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
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