This project is a Lisp interpreter written in Haskell, based on this book. It includes a general-purpose monadic parsing library based on this paper which implements a subset of the Parsec library. The Lisp interpreter is built on top of these parsing functions.
The parsing library in Parsing.hs features a number of simple and useful combinators that are used to make more powerful parsers. For example:
sat :: (Char -> Bool) -> Parser Char consumes characters if they satisfy a predicate.
We define char :: Char -> Parser Char as char c = sat (c ==) as a parser to match against a specific letter.
We use char to define string :: String -> Parser String as string (c:cs) = do {char c; string cs; return (c:cs)} to recursively match against a specific string.
The parsing library also takes advantage of monads to chain together parsers easily. For example:
We define a utility function many :: Parser a -> Parser [a] that parses repeated applications of a parser p. many relies on a helper function many1 :: Parser a -> Parser [a] which is defined as many1 p = do {a <- p; as <- manyp; return (a:as)}. many is the main way we chain together parsers.
There are many examples of this in Parsing.hs, namely sepby, chainr and chainl.
This interpreter supports many features of the Lisp language.
