interpreter first version

app/Main.hs

@@ -1,10 +1,12 @@
-module Main where
-
 import Lox.Scanner
+import Lox.Parser
+import Lox.Interpreter
 
 run :: String -> IO ()
-run source = mapM_ print tokens
+run source = print result
-    where tokens = scanTokensFromSource source
+    where result = eval expr
+          expr = parse tokens
+          tokens = scanTokensFromSource source
 
 main :: IO ()
 main = getLine >>= run 

crafting-interpreters-hs.cabal

@@ -54,7 +54,7 @@ common warnings
     ghc-options: -Wall
 
 library
-    exposed-modules:  Lox.Scanner, Lox.Expr, Lox.Parser
+    exposed-modules:  Lox.Scanner, Lox.Expr, Lox.Parser, Lox.Interpreter
     build-depends:    base ^>=4.18.3.0, mtl, extra
     hs-source-dirs:   src
     default-language: Haskell2010

src/Lox/Interpreter.hs

@@ -0,0 +1,44 @@
+module Lox.Interpreter (
+    eval   
+) where
+
+import Lox.Expr
+import Lox.Scanner
+import Control.Monad.State
+
+data InterpreterState = InterpreterState
+
+eval :: Expr -> Object
+eval expr = evalState (interpret expr) InterpreterState
+
+interpret :: Expr -> State InterpreterState Object
+interpret (Literal value) = return value
+interpret (Grouping expr) = interpret expr
+interpret (Unary op expr) = do 
+    right <- interpret expr
+    case (getType op, right) of
+        (MINUS, NumberObject x) -> return $ NumberObject (-x)
+        (BANG, NullObject) -> return $ BoolObject False
+        (BANG, BoolObject x) -> return $ BoolObject (not x)
+        (BANG, _) -> return $ BoolObject True
+        _ -> error "Type error"
+interpret (Binary leftExpr op rightExpr) = do
+    left <- interpret leftExpr
+    right <- interpret rightExpr
+    case (getType op, left, right) of
+        (PLUS, NumberObject x, NumberObject y) -> return $ NumberObject (x + y)
+        (MINUS, NumberObject x, NumberObject y) -> return $ NumberObject (x - y)
+        (SLASH, NumberObject x, NumberObject y) -> return $ NumberObject (x / y)
+        (STAR, NumberObject x, NumberObject y) -> return $ NumberObject (x * y)
+
+        (GREATER, NumberObject x, NumberObject y) -> return $ BoolObject (x > y)
+        (GREATER_EQUAL, NumberObject x, NumberObject y) -> return $ BoolObject (x >= y)
+        (LESS, NumberObject x, NumberObject y) -> return $ BoolObject (x < y)
+        (LESS_EQUAL, NumberObject x, NumberObject y) -> return $ BoolObject (x <= y)
+
+        (PLUS, StringObject s, StringObject t) -> return $ StringObject (s ++ t)
+
+        (EQUAL_EQUAL, x, y) -> return $ BoolObject (x == y)
+        
+        _ -> error "Type error"
+

src/Lox/Scanner.hs

@@ -22,7 +22,18 @@ data TokenType = LEFT_PAREN | RIGHT_PAREN | LEFT_BRACE | RIGHT_BRACE
                | EOF
     deriving (Show, Eq)
 
-data Object = NullObject | StringObject String | NumberObject Double | BoolObject Bool deriving Show
+data Object = NullObject 
+            | StringObject String 
+            | NumberObject Double 
+            | BoolObject Bool 
+    deriving (Eq)
+
+instance Show Object where
+    show NullObject = "Nil"
+    show (StringObject s) = show s
+    show (NumberObject x) = show x
+    show (BoolObject False) = "false"
+    show (BoolObject True) = "true"
 
 data Token = Token {
     getType :: TokenType,