https://github.com/breckinloggins/erlisp
I'm currently working on writing one in Haskell using the Scheme48 tutorial [1], though I'd eventually like to replace the evaluator with an F-Algebra[2] so I can learn about that.
https://github.com/breckinloggins/scheme48
As a side note, would any Haskellers be willing to look at the abstraction I did for parsing #-forms? I created an adhoc data structure:
hashLiteralInfo = [
('f', (Nothing, \_ -> Bool False)),
('t', (Nothing, \_ -> Bool True)),
('b', (Just $ many $ oneOf "01", Number . toInteger . fromBase 2)),
('o', (Just $ many $ oneOf "01234567", Number . toInteger . fromBase 8)),
('d', (Just $ many $ oneOf "0123456789", Number . toInteger . fromBase 10)),
('h', (Just $ many $ oneOf "0123456789abcdefABCDEF", Number . toInteger . fromBase 16))
]
parseHashLiteral :: Parser LispVal
parseHashLiteral = do
char '#'
c <- oneOf $ map fst hashLiteralInfo
let literalInfo = lookup c hashLiteralInfo
case literalInfo of
Nothing -> fail "Internal parse error: unregistered literal info"
Just (Nothing, f) -> return $ f "unneeded" -- I know there's a more idiomatic way to do that
Just (Just literalParser, f) -> do
digits <- literalParser
return $ f digits
[1] http://en.wikibooks.org/wiki/Write_Yourself_a_Scheme_in_48_H...
[2] http://bartoszmilewski.com/2013/06/10/understanding-f-algebr...
hashLiteralInfo =
[ ('f', pure $ Bool False)
, ('t', pure $ Bool True)
, ('b', fmap (Number . toInteger . fromBase 2) (many $ oneOf "01"))
-- ... and so on
]
https://github.com/breckinloggins/scheme48/commit/60d7930b4c...
In particular, with hashLiteralInfo setup as [(Char, Parser LispVal)], the parseHashLiteral function became as simple as:
parseHashLiteral :: Parser LispVal
parseHashLiteral = do
char '#'
c <- oneOf $ map fst hashLiteralInfo
case lookup c hashLiteralInfo of
Just x -> x
Nothing -> fail "Internal parse error: unregistered literal info" data HashLiteral a = HashLiteral { _character :: Char, _trailingChars :: [Char], _representation :: [Char] -> a}
Then you could do something like
binHashLiteral = HashLiteral { _char = 'b', _parser = Just . many . oneOf "01", _representation = Number . toInteger . fromBase 2 }
However, if I decide to go forward with some of the more advanced things in the tutorial like threading good error messages and such, I'm pretty sure some new data types are in my future.
Also, when you say "underscore for lenses", did you mean that your example would be pretty clean with the Lenses library? I've been looking for an excuse to learn that so if so I'd love to learn more!
Also, now you have to write your own lookup function instead of just using Prelude.lookup.
(pun intended)
http://michaux.ca/articles/scheme-from-scratch-introduction
I had fun following along with him. Now, if he (or I can get my shot at it finished) can ever finish working on the byte-code based version!
The book is in two parts. The first starts from a simple evaluation function and enriches it with multiple name spaces, continuations and side-effects with commented variants, while at the same time the language used to define these features is reduced to a simple lambda-calculus.
The second part focuses more on implementation techniques and discusses precompilation for fast interpretation: threaded code or bytecode; compilation towards C. Some extensions are also described such as dynamic evaluation, reflection, macros and objects.
I would also find very interesting to see a step-by-step series on compiling scheme (with macros included) using llvm. I never tried really hard to look for one, though.
I should finish my Ada version sometime... Or just start a new project haha
(Last time I tried the latter the more vocal members of the user base complained so much the entire effort died. That was in 2000.)
The business model was "gated open source", as in paying customers got source, the users were almost without exception system administrators, not programmers, and with one exception who was something of a programmer (at least he did more with the awful code base than any other user) and willing to give it a try, they demanded the "extension" language (actually the complicated business logic) be in Perl ... despite to our knowledge at there were no successful examples of this, whereas Lisp has many, with EMACS and AutoCAD being famous wild successes. (The whole thing couldn't be in Perl because of performance requirements.)
Despite being the only programmer working on it and not seriously experienced with Perl, I got overruled. The CEO, the only manager, was the sort who during this time of decreasing revenue felt compelled to move from Class B to Class A office space (https://en.wikipedia.org/wiki/Office#Grading), and more of it---yet somehow the 3 technical people supporting almost all revenue got allocated only one office, which was tight for 2 people, and I'm one of those who needs quiet for maximum productivity (the other two were sysadmins and technical support, and had to do a lot of the latter over the phone. Whine, whine.)
Basically, only with maximum productivity on my part was this possible to pull off (and I've done this sort of thing before), as the users were years overdue for real technical fixes and progress (long before I arrived).
The board got really upset, the executive director blamed me and they believed her (I after all had been there for less than 2 years and had been able to do only one month of programming in my first year due to Y2K and other sysadmin and tech support demands) ... I had to bail.
It was partly my fault, I was not very productive for several months after my major architectural decision was vetoed and that work was dumped, which happened roughly the time the office move made the company's financials likely terminal, and I should have outright refused the CEO when she demanded I drop everything to help try to sell more of the old (which her fantasy budget demanded) and pre-sell versions of the new (then again I have to wonder if that would have made any difference).
The company died an ugly death, but not before firing in revenge a close friend I'd hired to do system administration and tech support.
Bleah. Bottom lines: avoid recruiting your friends because the company may later go to hell, if you observe the former then don't stay at places that don't respect programmers, can't keep them, and who don't have their eye on the ball of what brings in the cash. None of this was apparent when I accepted their offer, but I'm sure most of us know that story.
Lisp implementation from one of the julialang devs
btw, not related to OP, those guys who think it is so clever to translate some primitive subset of Scheme into Haskell should appreciate that writing Scheme interpreter in Scheme is a part of CS61A course and it takes one page of much more readable code.
If, for some reason, you wish to write something other than toys, look at Mark Freeley, to realize that serious Scheme system is hard.
And does anyone else dislike web design that (1) limits the text to a column a third of the page wide, and then (2) because this is too small, requires a horizontal scrollbar on code segments?
For an explanation of a small Lisp interpreter (in Python) that uses sane web design, one might consider Norvig's http://norvig.com/lispy.html
I agree this page should be wider for posts like this.
Edit: Ok, ok. Now that the title of submission has been changed from "Toy Lisp Interpreter" to "Little Lisp Interpreter" my quip, having lost context doesn't make me smile anymore either.