Reverse Polish Notation Evaluator in Haskell
As you may have deduced from the title, I've created an rpn evaluator in Haskell, as an exercise because I'm new to this language.
It runs well:
$ ./rpn
2 3 4 5 + - +
-4
And the source code:
{-# LANGUAGE BangPatterns #-}
import Data.String
import System.IO
data Token = TNum Int | TOp Operator
data Operator = Add | Sub | Mul | Div
main :: IO ()
main = do
line <- getLine
let tokens = tokenise line
(numc, opc) = countTok tokens
!junk =
if numc == opc + 1
then ()
else error "Not a correct expression."
print $ eval tokens
tokenise :: String -> [Token]
tokenise = map str2tok . words
eval :: [Int] -> [Token] -> Int
eval (s:_) = s
eval stack (TNum t:ts) = eval (t : stack) ts
eval (x:y:stacknoxy) (TOp t:ts) = eval (applyOp t y x : stacknoxy) ts
str2tok :: String -> Token
str2tok tkn@(c:_)
| c `elem` ['0'..'9'] = TNum (read tkn :: Int)
| otherwise = TOp $ case tkn of
"+" -> Add
"-" -> Sub
"*" -> Mul
"/" -> Div
_ -> error $ "No such operator " ++ tkn
applyOp :: Operator -> Int -> Int -> Int
applyOp Add a b = a + b
applyOp Sub a b = a - b
applyOp Mul a b = a * b
applyOp Div a b = a `div` b
countTok :: [Token] -> (Int, Int)
countTok = (0, 0)
countTok (t:ts) =
let (x, y) = case t of
TNum _ -> (1, 0)
_ -> (0, 1)
in (x, y) `addPair` countTok ts
addPair :: (Num a, Num b) => (a, b) -> (a, b) -> (a, b)
addPair (x, y) (z, w) = (x + z, y + w)
How can this code be improved? I hope my implementation is elegant, and if it isn't - what are the ways to clean it up?
EDIT: I forgot to add! I really don't like the error's because they are just ugly:
./rpn
5 6 + +
rpn: Not a correct expression.
CallStack (from HasCallStack):
error, called at rpn.hs:16:22 in main:Main
I know they can be replaced with fail, which has a much nicer output, but from what I've read it can only be done inside a function that returns IO ().
haskell functional-programming math-expression-eval
asked 54 mins ago
atmostmediocre
233
add a comment |
As you may have deduced from the title, I've created an rpn evaluator in Haskell, as an exercise because I'm new to this language.
It runs well:
$ ./rpn
2 3 4 5 + - +
-4
And the source code:
{-# LANGUAGE BangPatterns #-}
import Data.String
import System.IO
data Token = TNum Int | TOp Operator
data Operator = Add | Sub | Mul | Div
main :: IO ()
main = do
line <- getLine
let tokens = tokenise line
(numc, opc) = countTok tokens
!junk =
if numc == opc + 1
then ()
else error "Not a correct expression."
print $ eval tokens
tokenise :: String -> [Token]
tokenise = map str2tok . words
eval :: [Int] -> [Token] -> Int
eval (s:_) = s
eval stack (TNum t:ts) = eval (t : stack) ts
eval (x:y:stacknoxy) (TOp t:ts) = eval (applyOp t y x : stacknoxy) ts
str2tok :: String -> Token
str2tok tkn@(c:_)
| c `elem` ['0'..'9'] = TNum (read tkn :: Int)
| otherwise = TOp $ case tkn of
"+" -> Add
"-" -> Sub
"*" -> Mul
"/" -> Div
_ -> error $ "No such operator " ++ tkn
applyOp :: Operator -> Int -> Int -> Int
applyOp Add a b = a + b
applyOp Sub a b = a - b
applyOp Mul a b = a * b
applyOp Div a b = a `div` b
countTok :: [Token] -> (Int, Int)
countTok = (0, 0)
countTok (t:ts) =
let (x, y) = case t of
TNum _ -> (1, 0)
_ -> (0, 1)
in (x, y) `addPair` countTok ts
addPair :: (Num a, Num b) => (a, b) -> (a, b) -> (a, b)
addPair (x, y) (z, w) = (x + z, y + w)
How can this code be improved? I hope my implementation is elegant, and if it isn't - what are the ways to clean it up?
EDIT: I forgot to add! I really don't like the error's because they are just ugly:
./rpn
5 6 + +
rpn: Not a correct expression.
CallStack (from HasCallStack):
error, called at rpn.hs:16:22 in main:Main
I know they can be replaced with fail, which has a much nicer output, but from what I've read it can only be done inside a function that returns IO ().
haskell functional-programming math-expression-eval
asked 54 mins ago
atmostmediocre
233
add a comment |
As you may have deduced from the title, I've created an rpn evaluator in Haskell, as an exercise because I'm new to this language.
