From 032e26cf9ed236889637e02e56aef7c22721cd23 Mon Sep 17 00:00:00 2001
From: Akshay <nerdy@peppe.rs>
Date: Sat, 17 Oct 2020 09:27:40 +0530
Subject: prefer applicative over monadic style

---
 bin/Main.hs        | 13 +++++--------
 shell.nix          |  1 +
 src/Environment.hs |  7 ++++---
 src/Evaluator.hs   | 14 +++++++-------
 src/Parser.hs      |  1 -
 5 files changed, 17 insertions(+), 19 deletions(-)

diff --git a/bin/Main.hs b/bin/Main.hs
index c66d328..18e1c53 100644
--- a/bin/Main.hs
+++ b/bin/Main.hs
@@ -20,7 +20,7 @@ readExpr inp =
 
 evalExpr :: Env -> String -> IO (LispResult String)
 evalExpr env inp = runExceptT $ fmap show $
-    (liftLispResult $ readExpr inp) >>= eval env
+    liftLispResult (readExpr inp) >>= eval env
 
 repl :: Env -> IO ()
 repl env = do
@@ -29,20 +29,17 @@ repl env = do
     case inp of
       Nothing -> return ()
       Just ",q" -> return ()
-      Just i -> do
-          out <- evalExpr env i
-          either (putStrLn . pp i) putStrLn out
-          repl env
+      Just i -> evalExpr env i >>= either (putStrLn . pp i) putStrLn >> repl env
 
 
 main :: IO ()
 main = do
     args <- getArgs
-    initEnv <- newEnv
+    env <- newEnv
     if null args
        then do
            putStrLn ";;; Entering lisk repl ..."
-           repl initEnv
+           repl env
        else do
            let pp = showError defaults "(lisk-repl)"
-           evalExpr initEnv (head args) >>= (either (putStrLn . pp) print)
+           evalExpr env (head args) >>= either (putStrLn . pp) print
diff --git a/shell.nix b/shell.nix
index 15334f5..0f07252 100644
--- a/shell.nix
+++ b/shell.nix
@@ -33,6 +33,7 @@ let
     pkgs.cabal2nix
     haskellPackages.cabal-install
     haskellPackages.hoogle
+    haskellPackages.hlint
   ];
 in
 pkgs.stdenv.mkDerivation {
diff --git a/src/Environment.hs b/src/Environment.hs
index 7b3f365..4c444b6 100644
--- a/src/Environment.hs
+++ b/src/Environment.hs
@@ -8,7 +8,8 @@ module Environment ( Env
                    , IOResult
                    ) where
 
-import           Control.Monad        (liftM, mapM)
+import           Control.Applicative  ((<$>))
+import           Control.Monad        (mapM)
 import           Control.Monad.Except
 import           Data.IORef
 import           Data.Maybe           (isJust)
@@ -44,7 +45,7 @@ setVar :: Env -> String -> Expr -> IOResult ()
 setVar env var val = do
     ptr <- liftIO $ readIORef env
     maybe (throwError $ UnboundVariable var)
-          (liftIO . (flip writeIORef val))
+          (liftIO . flip writeIORef val)
           $ lookup var ptr
 
 defineVar :: Env -> String -> Expr -> IOResult ()
@@ -65,5 +66,5 @@ makeBind (var, val) = do
 manyBindings :: Env -> [(String, Expr)] -> IO Env
 manyBindings env binds = do
     ptr <- readIORef env
-    extendedEnv <- liftM (++ ptr) $ mapM makeBind binds
+    extendedEnv <- (++ ptr) <$> mapM makeBind binds
     newIORef extendedEnv
diff --git a/src/Evaluator.hs b/src/Evaluator.hs
index 1cc9edd..3d3be02 100644
--- a/src/Evaluator.hs
+++ b/src/Evaluator.hs
@@ -19,9 +19,9 @@ apply fn args = maybe
 evalUnquote :: Env -> Expr -> IOResult Expr
 evalUnquote env (List [Id "unquote", vs]) = eval env vs
 evalUnquote env (List [Id "quote", vs]) =
-    liftM (List . ([Id "quote"] ++ ) . (: [])) (evalUnquote env vs)
-evalUnquote env (List vs)   = liftM List $ mapM (evalUnquote env) vs
-evalUnquote env (Vector vs) = liftM Vector $ mapM (evalUnquote env) vs
+    fmap (List . ([Id "quote"] ++ ) . (: [])) (evalUnquote env vs)
+evalUnquote env (List vs)   = List <$> mapM (evalUnquote env) vs
+evalUnquote env (Vector vs) = Vector <$> mapM (evalUnquote env) vs
 evalUnquote env literal = return literal
 
 evalQuasiQuote :: Env -> Expr -> IOResult Expr
@@ -35,12 +35,12 @@ eval _ v@(IntLiteral i)    = return v
 eval _ v@(BoolLiteral b)   = return v
 eval env (Id l)            = getVar env l
 eval _ v@(FloatLiteral f)  = return v
-eval env v@(Vector xs)     = liftM Vector $ mapM (eval env) xs
+eval env v@(Vector xs)     = Vector <$> mapM (eval env) xs
 eval env (List[Id "quote", val])      = return val
 eval env (List[Id "quasiquote", val]) = evalQuasiQuote env val
-eval env v@(List[Id "unquote", val])    = throwError $ BadForm "Cannot use `unquote` form outside quasiquote context" v
-eval env (List [Id "set!", Id var, val]) = eval env val >>= uncurry (*>) . ((setVar env var) &&& pure)
-eval env (List [Id "define", Id var, val]) = eval env val >>= uncurry (*>) . ((defineVar env var) &&& pure)
+eval env v@(List[Id "unquote", val])    = throwError $ BadForm "Cannot use `unquote` outside quasiquote form" v
+eval env (List [Id "set!", Id var, val]) = eval env val >>= uncurry (*>) . (setVar env var &&& pure)
+eval env (List [Id "define", Id var, val]) = eval env val >>= uncurry (*>) . (defineVar env var &&& pure)
 eval env (List (Id fn : args))        = mapM (eval env) args >>= liftLispResult . apply fn
 eval env NoReturn = throwError $ BadForm "Invalid usage of non-returning expression" NoReturn
 
diff --git a/src/Parser.hs b/src/Parser.hs
index a7f5571..6ec4ba0 100644
--- a/src/Parser.hs
+++ b/src/Parser.hs
@@ -8,7 +8,6 @@ module Parser ( parseLispValue
               ) where
 
 import           Control.Applicative           ((<$>))
-import           Control.Monad                 (liftM)
 import           Text.ParserCombinators.Parsec
 
 -- TODO: use LispNumber (src/Operators.hs) here instead of IntLiteral and FloatLiteral
-- 
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