Connecting the Wolfram Language to Haskell

[Ben Gamari]

Simon Peyton Jones via ghc-devs <ghc-devs at haskell.org> writes:

> Friends
>
> Short summary: can someone familiar with using the GHC API offer
> advice on getting the Wolfram Language connected to Haskell?
>
Hi Todd, et al.,

This sounds like a great project. I have fond memories of Mathematica
from my studies.

...

> We have the following basic line of code for evaluating a string of Haskell code:
>
>     r <- liftIO $ runInterpreter $ do { setImports ["Prelude"]; eval strToEvaluate}
>
> The problem is that this is a one-shot evaluation, and we want a
> long-lived interactive Haskell session, to which a series of inputs
> can be directed. We have been told that we have to use GHCi for that,
> but we don't know how to do it.
>
It appears that you are using the `hint` library [1] for evaluation.
I'll admit that I've not used hint; it looks quite sensible but I do not
know what limitations you might encounter. It looks like its approach to
error handling may leave something to be desired. Nevertheless, we can
work with it for now; if we run into its limitations then the
alternative is to use the GHC API directly, as suggested by Simon.

> The basic flow of our functionality is as follows:
>
> 1) User calls StartExternalSession["LanguageName"] to launch an
> interpreter for the language. This process remains running and can be
> used for multiple calls.
>
> 2) User calls ExternalEvaluate[session, "some code"] to execute the
> given code in the external language and return a result converted into
> native Wolfram Language types (strings, numbers, lists, associations,
> etc.)
>
Sure.

...

> We have attached a simple file of Haskell code that one of our
> engineers has successfully used to get a basic evaluation of Haskell
> code from the Wolfram Language, but it uses the single-shot evaluation
> code that was given above, and so is not suitable. We would appreciate
> any help that you can give us, or developers or resources you can
> point us at, to assist in integrating Haskell into our
> ExternalEvaluate system.
>
It looks like you will want to push the `runInterpreter` out of the
`forever`. Afterall, you want the interpreter session to persist over
multiple requests. Doing this isn't difficult but does require some
monad transformer shuffling, which may be unfamiliar to someone coming
from another language. I've put up a cleaned up version of your program
here [1]; hopefully this is enough to get you started. Do note that this
requires a patched version of zeromq4-haskell due to a minor bug [2]
which I have fixed [3].

Do note that there is a related effort, iHaskell [4], which provides a
Haskell kernel for Jupyter Notebook. This might be a place to draw
inspiration from.

Let us know how things go and don't hesitate to be in touch if you have
questions regarding the GHC API.

Cheers,

- Ben


[1] https://github.com/bgamari/zeromq-hint
[2] https://gitlab.com/twittner/zeromq-haskell/-/issues/66
[3] https://gitlab.com/twittner/zeromq-haskell/-/merge_requests/6
[4] https://github.com/gibiansky/IHaskell
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