parsing, update readme

README.md

@@ -4,7 +4,8 @@ A (very WIP) lambda expression evaluator and debugger.
 
 I'd like to build the following:
 
-- [ ] parsing with parsec
+- [X] parsing with parsec
+  - need to harden tests a bit more but existing tests pass
 - [ ] alpha/beta reduction to beta normal
 - [ ] substitutions for known abstractions (numbers can be written and are displayed as numbers rather than lambda terms)
 - [ ] persisted variables

src/Parser.hs

@@ -1,36 +1,47 @@
 module Parser where
 
 import Control.Monad (void)
+import Data.Char (isLetter)
 import Text.Parsec
 import Text.Parsec.String (Parser)
 
-data Expr = Variable String | Abstraction (String, Expr) | Application (Expr, Expr)
+data Expr = Variable String | Abstraction String Expr | Application Expr Expr
   deriving (Eq)
 
 instance Show Expr where
   show (Variable name) = name
-  show (Abstraction (arg, body)) = "λ" ++ arg ++ ". " ++ show body
-  show (Application (f, x)) = "(" ++ show f ++ " " ++ show x ++ ")"
+  show (Abstraction arg body) = "λ" ++ arg ++ ". " ++ show body
+  show (Application f x) = "(" ++ show f ++ " " ++ show x ++ ")"
 
 ws :: Parser ()
 ws = skipMany (void space)
 
+lexeme :: Parser a -> Parser a
+lexeme p = p <* ws
+
 variable :: Parser Expr
-variable = do
-  name <- many letter
+variable = lexeme $ do
+  name <- many1 (satisfy (\c -> isLetter c && c /= 'λ'))
   return $ Variable name
 
 abstraction :: Parser Expr
-abstraction = do
+abstraction = lexeme $ do
   _ <- void (char 'λ')
-  variable <- many letter
-  body <- parseExpr
-  return $ Abstraction (variable, body)
+  name <- many letter
+  _ <- void (char '.' >> ws)
+  body <- application
+  return $ Abstraction name body
 
-parseExpr :: Parser Expr
-parseExpr = variable <|> abstraction
+application :: Parser Expr
+application = lexeme $ chainl1 atom (return Application)
+
+parens :: Parser Expr
+parens = lexeme $ between (char '(') (char ')') application
+
+atom :: Parser Expr
+atom = variable <|> abstraction <|> parens
 
 parse :: String -> Either String Expr
-parse content = case Text.Parsec.parse parseExpr "<input>" content of
+parse content = case Text.Parsec.parse application "<input>" content of
   Left err -> Left (show err)
   Right val -> Right val

test/ParserSpec.hs

@@ -9,39 +9,37 @@ spec = do
     it "can parse variable" $
       (parse "x") `shouldBe` Right (Variable "x")
     it "can parse identity abstraction" $
-      (parse "λx.x") `shouldBe` Right (Abstraction ("x", Variable "x"))
+      (parse "λx.x") `shouldBe` Right (Abstraction "x" (Variable "x"))
     it "can parse mockingbird of identity" $
       (parse "(λx.x x) (λx.x)") -- (λx.x) (λx.x) -> (λx.x)
         `shouldBe` Right
           ( Application
               ( Abstraction
-                  ( "x",
-                    Application
-                      ( Variable "x",
-                        Variable "x"
-                      )
-                  ),
-                Abstraction ("x", Variable "x")
+                  "x"
+                  ( Application
+                      (Variable "x")
+                      (Variable "x")
+                  )
               )
+              (Abstraction "x" (Variable "x"))
           )
     it "it can parse successor" $
       (parse "λn.λf.λx.f (n f x)")
         `shouldBe` Right
           ( Abstraction
-              ( "n",
-                Abstraction
-                  ( "f",
-                    Abstraction
-                      ( "x",
-                        Application
-                          ( Variable "f",
-                            Application
+              "n"
+              ( Abstraction
+                  "f"
+                  ( Abstraction
+                      "x"
+                      ( Application
+                          (Variable "f")
+                          ( Application
                               ( Application
-                                  ( Variable "n",
-                                    Variable "f"
-                                  ),
-                                Variable "x"
+                                  (Variable "n")
+                                  (Variable "f")
                               )
+                              (Variable "x")
                           )
                       )
                   )