Here is an informal subjective comparison of Sieve and P, based on a
quick reading of RFC 3028 and RFC 3431.
Note that a few items are related as they look at the same language
property from different angles.
1. P has a much shorter, easier to spell name.
2. Both languages have similar design goals and execution environment
as far as email filtering is concerned.
3. P has a much broader scope, including facilitating email adaptation
(not just filtering) and adaptation of other protocols such as HTTP.
4. Sieve contains commands to redirect or respond to messages. IIRC, P
scope was limited to directing messages to services (that, in turn,
could redirect or respond to messages, among other things). On the
other hand, if we follow the service-is-a-method approach we have
discussed, then there may be no difference from user point of view.
5. Sieve scripts are executed during final delivery, when the message
is moved to the user-accessible mailbox. P can be used during final
delivery or anywhere message adaptation is needed.
6. P Core makes rich extensions easier, but Sieve was not designed
with rich extensions in mind (Sieve scope is narrow).
7. If we "extend" Sieve to apply it to HTTP adaptations, we (a) would
not be able to reuse much and (b) may have to violate a couple of
existing Sieve Core rules. Thus, the argument "why do we need
another filtering language?" is not that strong because it is, in
theory, better to have a single common language for several key
protocols than multiple similar languages, one for each key
protocol. It is sad if P/SMTP is Sieve, but one can argue that it is
not OPES fault that Sieve team did not want to look beyond email
8. Sieve does not allow for code or computation reuse (i.e., functions
and variables) because creators view reuse as a security model
violation (not sure why, but it seems to be a common misconception)
P may allow for some code and computation reuse, hopefully without
significantly increasing vulnerability of compliant interpreters.
9. Unlike P, 06 does not appear to support modern error handling --
if something fails, the whole script fails. This might be OK for
simple/core applications but sounds like a serious limitation if
non-trivial extension actions are to be supported. It would be
difficult to write a script that never fails but tries to access
things deep inside message structure or tries to execute non-trivial
commands. Even trivial commands like redirecting a message to a
folder can fail; I am not sure exactly what would happen with
Sieve-controlled message then; the outcome is likely to be
10. Sieve has two basic data types: integers and strings. Sieve also
has lists of strings. For example, MIME headers and even MIME body
parts are strings. P has many types representing messages and their
components. Thus, P might support more intuitive, protocol- and
context-dependent access to data, making some tests faster or more
11. I am not positive, but it seems that Sieve provides limited access
to message parts, assuming some sort of common tests or expressions.
It seems that some tests may not be possible with Sieve because the
documented tests hide some details. Perhaps extensions can cover for
that, or perhaps nobody needs full access to some message parts.
12. 06 does not use XML, but its expression syntax is not
human-friendly. The assumption is that most scripts will be written
by programs controlled via user-friendly GUIs.
13. 06 supports a simple subset of regular expressions. We have
not decided what regular expressions P should support.
14. 06 Core does not support relational operators other than a
match (except for message size comparisons?), but extensions such as
RFC 3431 can add those. I do not know how portable use of extensions
for operators makes one's code. It is possible that RFC 3431 is
widely supported so its "extension" status is not an issue.
15. Both languages can require support for a given extension from the
interpreter (but Sieve scripts cannot recover if such support is
16. Sieve does not define a common way to define extensions or load
extensions into an interpreter. We have argued that P should at
least provide a way to do the former so that scripts can be
thoroughly checked against public extension interface, and without
knowing extension implementation details.
Please correct any factual errors. I am not a Sieve expert or even
casual user; I just read the two RFCs quoted above! Also, it would be
great if somebody can describe Sieve popularity in real world. Are we
looking at a well-established and widely implemented standard? Like
Finally, I realize that it is easy to portrait partially-defined P in
a better light than already "frozen" Sieve -- it is easy to claim that
P will gain all necessary features it is missing now.
I would prefer to hear from other folks before sharing my half-baked
ideas of where this quick comparison leaves us.
P.S. I agree with Abbie that to compare we need a set of criteria.
The above items roughly outline the set I have in my mind. I
hope the set is visible enough through the details, and I
hesitate spending a few months on a good criteria document
instead. This informal approach may backfire, of course.