Kindof vaguely made a start on this, but cant quite see how to handle
variables.
I guess variables can be stored as a (Map.Map String Double), at least for a
first draft?
Then, I'm building up two hierarchies in parallel:
- a set of parsec functions to parse the incoming string into a Program
hierarchy
- a set of data types to represent a program
Then, there's a class called "Eval" containing a function "eval" which is
instanced for each bit of the program hierarchy, so we simply call "eval" on
the top level, and the program is executed.
That works just fine as long as the only thing eval has to cope with is
print statements (so eval has type IO ()), but I'm guessing the clean
solution is to thread a Map.Map through that somehow?
Solution so far:
-- parsing hierarchy (pretty basic, but this bit doesnt seem particularly
scary)
string :: Parsec.Parser String
string = Parsec.many1 Parsec.letter
minimprint = do Parsec.string "print"
Parsec.many1 (Parsec.char ' ')
Parsec.char '"'
stringvalue <- string
Parsec.char '"'
return (Print stringvalue)
-- program data type hierarchy
data Program = ProgramLeaf Statement | ProgramTree Program Statement
deriving(Show)
data Statement = PrintStatement Print |
AssignmentStatement Assignment
deriving(Show)
data Print = Print String
deriving(Show)
data Assignment = VarAssignment Variable Value |
Increment Variable |
Decrement Variable
deriving(Show)
data Variable = Variable String
deriving(Show)
data Value = ValueFromConstant Constant | ValueFromVariable Variable
deriving(Show)
newtype Constant = Constant Int
deriving(Show)
-- eval instances
class Eval a where
eval :: a -> IO()
instance Eval Program where
eval (ProgramLeaf statement) = eval statement
eval (ProgramTree program statement) = do eval program
eval statement
instance Eval Statement where
eval ( PrintStatement print) = eval print
eval ( AssignmentStatement assignment) = return ()
instance Eval Print where
eval (Print value) = putStrLn value
-- some code to test this
minimparse minimsentence = case (Parsec.runParser minimprint () ""
minimsentence) of
(Right statement) -> eval statement
Left error -> putStrLn("error: " ++
show(error))
test = minimparse "print \"hello\""
Running "test" correctly gives an output of "hello", which is a good start.
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
