Skip to content

Latest commit

 

History

History
50 lines (39 loc) · 2.38 KB

README.md

File metadata and controls

50 lines (39 loc) · 2.38 KB
Raw

Write Yourself a Scheme

What is it?

This is my implementation of the Scheme programming language, based on a book called Write Yourself a Scheme in 48 Hours. It's essentially in line with the book, but most code snippets are rewritten and I take significant detours in the direction of completeness/compliance with the R7RS (which is not achieved by a long shot).

Build and run it

Stack is used for dependency and build management. I usually use it like so:

cd wya-scheme
stack run                # for a repl
stack run some-code.scm  # execute a file

Notable Features

  • full numeric tower (integer, rational, real, complex)
  • quote, quasiquote, unquote, unquote-splicing
  • define-macro
  • exceptions (raise, with-exception-handler)
  • I/O (ports)

Notable Missing Features

  • comments
  • exact/inexact numbers
  • continuations
  • user-defined exceptions
  • define-syntax-style macros
  • TCO
  • libraries
  • first class environments
  • parameter objects
  • records
  • vectors
  • bytevectors
  • numerous derived procedures and forms
  • more than a few primatives, probably

Confessions

IORef

IORef is used to implement the mutable Scheme environment, because that's what the book uses. The book justifies this in the first few paragraphs of chapter 7, in particular:

The IORef module lets you use stateful variables within the IO monad. Since our state has to be interleaved with IO anyway (it persists between lines in the REPL, and we will eventually have IO functions within the language itself), we'll be using IORefs.

But apparently there's a nicer way to handle this: Write You A Scheme, Version 2 (by a different author) does not use IORef, but still provides a REPL. Instead, the Reader monad (which handles lexical scope) and IO monad are combined using monad transformers.

Overall the original book struck me as friendlier introduction to Haskell than did Version 2, so I stuck with it and IORef, but I'd still like to explore the Reader/transformer approach at some point.

String

Another tidbit I learned from skimming Version 2: Text is better than String. Yet this project uses String.