Writing a Lisp: spit and slurp

, in Writing a Lisp, Haskell

To make my Lisp a more useful language, and to pave the road for a module system and package manager, I added the capability of performing IO.

I like the names spit and slurp for this functionality, and I think using slurp as a unified interface for network and file IO is an elegant idea. Let’s see them in action:

(spit "todo.txt" "Remember the milk") ;=> nil
(slurp "todo.txt") ;=> "Remember the milk"

(slurp "https://httpbin.org/get") 
;=> "{"args": {}, "headers": { "Accept-Encoding": ..."

Implementation

Spit simply writes a string to a file, returning nil.

spit :: Env -> [LispVal] -> IO LispVal
spit _ [String f, String s] = do
  writeFile f s
  return Nil

Slurp uses the uri package to check if the given location is a network resource. If it is, it uses wreq to make a get request, extracting the response body and turning it into a string. Otherwise, it reads the file at the given location. The result is turned into a Lisp string either way.

In case you’re wondering, the <&> operator comes from the lens package, and is defined as flip fmap.

slurp :: Env -> [LispVal] -> IO LispVal
slurp _ [String s] =
  String <$> case parseURI s >>= uriScheme of
    Just _ -> get s <&> (^. responseBody) <&> toString
    _      -> readFile s

Finally, the functions are wrapped in a LispVal constructor, and defined in the enviroment.

impurePrimitives :: [(String, LispVal)]
impurePrimitives = wrapPrimitives False Impure 
  [ --
   ("slurp", slurp), 
   ("spit", spit)]