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|
{-
frown Simple.g
Try
expr (lexer "3+(5)") :: IO Expr
-}
module Simple
where
import Char
data Expr = Const Int
| Add Expr Expr
deriving (Show)
type Result = IO
%{
Terminal = NAT {Int} | PLUS | LPAR | RPAR;
Nonterminal = *expr {Expr} | term {Expr};
expr {Add e1 e2} : expr {e1}, PLUS, term {e2};
{e} | term {e};
term {Const n} : NAT {n};
{e} | LPAR, expr {e}, RPAR;
}%
frown ts = fail ("syntax error: " ++ unlex ts ++ ".")
data Terminal = NAT Int | PLUS | LPAR | RPAR | EOF
deriving (Show)
lexer :: String -> [Terminal]
lexer [] = []
lexer ('+' : cs) = PLUS : lexer cs
lexer ('(' : cs) = LPAR : lexer cs
lexer (')' : cs) = RPAR : lexer cs
lexer (c : cs)
| isDigit c = let (n, cs') = span isDigit cs
in NAT (read (c : n)) : lexer cs'
| otherwise = lexer cs
unlex [] = []
unlex (PLUS : ts) = '+' : unlex ts
unlex (LPAR : ts) = '(' : unlex ts
unlex (RPAR : ts) = ')' : unlex ts
unlex (NAT n : ts) = show n ++ unlex ts
|
