- Def \lit = Int
- |. Lit "(" -. expr -. Lit ")"
- Def \fac = lit -. Lit "*" -. fac
- |. lit -. Lit "/" -. fac
- |. lit :.
- Def \expr= fac -. Lit "+" -. expr
- |. fac -. Lit "-" -. expr
- |. fac :.
- expr
+ let lit = Lit "(" -. expr -. Lit ")"
+ |. Int
+ pow = lit -. ?! (Lit "^" -. pow)
+ fac = pow -. *! (Lits ["*","/"] -. pow)
+ expr = fac -. *! (Lits ["+","-","%"] -. fac)
+ in expr
+
+ printeval a = (eval a, print a)
+
+ eval :: Gast -> Maybe Int
+ eval (BIN (LIT "(") (BIN e (LIT ")"))) = eval e
+ eval (INT i) = Just i
+ eval (LIT _) = Nothing
+ eval (BIN l (OPT Nothing)) = eval l
+ eval (BIN l (OPT (Just a))) = eval (BIN l a)
+ //Right associative operators
+ eval (BIN l (BIN (LIT op) r)) = op2op op <*> eval l <*> eval r
+ //Left associative operators
+ eval (BIN l (MANY [])) = eval l
+ eval (BIN l (MANY [BIN (LIT op) r:rest]))
+ = eval (BIN (BIN l (BIN (LIT op) r)) (MANY rest))
+ eval e = abort ("eval: " +++ printToString e +++ "\n")
+
+ print :: Gast -> String
+ print (BIN (LIT "(") (BIN e (LIT ")"))) = "(" +++ print e +++ ")"
+ print (INT i) = toString i
+ print (LIT l) = l
+ print (BIN l (OPT Nothing)) = print l
+ print (BIN l (OPT (Just a))) = print (BIN l a)
+ //Right associative operators
+ print (BIN l (BIN (LIT op) r)) = "(" +++ print l +++ op +++ print r +++ ")"
+ //Left associative operators
+ print (BIN l (MANY [])) = print l
+ print (BIN l (MANY [BIN (LIT op) r:rest]))
+ = print (BIN (BIN l (BIN (LIT op) r)) (MANY rest))
+ print e = printToString e +++ "\n"
+
+ op2op "+" = Just (+)
+ op2op "-" = Just (-)
+ op2op "*" = Just (*)
+ op2op "/" = Just (/)
+ op2op "%" = Just (rem)
+ op2op "^" = Just (^)
+ op2op _ = Nothing
+
+import Text.GenPrint
+derive gPrint Gast, Maybe