Allow defining multiple variables with one `let`.

.editorconfig

@@ -0,0 +1,11 @@
+root = true
+
+[*]
+charset = utf-8
+indent_style = space
+indent_size = 4
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+[*.yml]
+indent_size = 2

README.md

@@ -9,7 +9,7 @@ Exit the prompt with `Ctrl-c` (or equivalent).
 
 ### Example session
 ```
->> let D = (\x. x x) in let F = (\f. f (f y)) in D (F (\x. x))
+>> let D = (\x. x x); F = (\f. f (f y)) in D (F (\x. x))
 (y y)
 >> let T = (\f x. f (f x)) in (\f x. T (T (T (T T))) f x) (\x. x) y
 y
@@ -29,4 +29,5 @@ and spaces are used to separate variables rather than commas.
 * Applications are left-associative: `M N P` may be written instead of `((M N) P)`.
 * The body of an abstraction or let expression extends as far right as possible: `\x. M N` means `\x.(M N)` and not `(\x. M) N`.
 * A sequence of abstractions may be contracted: `\foo. \bar. \baz. N` may be abbreviated as `\foo bar baz. N`.
-* Variables may be bound using let expressions: `let x = N in M` is syntactic sugar for `(\x. N) M`.
+* Variables may be bound using let expressions: `let x = N in M` is syntactic sugar for `(\x. N) M`.
+* Multiple variables may be defined in one let expression: `let x = N; y = O in M`

src/LambdaCalculus/Expression.hs

@@ -19,7 +19,7 @@ data Expression
 instance TextShow Expression where
   showb (Variable var) = fromText var
   showb (Application ef ex) = "(" <> showb ef <> " " <> showb ex <> ")"
-  showb (Abstraction var body) = "(\\" <> fromText var <> ". " <> showb body <> ")"
+  showb (Abstraction var body) = "(λ" <> fromText var <> ". " <> showb body <> ")"
 
 instance Show Expression where
   show = T.unpack . showt

src/LambdaCalculus/Parser.hs

@@ -8,52 +8,58 @@ import LambdaCalculus.Expression
 import Text.Parsec hiding (spaces)
 import Text.Parsec.Text
 
-spaces :: Parser ()
-spaces = void $ many1 space
+keywords :: [Text]
+keywords = ["let", "in"]
 
-variableName :: Parser Text
-variableName = do
-  notFollowedBy anyKeyword
-  T.pack <$> many1 letter
-  where anyKeyword = choice $ map keyword keywords
-        keywords = ["let", "in"]
-
--- | Match an exact string which is not just a substring
--- of a larger variable name.
+-- | A keyword is an exact string which is not part of an identifier.
 keyword :: Text -> Parser ()
-keyword kwd = try $ do
-  void $ string (T.unpack kwd)
-  notFollowedBy letter
+keyword kwd = do
+    void $ string (T.unpack kwd)
+    notFollowedBy letter
+
+-- | An identifier is a sequence of letters which is not a keyword.
+identifier :: Parser Text
+identifier = do
+    notFollowedBy anyKeyword
+    T.pack <$> many1 letter
+  where anyKeyword = choice $ map (try . keyword) keywords
 
 variable :: Parser Expression
-variable = Variable <$> variableName
+variable = Variable <$> identifier
+
+spaces :: Parser ()
+spaces = skipMany1 space
 
 application :: Parser Expression
 application = foldl1 Application <$> sepEndBy1 applicationTerm spaces
   where applicationTerm :: Parser Expression
-        applicationTerm = variable <|> abstraction <|> let_ <|> grouping
+        applicationTerm = abstraction <|> grouping <|> let_ <|> variable
           where grouping :: Parser Expression
                 grouping = between (char '(') (char ')') expression
 
 abstraction :: Parser Expression
 abstraction = do
-  char '\\'
-  optional spaces
-  names <- sepEndBy1 variableName spaces
-  char '.'
-  optional spaces
-  body <- expression
-  pure $ foldr Abstraction body names
+    char '\\' <|> char 'λ' ; optional spaces
+    names <- sepEndBy1 identifier spaces
+    char '.'
+    body <- expression
+    pure $ foldr Abstraction body names
 
 let_ :: Parser Expression
 let_ = do
-  keyword "let"
-  name <- between spaces (optional spaces) variableName
-  char '='
-  value <- expression
-  keyword "in"
-  body <- expression
-  pure $ Application (Abstraction name body) value
+    try (keyword "let") ; spaces
+    defs <- sepBy1 definition (char ';' *> optional spaces)
+    keyword "in"
+    body <- expression
+    pure $ foldr (uncurry letExpr) body defs
+  where definition :: Parser (Text, Expression)
+        definition = do
+            name <- identifier ; optional spaces
+            char '='
+            value <- expression
+            pure (name, value)
+        letExpr :: Text -> Expression -> Expression -> Expression
+        letExpr name value body = Application (Abstraction name body) value
 
 expression :: Parser Expression
 expression = optional spaces *> (abstraction <|> let_ <|> application <|> variable) <* optional spaces

test/Spec.hs

@@ -60,6 +60,7 @@ main = defaultMain $
       , testCase "unary application" $ parseExpression "(x)" @?= Right (Variable "x")
       , testCase "application shorthand" $ parseExpression "a b c d" @?= Right (Application (Application (Application (Variable "a") (Variable "b")) (Variable "c")) (Variable "d"))
       , testCase "let" $ parseExpression "let x = \\y.y in x" @?= Right (Application (Abstraction "x" (Variable "x")) (Abstraction "y" (Variable "y")))
+      , testCase "multi-let" $ parseExpression "let x = y; y = z in x y" @?= Right (Application (Abstraction "x" (Application (Abstraction "y" (Application (Variable "x") (Variable "y"))) (Variable "z"))) (Variable "y"))
       , testCase "ttttt" $ parseExpression "(\\T f x.(T (T (T (T T)))) f x) (\\f x.f (f x)) (\\x.x) y"
           @?= Right ttttt
       , testGroup "Redundant whitespace"