Parsing/RPN to infix conversion: Difference between revisions

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@@ Line 1,915: / Line 1,915: @@
 data Expression = Const String | Exp Expression String Expression
+------------- INFIX EXPRESSION FROM RPN STRING -----------
-precedence :: Expression -> Int
-precedence (Const _) = 5
-precedence (Exp _ op _)
-  | op == "^" = 4
-  | op `elem` ["*", "/"] = 3
-  | op `elem` ["+", "-"] = 2
-  | otherwise = 0
+infixFromRPN :: String -> Expression
-leftAssoc :: Expression -> Bool
+infixFromRPN = head . foldl buildExp [] . words
-leftAssoc (Const _) = False
-leftAssoc (Exp _ op _) = op `notElem` ["^", "*", "+"]
-rightAssoc :: Expression -> Bool
-rightAssoc (Const _) = False
-rightAssoc (Exp _ op _) = op == "^"
-instance Show Expression where
-  show (Const x) = x
-  show exp@(Exp l op r) = left <> " " <> op <> " " <> right
-    where
-      left
-        | leftNeedParen = "( " <> show l <> " )"
-        | otherwise = show l
-      right
-        | rightNeedParen = "( " <> show r <> " )"
-        | otherwise = show r
-      leftNeedParen =
-        (leftPrec < opPrec)
-          || ((leftPrec == opPrec) && rightAssoc exp)
-      rightNeedParen =
-        (rightPrec < opPrec)
-          || ((rightPrec == opPrec) && leftAssoc exp)
-      leftPrec = precedence l
-      rightPrec = precedence r
-      opPrec = precedence exp
 buildExp :: [Expression] -> String -> [Expression]
@@ Line 1,963: / Line 1,932: @@
     isOp = (`elem` ["^", "*", "/", "+", "-"])
+--------------------------- TEST -------------------------
 main :: IO ()
 main =
@@ Line 1,968: / Line 1,939: @@
     ( \s ->
         putStr (s <> "\n-->\n")
-          >> (print . head . foldl buildExp [] . words)
+          >> (print . infixFromRPN)
             s
           >> putStrLn []
@@ Line 1,977: / Line 1,948: @@
       "1 2 * 3 4 * *",
       "1 2 + 3 4 + +"
-    ]</lang>
+    ]
+---------------------- SHOW INSTANCE ---------------------
+instance Show Expression where
+  show (Const x) = x
+  show exp@(Exp l op r) = left <> " " <> op <> " " <> right
+    where
+      left
+        | leftNeedParen = "( " <> show l <> " )"
+        | otherwise = show l
+      right
+        | rightNeedParen = "( " <> show r <> " )"
+        | otherwise = show r
+      leftNeedParen =
+        (leftPrec < opPrec)
+          || ((leftPrec == opPrec) && rightAssoc exp)
+      rightNeedParen =
+        (rightPrec < opPrec)
+          || ((rightPrec == opPrec) && leftAssoc exp)
+      leftPrec = precedence l
+      rightPrec = precedence r
+      opPrec = precedence exp
+leftAssoc :: Expression -> Bool
+leftAssoc (Const _) = False
+leftAssoc (Exp _ op _) = op `notElem` ["^", "*", "+"]
+rightAssoc :: Expression -> Bool
+rightAssoc (Const _) = False
+rightAssoc (Exp _ op _) = op == "^"
+precedence :: Expression -> Int
+precedence (Const _) = 5
+precedence (Exp _ op _)
+  | op == "^" = 4
+  | op `elem` ["*", "/"] = 3
+  | op `elem` ["+", "-"] = 2
+  | otherwise = 0</lang>
 {{Out}}
 <pre>3 4 2 * 1 5 - 2 3 ^ ^ / +