Re: [Haskell-cafe] Writing an interpreter for a language with mutability

"Write Yourself a Scheme in 48 Hours"[1] is a tutorial about implementing Scheme in Haskell. It takes the approach of modeling mutable variables in Scheme using IORefs. It's a good place to start learning about these ideas—in fact, it's pretty much how I learned Haskell. Looking back at it now I'm not sure I would make the same decisions as the author any more, but it worked and the tutorial does a good job of guiding you through what the code does. After you follow that, you should have a good feeling for how IORefs work in Haskell and how they fit into an interpreter for a mutable language. [1]: https://en.wikibooks.org/wiki/Write_Yourself_a_Scheme_in_48_Hours On Tue, Dec 19, 2017 at 9:37 AM, Jeremy Mikkola wrote:

Hi all, thank you for the replies!

The language this will interpret does not expose pointers (so it acts like the more modern languages).

I have started to try to wrap my head around IORef and that monad (anyone know of a "for dummies" guide to those?). I think that they will likely be exactly what I need.

Thanks again,

- Jeremy Mikkola

On Mon, Dec 18, 2017 at 12:31 PM, Joachim Durchholz wrote:

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Am 17.12.2017 um 22:03 schrieb Jeremy Mikkola:

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How does one go about interpreting a language with mutable objects in Haskell?

The best approach I can think of is to represent the language's memory as a `Data.Map.Map RefID LanguageObject` where RefID is some type (probably Int) used as a reference.

It depends on how data is addressed in the language. If you wanto interpret a C-style language where every address can be cast to an int, then that's the most straightforward (though not necessarily best) approach. If it is just references to objects as in most, erm, "more modern" languages, the Id can be any type. E.g. something based on the language's data model - Int | Real | Record | Array, with type parameters as appropriate. Upside is that you leverage the Haskell GC that way, downside is that you'll need a recursive type (Array is really Array RefId, and my Haskell-fu is hilariously inadequate to properly flesh out that recursion).

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