It runs well:
$ ./rpn
2 3 4 5 + - +
-4
And the source code:
{-# LANGUAGE BangPatterns #-}
import Data.String
import System.IO
data Token = TNum Int | TOp Operator
data Operator = Add | Sub | Mul | Div
main :: IO ()
main = do
line <- getLine
let tokens = tokenise line
(numc, opc) = countTok tokens
!junk =
if numc == opc + 1
then ()
else error "Not a correct expression."
print $ eval tokens
tokenise :: String -> [Token]
tokenise = map str2tok . words
eval :: [Int] -> [Token] -> Int
eval (s:_) = s
eval stack (TNum t:ts) = eval (t : stack) ts
eval (x:y:stacknoxy) (TOp t:ts) = eval (applyOp t y x : stacknoxy) ts
str2tok :: String -> Token
str2tok tkn@(c:_)
| c `elem` ['0'..'9'] = TNum (read tkn :: Int)
| otherwise = TOp $ case tkn of
"+" -> Add
"-" -> Sub
"*" -> Mul
"/" -> Div
_ -> error $ "No such operator " ++ tkn
applyOp :: Operator -> Int -> Int -> Int
applyOp Add a b = a + b
applyOp Sub a b = a - b
applyOp Mul a b = a * b
applyOp Div a b = a `div` b
countTok :: [Token] -> (Int, Int)
countTok = (0, 0)
countTok (t:ts) =
let (x, y) = case t of
TNum _ -> (1, 0)
_ -> (0, 1)
in (x, y) `addPair` countTok ts
addPair :: (Num a, Num b) => (a, b) -> (a, b) -> (a, b)
addPair (x, y) (z, w) = (x + z, y + w)
How can this code be improved? I hope my implementation is elegant, and if it isn't - what are the ways to clean it up?
EDIT: I forgot to add! I really don't like the error's because they are just ugly:
./rpn
5 6 + +
rpn: Not a correct expression.
CallStack (from HasCallStack):
error, called at rpn.hs:16:22 in main:Main
I know they can be replaced with fail, which has a much nicer output, but from what I've read it can only be done inside a function that returns IO ().
haskell functional-programming math-expression-eval
asked 54 mins ago
atmostmediocre
233
As you may have deduced from the title, I've created an rpn evaluator in Haskell, as an exercise because I'm new to this language.
It runs well:
$ ./rpn
2 3 4 5 + - +
-4
And the source code:
{-# LANGUAGE BangPatterns #-}
import Data.String
import System.IO
data Token = TNum Int | TOp Operator
data Operator = Add | Sub | Mul | Div
main :: IO ()
main = do
line <- getLine
let tokens = tokenise line
(numc, opc) = countTok tokens
!junk =
if numc == opc + 1
then ()
else error "Not a correct expression."
print $ eval tokens
tokenise :: String -> [Token]
tokenise = map str2tok . words
eval :: [Int] -> [Token] -> Int
eval (s:_) = s
eval stack (TNum t:ts) = eval (t : stack) ts
eval (x:y:stacknoxy) (TOp t:ts) = eval (applyOp t y x : stacknoxy) ts
str2tok :: String -> Token
str2tok tkn@(c:_)
| c `elem` ['0'..'9'] = TNum (read tkn :: Int)
| otherwise = TOp $ case tkn of
"+" -> Add
"-" -> Sub
"*" -> Mul
"/" -> Div
_ -> error $ "No such operator " ++ tkn
applyOp :: Operator -> Int -> Int -> Int
applyOp Add a b = a + b
applyOp Sub a b = a - b
applyOp Mul a b = a * b
applyOp Div a b = a `div` b
countTok :: [Token] -> (Int, Int)
countTok = (0, 0)
countTok (t:ts) =
let (x, y) = case t of
TNum _ -> (1, 0)
_ -> (0, 1)
in (x, y) `addPair` countTok ts
addPair :: (Num a, Num b) => (a, b) -> (a, b) -> (a, b)
addPair (x, y) (z, w) = (x + z, y + w)
How can this code be improved? I hope my implementation is elegant, and if it isn't - what are the ways to clean it up?
EDIT: I forgot to add! I really don't like the error's because they are just ugly:
./rpn
5 6 + +
rpn: Not a correct expression.
CallStack (from HasCallStack):
error, called at rpn.hs:16:22 in main:Main
I know they can be replaced with fail, which has a much nicer output, but from what I've read it can only be done inside a function that returns IO ().
haskell functional-programming math-expression-eval
haskell functional-programming math-expression-eval
asked 54 mins ago
atmostmediocre
233
asked 54 mins ago
atmostmediocre
233
edited 48 mins ago
asked 54 mins ago
atmostmediocre
233
asked 54 mins ago
atmostmediocre
233
asked 54 mins ago
atmostmediocre
233
233
add a comment |
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