bothered to define "List" protocol as
the methods supported by list, you might very well 
make stack a base interface.
 
>  How about FileInput? That's a sequenceish protocol. A read-only,
>  forward-only, sequential-only sequence. How do we (sensibly and
>  comprehensibly) capture the fact that most sequenceish protocols will
>  suffice where a FileInput protocol is required, with the exception of
>  those stack-ish style sequences?

I don't know what FileInput is.
 
>  In fact (as Barry was supposed to point out at the conference for me,
>  harrumph, harrumph), the set of all interfaces / protocols is the set
>  of all subsets of the set of all signatures - without taking behavior
>  into account. That is, the set of all interfaces has a cardinality
>  that I don't particularly want to contemplate.

But one would not bother to enumerate all possible interfaces.

>  At the moment "sequence protocol" means 2 different things. It means
>  Py_SequenceCheck(o) == TRUE (which I hope the type / class
>  unification fixes),

No.  This is a purely implementation check.  It doesn't, for example, 
capture instances that implement the sequence protocol.

>  and it means that if you squint a little and wave
>  your hands alot, this thing could be considered a sequence. My worry
>  is that if you attempt to make the latter definition more precise,
>  you in fact increase complexity.

Interfaces give you the ability to express more.  In a sense this
adds complexity, but it also lends order to existing complexity
(e.g., as you point out, Tuples are sequences, sort of)
 
Jim

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