expressions

app/Main.hs

@@ -9,13 +9,12 @@ run source = do
     case tokensMaybe of
         Left UnexpectedCharacterError -> putStrLn "Unexpected character"
         Right tokens -> do
-            let exprMaybe = parse tokens
-            case exprMaybe of
+            let stmtMaybe = parse tokens
+            case stmtMaybe of
                 Left ExpectedExpressionError -> putStrLn "Expected expression"
                 Left MismatchedParenthesesError -> putStrLn "Mismatched parentheses"
-                Right expr -> do
-                    result <- eval expr
-                    print result
+                Left ExpectedSemicolonError -> putStrLn "Expected semicolon"
+                Right statements -> runStatements statements
 
 main :: IO ()
 main = putStr ">> " >> hFlush stdout >> getLine >>= run 

src/Lox/Expr.hs

@@ -1,9 +1,15 @@
 module Lox.Expr (
+    Stmt (..),
     Expr (..)
 ) where
 
 import Lox.Scanner
 
+data Stmt =
+    Expression Expr |
+    Print Expr
+    deriving Show
+
 data Expr = 
     Literal Object | 
     Unary Token Expr | 

src/Lox/Interpreter.hs

@@ -1,30 +1,48 @@
 module Lox.Interpreter (
+    runStatements,
     eval   
 ) where
 
 import Lox.Expr
 import Lox.Scanner
 import Control.Monad.State
+import Control.Monad
 
-data InterpreterState = InterpreterState
+data InterpreterState = InterpreterState (IO ())
+
+emptyInterpreter :: InterpreterState
+emptyInterpreter = InterpreterState (return ())
+
+runStatements :: [Stmt] -> IO ()
+runStatements s = io
+    where InterpreterState io = execState (interpret s) emptyInterpreter
+
+interpret :: [Stmt] -> State InterpreterState ()
+interpret = foldr ((>>) . execute) (return ())
+
+execute :: Stmt -> State InterpreterState ()
+execute (Print expr) = do
+    value <- evalFrom expr
+    modify (\(InterpreterState s) -> InterpreterState (s >> print value))
+execute (Expression value) = void $ evalFrom value
 
 eval :: Expr -> IO Object
-eval expr = return $ evalState (interpret expr) InterpreterState
+eval expr = return $ evalState (evalFrom expr) $ InterpreterState (return ())
 
-interpret :: Expr -> State InterpreterState Object
-interpret (Literal value) = return value
-interpret (Grouping expr) = interpret expr
-interpret (Unary op expr) = do 
-    right <- interpret expr
+evalFrom :: Expr -> State InterpreterState Object
+evalFrom (Literal value) = return value
+evalFrom (Grouping expr) = evalFrom expr
+evalFrom (Unary op expr) = do 
+    right <- evalFrom expr
     case (tokenType 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
+evalFrom (Binary leftExpr op rightExpr) = do
+    left <- evalFrom leftExpr
+    right <- evalFrom rightExpr
     case (tokenType op, left, right) of
         (PLUS, NumberObject x, NumberObject y) -> return $ NumberObject (x + y)
         (MINUS, NumberObject x, NumberObject y) -> return $ NumberObject (x - y)

src/Lox/Parser.hs

@@ -10,7 +10,19 @@ import Lox.Expr
 
 data ParserState = ParserState {tokens :: [Token]}
 
-data ParserError = MismatchedParenthesesError | ExpectedExpressionError
+data ParserError = MismatchedParenthesesError 
+                 | ExpectedExpressionError 
+                 | ExpectedSemicolonError
+    deriving Show
+
+-- program        → statement* EOF ;
+--
+-- statement      → exprStmt
+--                | printStmt ;
+--
+-- exprStmt       → expression ";" ;
+-- printStmt      → "print" expression ";" ;
+
 
 -- expression     → equality ;
 -- equality       → comparison ( ( "!=" | "==" ) comparison )* ;
@@ -22,8 +34,47 @@ data ParserError = MismatchedParenthesesError | ExpectedExpressionError
 -- primary        → NUMBER | STRING | "true" | "false" | "nil"
 --                | "(" expression ")" ;
 
-parse :: [Token] -> Either ParserError Expr
-parse tokens = evalState expression (ParserState {tokens=tokens})
+parse :: [Token] -> Either ParserError [Stmt]
+parse tokens = evalState program (ParserState {tokens=tokens})
+
+program :: State ParserState (Either ParserError [Stmt])
+program = do
+    atEnd <- isAtEnd
+    if atEnd then return $ Right [] else do
+        headMaybe <- statement
+        case headMaybe of
+            Left err -> return $ Left err
+            Right head -> do
+                tailMaybe <- program
+                case tailMaybe of
+                    Left err -> return $ Left err
+                    Right tail -> return $ Right $ head : tail
+
+statement :: State ParserState (Either ParserError Stmt)
+statement = do
+    printMaybe <- matchToken [PRINT]
+    case printMaybe of
+        Just _ -> printStatement
+        _ -> expressionStatement
+
+printStatement :: State ParserState (Either ParserError Stmt)
+printStatement = do
+    valueMaybe <- expression
+    semicolonMaybe <- consume SEMICOLON ExpectedSemicolonError
+    case (valueMaybe, semicolonMaybe) of
+        (Left err, _) -> return $ Left err
+        (_, Left err) -> return $ Left err
+        (Right value, Right _) -> return $ Right $ Print value
+
+expressionStatement :: State ParserState (Either ParserError Stmt)
+expressionStatement = do
+    valueMaybe <- expression
+    semicolonMaybe <- consume SEMICOLON ExpectedSemicolonError
+    case (valueMaybe, semicolonMaybe) of
+        (Left err, _) -> return $ Left err
+        (_, Left err) -> return $ Left err
+        (Right value, Right _) -> return $ Right $ Expression value
+    
 
 expression :: State ParserState (Either ParserError Expr)
 expression = equality

src/Lox/Scanner.hs

@@ -31,7 +31,7 @@ data Object = NullObject
 
 instance Show Object where
     show NullObject = "Nil"
-    show (StringObject s) = show s
+    show (StringObject s) = s
     show (NumberObject x) = show x
     show (BoolObject False) = "false"
     show (BoolObject True) = "true"
@@ -45,7 +45,7 @@ data Token = Token {
 
 data ScannerState = ScannerState {source :: String, current :: String, lineNumber :: Int}
 
-data ScannerError = UnexpectedCharacterError
+data ScannerError = UnexpectedCharacterError deriving Show
 
 emptyScannerState :: String -> ScannerState
 emptyScannerState source = 
@@ -96,7 +96,7 @@ scanToken = do
         ' ' -> return nothing
         '\r' -> return nothing
         '\t' -> return nothing
-        '\n' -> return nothing
+        '\n' -> modify (\s@(ScannerState {lineNumber=n}) -> s {lineNumber=n+1}) >> return nothing
         c -> if isDigit c then ok <$> scanNumber else if isAlpha c then ok <$> scanIdentifier else return $ Left UnexpectedCharacterError
 
 scanString :: State ScannerState Token