Question

EDIT: problem partially solved, skip to the bottom for update.

I’m writing a small language using haskell, and I’ve made a lot of progress, but I am having trouble implementing statements that use blocks, like “{ … }”. I’ve implemented support for If statements like so in my parser file:

stmt = skip +++ ifstmt +++ assignment +++ whilestmt

ifstmt = symbol "if" >>
         parens expr >>= \c ->
         stmt >>= \t ->
         symbol "else" >>
         stmt >>= \e ->
         return $ If c t e

whilestmt = symbol "while" >>
            parens expr >>= \c ->
        symbol "\n" >>
        symbol "{" >>
        stmt >>= \t ->
        symbol "}" >>
        return $ While c t

expr = composite +++ atomic

And in the Syntax file:

class PP a where 
  pp :: Int -> a -> String

instance PP Stmt where
  pp ind (If c t e) = indent ind ++ 
                      "if (" ++ show c ++ ") \n" ++ 
                      pp (ind + 2) t ++
                      indent ind ++ "else\n" ++
                      pp (ind + 2) e
  pp ind (While c t) = indent ind ++
                   "while (" ++ show c ++") \n" ++
                   "{" ++ pp (ind + 2) t ++ "}" ++
                   indent ind

Something is wrong with the while statement, and I don’t understand what. The logic seems correct, but when I run the code I get the following error:

EDIT: Fixed the first problem based on the first reply, now it is not recognizing my while statment which I assume comes from this:
exec :: Env -> Stmt -> Env
exec env (If c t e) = 
    exec env ( if eval env c == BoolLit True then t else e )
exec env (While c t) =
    exec env ( if eval env c == BoolLit True then t )

The file being read from looks like this:

x = 1; c = 0;
if (x < 2) c = c + 1; else ;
-- SEPARATE FILES FOR EACH
x = 1; c = 1;
while (x < 10)
{
  c = c * x;
  x = x + 1;
}
c

I’ve tried to understand the error report but nothing I’ve tried solves the problem.

Answer 1

>> and >>= bind tighter than $. Try using

return (While c t)

instead of

return $ While c t

And also, put parenthesis around the lambda expression on the right hand side of >>=.

Or just use do-notation:

whilestmt = do
    symbol "while"
    c <- parens expr
    symbol "\n"
    symbol "{"
    t <- stmt
    symbol "}"
    return $ While c t