Write a Haskell interpreter in Haskell

I love your goal, but it's a big job. A couple of hints:

I've worked on GHC, and you don't want any part of the sources. Hugs is a much simpler, cleaner implementation but unfortunately it's in C.
It's a small piece of the puzzle, but Mark Jones wrote a beautiful paper called Typing Haskell in Haskell which would be a great starting point for your front end.

Good luck! Identifying language levels for Haskell, with supporting evidence from the classroom, would be of great benefit to the community and definitely a publishable result!

There is a complete Haskell parser: http://hackage.haskell.org/package/haskell-src-exts

Once you've parsed it, stripping out or disallowing certain things is easy. I did this for tryhaskell.org to disallow import statements, to support top-level definitions, etc.

Just parse the module:

parseModule :: String -> ParseResult Module

Then you have an AST for a module:

Module SrcLoc ModuleName [ModulePragma] (Maybe WarningText) (Maybe [ExportSpec]) [ImportDecl] [Decl]

The Decl type is extensive: http://hackage.haskell.org/packages/archive/haskell-src-exts/1.9.0/doc/html/Language-Haskell-Exts-Syntax.html#t%3ADecl

All you need to do is define a white-list -- of what declarations, imports, symbols, syntax is available, then walk the AST and throw a "parse error" on anything you don't want them to be aware of yet. You can use the SrcLoc value attached to every node in the AST:

data SrcLoc = SrcLoc
     { srcFilename :: String
     , srcLine :: Int
     , srcColumn :: Int
     }

There's no need to re-implement Haskell. If you want to provide more friendly compile errors, just parse the code, filter it, send it to the compiler, and parse the compiler output. If it's a "couldn't match expected type a against inferred a -> b" then you know it's probably too few arguments to a function.

Unless you really really want to spend time implementing Haskell from scratch or messing with the internals of Hugs, or some dumb implementation, I think you should just filter what gets passed to GHC. That way, if your students want to take their code-base and take it to the next step and write some real fully fledged Haskell code, the transition is transparent.

Write a Haskell interpreter in Haskell

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