Bulat Ziganshin wrote:

Hello Michal,

Monday, March 24, 2008, 11:38:07 PM, you wrote:

...

Python-Haskell bridge

seems interesting

This is indeed interesting for those (like me) wanting to introduce Haskell stealthily into a Python-based facility: essentially, leave the IO monad in Python but invoke non-IO Haskell functions.

...

Benefits for Haskell

you forget about ability to use Python as scripting language inside Haskell programs. look at HsLua library as example of this

This is much less interesting for those (like me) who, once in Haskell, don't feel the least inclined to go back to Python. Missing libraries in Haskell (for my applications) are usually also missing in Python and need FFI to some (usually numeric) library written in C/C++ anyway. Why do data marshalling twice? Dan

westondan:

Bulat Ziganshin wrote:

...

Hello Michal,

Monday, March 24, 2008, 11:38:07 PM, you wrote:

...

Python-Haskell bridge

seems interesting

This is indeed interesting for those (like me) wanting to introduce Haskell stealthily into a Python-based facility: essentially, leave the IO monad in Python but invoke non-IO Haskell functions.

Yes, exactly. The Haskell risk is mitigated, and you can use it exactly as you wish, for strongly typed, purely functionaly, robust components held together with Python glue. And then its easier to sneak Haskell into an existing python toolchain.

...

...

Benefits for Haskell

you forget about ability to use Python as scripting language inside Haskell programs. look at HsLua library as example of this

This is much less interesting for those (like me) who, once in Haskell, don't feel the least inclined to go back to Python. Missing libraries in Haskell (for my applications) are usually also missing in Python and need FFI to some (usually numeric) library written in C/C++ anyway. Why do data marshalling twice?

Agreed. There are some libraries we don't have (pygments is one). -- Don

I notice that you omit from the advantages of calling Haskell from Python what I consider the most important reason of all (at least from the Haskell community side). The choice of programming language (at least at the top level) is primarily a political or managerial choice, not a technical one. Python is not merely a "terrific glue language", it is often the "blessed" language, and currently the language of choice in some industries, including my own. This is not just "glue code" either: one in-house application has about 100,000 lines of Python code (calling a comparable amount of C++ code)! Any hope of introducing Haskell into this realm requires a way to work Haskell in slowly and from the bottom. As such I view your proposed project as quite valuable, a robust and easy-to-use Python-Haskell bridge (Python on top) being essential to growing the presence of Haskell in non-academic circles. Of course, it may be for this very political reason that you have omitted the above from your list, in which case I completely understand. :) Dan Michał Janeczek wrote:

Hi,

This is my second take on the project proposal. I have expanded on a few points, and hopefully also clarified a little bit.

Please comment :)

Regards, Michal

Python-Haskell bridge =====================

Description -----------

This project will seek to provide a comprehensive, high level (and thus easy to use) binding between Haskell and Python programming languages. This will allow using libraries of either side from each language.

If we decide to support function callbacks, these two functionalities (embedding Haskell in Python, and vice versa) become interrelated. In this light, it makes sense to implement them in the scope of the same project.

Advantages of calling Haskell from Python -----------------------------------------

* Robust components

It might be beneficial to implement safety-critical componenets in a strongly, statically typed language. Since Python itself is a terrific "glue language", such components would usually be purely functional, leaving IO to the Python side. Embedding Haskell code in this way is of particular interest when there already is a large existing Python infrastructure. One can implement new functionality in a form of Haskell plugin/component, even if complete rewrite is not feasible.

* Performance improvements for speed-critical code

Haskell compiled to native code is typically an order of magnitude faster than Python. Aside from that, advanced language features (such as multicore parallel runtime, very lightweight threads and software transactional memory) further serve to improve the performance. Haskell could become a safe, high level alternative to commonly used C extensions.

* Access to sophisticated libraries

While its set of libraries is not as comprehensive as that of Python, Haskell can still offer some well tested, efficient libraries. Some of the examples might be:

* rich parser combinator libraries (like Parsec) * persistent, functional data structures (i.e. Data.Map, Data.IntMap, Data.Sequence, Data.ByteString) * QuickCheck testing library to drive analysis of Python code

Advantages of calling Python from Haskell -----------------------------------------

* Python as a scripting language for Haskell applications

Python is widely considered to be more approachable for regular users. As such, it could be used as a configuration/extension language for applications that benefit from extra flexibility. One example of such application is xmonad window manager.

* Access to a large number of libraries

As a much more popular language, Python has built up an impressive suite of libraries. There already are Haskell projects which rely on Python code to implement missing functionality, for example a paste bin application hpaste, which uses Pygments syntax coloring library.

Deliverables ------------

* A low level library to access and manage Python objects from Haskell

* Library support for wrapping Haskell values in Python objects. This is necessary to allow function callbacks. In addition, thanks to that feature large and/or lazy data structures can be efficiently passed from Haskell to Python

* A set of low level functions to convert built-in data types between Haskell and Python (strings, numbers, lists, dictionaries, generators etc.)

* A high level wrapper library for Haskell that simplifies embedding and calling Python code

* A set of functions, and possibly a tool, to wrap up monomorphic functions and expose them to Python through foreign export facility

* A way (an external tool, or a Template Haskell macro) to easily derive conversion functions for user-defined data types/objects

* Documentation and a set of examples for all of above

Optional goals --------------

In order of increasing amount of work and decreasing priority:

* Exporting a wider class of functions to Python

* A Python module that inspects a compiled Haskell module and transparently exposes functions within. This might be possible thanks to GHC API _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

FWIW, my MissingPy project accomplishes part of this (calling Python from Haskell) already. -- John On 2008-03-27, Dan Weston wrote:

I notice that you omit from the advantages of calling Haskell from Python what I consider the most important reason of all (at least from the Haskell community side).

On Thursday 27 March 2008 04:07:23 pm Dan Weston wrote:

I did not see MissingPy on Hackage (presumably it would be next to MissingH?)

I found it (listed on http://www.complete.org/jgoerzen/softindex.html) at http://darcs.complete.org/missingpy

Is this the right place to get it?

Yes, it is. I have not had the need for it lately, and thus haven't made recent releases to upload to Hackage. It did work solidly when I used it, so hopefully it can be at least some value to someone. -- John

Oh, I thought I added that as "related work" to the ticket page. If its not, couldyou add it? jgoerzen:

FWIW, my MissingPy project accomplishes part of this (calling Python from Haskell) already.

-- John

On 2008-03-27, Dan Weston wrote:

...

I notice that you omit from the advantages of calling Haskell from Python what I consider the most important reason of all (at least from the Haskell community side).

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