Mixing Haskell and C++

Question

Hi

If you had the possibility of having an application that would use both Haskell and C++. What layers would you let Haskell-managed and what layers would you let C++-managed ?

Has any one ever done such an association, (surely) ?

(the Haskell site tells it's really easy because Haskell has a mode where it can be compiled in C by gcc)

At first I think I would keep all I/O operations in the C++ layers. As well as GUI management.

It is pretty vague a question, but as I am planning to learn Haskell, I was thinking about delegating some work to Haskell-code (I learn in actually coding), and I want to choose some part where I will see Haskell benefits.

Answer 1

I have never mixed both languages but your approach feels a little upside down to me. Haskell is more apt at high-level operations while C++ can be optimized and can be most beneficial for tight loops and other performance critical code.

One of the largest benefits of Haskell is the encapsulation of IO into monads. As long as this IO isn't time critical I don't see any reason to do it in C++.

For the GUI part you are probably right. There is a plethora of Haskell GUI libraries but C++ has powerful tools such as QtCreator which simplify the tedious tasks a lot.

Answer 2

The benefit of Haskell is the powerful abstractions it allows you to use. You're not thinking in terms of ones and zeros and addresses and registers but computations and type properties and continuations.

The benefit of C++ is how tightly you can optimize it when necessary. You aren't thinking about high-minded monads, arrows, partial application, and composing pure functions: with C++, you can get right down to the bare metal!

There's tension between these two statements. In his paper “Structured Programming with go to statements,” Donald Knuth wrote

I have felt for a long time that a talent for programming consists largely of the ability to switch readily from microscopic to macroscopic views of things, i.e., to change levels of abstraction fluently.

Knowing how to use Haskell and C++ but also how and when to combine them well will knock down all sorts of problems.

The last big project I wrote that used FFI involved using an in-house radar modeling library written in C. Reimplementing it would have been silly, and expressing the high-level logic of the rest of the application would have been a pain. I kept the “brains” of it in Haskell and called the C library when I needed it.

You're wanting to do this as an exercise, so I'd recommend the same approach: write the smarts in Haskell. Shackling Haskell as a slave to C++ will probably end up frustrating you or making you feel as though you wasted your time. Use each language where its strengths lie.

Answer 3

This answer is more a story than a comprehensive answer, but I used a mix of Haskell, Python and C++ for my dissertation in computational linguistics, as well as several C and Java tools that I didn't write. I found it simplest to run everything as a separate process, using Python as glue code to start the Haskell, C++ and Java programs.

The C++ was a fairly simple, tight loop that counted feature occurrences. Basically all it did was math and simple I/O. I actually controlled options by having the Python glue code write out a header full of #defines and recompiling. Kind of hacky, but it worked.

The Haskell was all the intermediate processing: taking the complex output from the various C and Java parsers that I used, filtering extraneous data, and transforming it the simple format the C++ code expected. Then I took the C++ output and transformed it into LaTeX markup (among other formats).

This is an area that you would expect Python to be strong, but I found that Haskell makes manipulation of complex structures easier; Python is probably better for simple line-to-line transformations, but I was slicing and dicing parse trees and I found that I forgot the input and output types when I wrote code in Python.

Since I was using Haskell a lot like a more-structured scripting language, I ended up writing a few file I/O utilities, but beyond that, the built in libraries for tree and list manipulation sufficed.

In summary, if you have a problem like mine, I would suggest C++ for the memory-constrained, speed-critical part, Haskell for the high-level transformations, and Python to run it all.