Haschoo: the Scheme interpreter to be sneezed at

Haschoo is a little R5RS 1 interpreter written in Haskell for a university course. The name, if it's not obvious, is a portmanteau of "Haskell", "Scheme", and "achoo", the last of which is meant to signify something along the lines of it being a sneeze's worth of code and not a particularly serious endeavour.

For licensing information, see LICENSE.txt.

Usage

Haschoo is quite spartan. It doesn't understand any command line options: if given no parameters, it becomes a REPL which terminates on end-of-file, otherwise it runs the file named by each argument as a standalone Scheme program.

Building

Haschoo is not written in standard Haskell 98, but depends on GHC-only (at the time of writing) compiler extensions, so you'll need GHC to build it. Get it at http://www.haskell.org/ghc/; version 6.10.2 was used for testing, but later 6.10 versions should work as well.

Nowadays you can also try the Haskell Platform, intended as a simple installer to get you started quickly: http://hackage.haskell.org/platform/

If you have the cabal-install tool installed (2 or the Haskell Platform), installing Haschoo without downloading it manually should be as simple as:

cabal install Haschoo

If you've already got Haschoo downloaded, and you have cabal-install, try:

cabal configure cabal build cabal install

If you don't have cabal-install, the following commands, run from Haschoo's root directory, should also work:

runhaskell Setup.hs configure runhaskell Setup.hs build runhaskell Setup.hs install

Goal

If Haschoo can be said to have had a goal, it is minimalism: it tries to be a bit of a DeathStation 9000 (see e.g. 3) for R5RS. Currently it achieves this in the following ways:

• No non-R5RS procedures are implemented or recognized.
• No extensions are made to existing R5RS procedures.
• Only two optional procedures are implemented: - and / for more than two arguments.
• Literal lists, strings, and vectors are immutable, as allowed by R5RS 3.4.

The basic idea was that if something works in Haschoo, it should work in any R5RS system. That's not quite the current situation, but it seems to be mostly true, at least based on my limited testing.

Known bugs

Due to time and energy constraints I didn't manage to iron out the following:

• Continuations, regretfully, are not implemented at all.

• Nested ellipses in macros don't always work correctly. (e.g. tests/macros/nested-ellipses.scm)

• The "read" procedure doesn't work very well: due to annoying technical issues, it works by reading one character at a time and then trying to parse the string gathered thus far, stopping on a successful parse. Hence reading "-1" gives "-" followed by "1".

• Quasiquotation has some silly bugs. For example, `(1 2 . ,(list 3 4)) results in '(1 2 unquote (list 3 4)) instead of '(1 2 (list 3 4)).

No doubt there are many other lurking bugs. Time and energy constraints tend to limit testing as well.

References