Ran a Language server on my old code

app/Main.hs

@@ -34,6 +34,7 @@ main = ioLoop initVars
 precision = 5 :: Int
 
 showRatio :: Int -> Rational -> String
+showRatio _ 0 = "0"
 showRatio p r = (if (r < 0) then "-" else "") ++ prepoint_digits ++ (if (length postpoint_digits > 0) then ("." ++ postpoint_digits) else "")
     where
         prepoint_digits  = init . show . round . abs $ (r * 10)

src/Lib.hs

@@ -18,26 +18,46 @@
 module Lib
     ( exprparser, evaluate, replaceVars, Expr, extractVariableDefinitions, updateVariables, parseFullString
     ) where
-import Control.Applicative((<*))
 
-import Data.Ratio
+import Data.Functor.Identity
+
+import Data.Ratio ( (%), denominator, numerator )
 import Data.Map (Map)
 import qualified Data.Map as Map
 
 import Text.Parsec
-import Text.Parsec.Char
+    ( alphaNum,
-import Text.Parsec.String
+      char,
+      digit,
+      letter,
+      oneOf,
+      eof,
+      notFollowedBy,
+      (<?>),
+      (<|>),
+      many1,
+      parse,
+      try,
+      ParseError,
+      ParsecT )
+import Text.Parsec.Char ()
+import Text.Parsec.String ( Parser )
 import Text.Parsec.Expr
+    ( buildExpressionParser, Assoc(AssocLeft), Operator(Infix) )
 import Text.Parsec.Token
-import Text.Parsec.Language
+    ( makeTokenParser,
+      GenLanguageDef(reservedNames, commentStart, commentEnd, identStart,
+                     identLetter, opStart, opLetter, reservedOpNames),
+      GenTokenParser(whiteSpace, TokenParser, parens, identifier,
+                     reservedOp, natural, integer) )
+import Text.Parsec.Language ( emptyDef )
 
 data Expr = Variable String | Constant Rational | Binary BinaryOperator Expr Expr | VariableDefinition String Expr
     deriving Show
 data BinaryOperator = Plus | Minus | Multiply | Divide | Power
     deriving Show
 
-naturalRatio a = a % 1
+def :: Monad m => GenLanguageDef String u m
-
 def = emptyDef{ commentStart = ""
               , commentEnd = ""
               , identStart = letter <|> char '_'
@@ -48,20 +68,26 @@ def = emptyDef{ commentStart = ""
               , reservedNames = []
               }
 
+m_parens :: ParsecT String u Identity a -> ParsecT String u Identity a
+m_identifier :: ParsecT String u Identity String
+m_whiteSpace :: ParsecT String u Identity ()
+m_reservedOp :: String -> ParsecT String u Identity ()
+m_natural :: ParsecT String u Identity Integer
+m_integer :: ParsecT String u Identity Integer
 TokenParser{ parens = m_parens
            , identifier = m_identifier
            , reservedOp = m_reservedOp
-           , reserved   = m_reserved
-           , semiSep1   = m_semiSep1
            , natural    = m_natural
            , integer    = m_integer
            , whiteSpace = m_whiteSpace } = makeTokenParser def
 
+parseFullString :: String -> Either ParseError Expr
 parseFullString s = parse (m_whiteSpace *> exprparser <* eof) "<stdin>" s
 
 exprparser :: Parser Expr
 exprparser = buildExpressionParser table term <?> "expression"
 
+table :: [[Operator String u Identity Expr]]
 table = [
             [
                 Infix (m_reservedOp "^" >> return (Binary Power)) AssocLeft,
@@ -94,7 +120,6 @@ constantRational = do
     decimal_digits <- many1 digit
     _              <- m_whiteSpace
     let decimal = read decimal_digits :: Integer
-    let natural_length = length . show $ natural
     let decimal_length = length decimal_digits
     let numerator   = natural * (10 ^ decimal_length) + decimal
     let denominator = 10 ^ decimal_length
@@ -108,6 +133,7 @@ variableDefinition = try (do
         return (VariableDefinition name value)
     )
 
+term :: ParsecT String () Identity Expr
 term = m_parens exprparser
        <|> variableDefinition
        <|> fmap Variable m_identifier
@@ -121,7 +147,7 @@ extractVariableDefinitions (Constant _)                    = []
 extractVariableDefinitions (Variable _)                    = []
 
 updateVariables :: [(String, Expr)] -> Map String Rational -> Either String (Map String Rational)
-updateVariables ds vs = foldl updateVariable (Right vs) ds
+updateVariables ds vs = foldl' updateVariable (Right vs) ds
 
 updateVariable :: Either String (Map String Rational) -> (String, Expr) -> Either String (Map String Rational)
 updateVariable (Left  e)  _         = Left e
@@ -135,16 +161,14 @@ updateVariable (Right vs) (name, e) = case updateVariables nvs vs of
 
 replaceVars :: Expr -> Map.Map String Rational -> Either String Expr
 replaceVars (Constant c)                  vs = Right $ Constant c
-replaceVars (Variable name)               vs = maybe (Left ("Usage of unknown variable: '" ++ name ++ "'\nSuggestion: define the variable\n++  " ++ name ++ "=42")) (Right . Constant . id) $ Map.lookup name vs
+replaceVars (Variable name)               vs = case Map.lookup name vs of
+  Just value -> Right $ Constant value
+  Nothing -> Left ("Usage of unknown variable: '" ++ name ++ "'\nSuggestion: define the variable\n++  " ++ name ++ "=42")
 replaceVars (VariableDefinition name _)   vs = replaceVars (Variable name) vs
-replaceVars (Binary op l r)               vs = case leftBranch of
+replaceVars (Binary op l r)               vs = do
-        Left  s -> Left  s
+  leftBranch <- replaceVars l vs
-        Right a -> case rightBranch of
+  rightBranch <- replaceVars r vs
-            Left s  -> Left s
+  pure $ Binary op leftBranch rightBranch
-            Right b -> Right $ Binary op a b
-    where
-        leftBranch  = replaceVars l vs
-        rightBranch = replaceVars r vs
 
 {-
  - a/b ^ c/d

stack.yaml

@@ -17,8 +17,7 @@
 #
 # resolver: ./custom-snapshot.yaml
 # resolver: https://example.com/snapshots/2018-01-01.yaml
-resolver:
+resolver: nightly-2025-07-04
-  url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/22/34.yaml
 
 # User packages to be built.
 # Various formats can be used as shown in the example below.

stack.yaml.lock

@@ -1,13 +1,12 @@
 # This file was autogenerated by Stack.
 # You should not edit this file by hand.
 # For more information, please see the documentation at:
-#   https://docs.haskellstack.org/en/stable/lock_files
+#   https://docs.haskellstack.org/en/stable/topics/lock_files
 
 packages: []
 snapshots:
 - completed:
-    sha256: edbd50d7e7c85c13ad5f5835ae2db92fab1e9cf05ecf85340e2622ec0a303df1
+    sha256: f519f0130813ac275982b1a378f4ffd38680394a98ce27c76341e7f7e94ee0a8
-    size: 720020
+    size: 722616
-    url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/22/34.yaml
+    url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/nightly/2025/7/4.yaml
-  original:
+  original: nightly-2025-07-04
-    url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/22/34.yaml