Make the printer smarter, separate intermediate AST data type.

* The expression printer now knows how to use `let`, multi-argument lambdas and applications, and block arguments when appropriate.
* There is a separate type, AbstractSyntax, which separates parsing/printing logic from removing/reintroducing the more advanced syntax described above.
* Expression is now its own module because its 'show' depends on AbstractSyntax,
  and I don't want the ast2expr/expr2ast stuff to be in the same module as the real lambda calculus stuff.

README.md

@@ -9,12 +9,12 @@ Exit the prompt with `Ctrl-c` (or equivalent).
 
 ### Example session
 ```
->> 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
+>> 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
 >> (\x y z. x y) y
-(\y'. (\z. (y y')))
+λy' z. y y'
 >> ^C
 ```
 

app/Main.hs

@@ -4,7 +4,7 @@ module Main where
 import Control.Monad (forever)
 import Data.Text
 import qualified Data.Text.IO as TIO
-import LambdaCalculus.Expression (eagerEval)
+import LambdaCalculus (eagerEval)
 import LambdaCalculus.Parser (parseExpression)
 import System.IO (hFlush, stdout)
 

package.yaml

@@ -13,7 +13,9 @@ extra-source-files:
 - README.md
 
 default-extensions:
+- ImportQualifiedPost
 - OverloadedStrings
+- ViewPatterns
 
 dependencies:
 - base >= 4.13 && < 5

src/LambdaCalculus.hs

@@ -0,0 +1,136 @@
+module LambdaCalculus
+    ( module LambdaCalculus.Expression
+    , eagerEval, lazyEval
+    ) where
+
+import Data.List (elemIndex, find)
+import Data.Maybe (fromJust)
+import Data.HashSet (HashSet)
+import Data.HashSet qualified as HS
+import Data.Text (Text)
+import Data.Text qualified as T
+import LambdaCalculus.Expression (Expression (..))
+
+-- | Free variables are variables which are present in an expression but not bound by any abstraction.
+freeVariables :: Expression -> HashSet Text
+freeVariables (Variable variable) = HS.singleton variable
+freeVariables (Application ef ex) = freeVariables ef `HS.union` freeVariables ex
+freeVariables (Abstraction variable body) = HS.delete variable $ freeVariables body
+
+-- | Return True if the given variable is free in the given expression.
+freeIn :: Text -> Expression -> Bool
+freeIn var1 (Variable var2) = var1 == var2
+freeIn var (Application ef ex) = var `freeIn` ef && var `freeIn` ex
+freeIn var1 (Abstraction var2 body) = var1 == var2 || var1 `freeIn` body
+
+-- | Bound variables are variables which are bound by any abstraction in an expression.
+boundVariables :: Expression -> HashSet Text
+boundVariables (Variable _) = HS.empty
+boundVariables (Application ef ex) = boundVariables ef `HS.union` boundVariables ex
+boundVariables (Abstraction variable body) = HS.insert variable $ boundVariables body
+
+-- | A closed expression is an expression with no free variables.
+-- Closed expressions are also known as combinators and are equivalent to terms in combinatory logic.
+closed :: Expression -> Bool
+closed = HS.null . freeVariables
+
+-- | Alpha-equivalent terms differ only by the names of bound variables,
+-- i.e. one can be converted to the other using only alpha-conversion.
+alphaEquivalent :: Expression -> Expression -> Bool
+alphaEquivalent = alphaEquivalent' [] []
+  where alphaEquivalent' :: [Text] -> [Text] -> Expression -> Expression -> Bool
+        alphaEquivalent' ctx1 ctx2 (Variable v1) (Variable v2)
+          -- Two variables are alpha-equivalent if they are bound in the same location.
+          = bindingSite ctx1 v1 == bindingSite ctx2 v2
+        alphaEquivalent' ctx1 ctx2 (Application ef1 ex1) (Application ef2 ex2)
+          -- Two applications are alpha-equivalent if their components are alpha-equivalent.
+          = alphaEquivalent' ctx1 ctx2 ef1 ef2
+          && alphaEquivalent' ctx1 ctx2 ex1 ex2
+        alphaEquivalent' ctx1 ctx2 (Abstraction v1 b1) (Abstraction v2 b2)
+          -- Two abstractions are alpha-equivalent if their bodies are alpha-equivalent.
+          = alphaEquivalent' (v1 : ctx1) (v2 : ctx2) b1 b2
+
+        -- | The binding site of a variable is either the index of its binder
+        -- or, if it is unbound, the name of the free variable.
+        bindingSite :: [Text] -> Text -> Either Text Int
+        bindingSite ctx var = maybeToRight var $ var `elemIndex` ctx
+          where maybeToRight :: b -> Maybe a -> Either b a
+                maybeToRight default_ = maybe (Left default_) Right
+
+-- | Substitution is the process of replacing all free occurrences of a variable in one expression with another expression.
+substitute :: Text -> Expression -> Expression -> Expression
+substitute var1 value unmodified@(Variable var2)
+  | var1 == var2 = value
+  | otherwise = unmodified
+substitute var value (Application ef ex)
+  = Application (substitute var value ef) (substitute var value ex)
+substitute var1 value unmodified@(Abstraction var2 body)
+  | var1 == var2 = unmodified
+  | otherwise = Abstraction var2' $ substitute var1 value $ alphaConvert var2 var2' body
+  where var2' :: Text
+        var2' = escapeName (freeVariables value) var2
+
+        alphaConvert :: Text -> Text -> Expression -> Expression
+        alphaConvert oldName newName expr = substitute oldName (Variable newName) expr
+        -- | Generate a new name which isn't present in the set, based on the old name.
+        escapeName :: HashSet Text -> Text -> Text
+        escapeName env name = fromJust $ find (not . free) names
+          where names :: [Text]
+                names = name : map (`T.snoc` '\'') names
+                free :: Text -> Bool
+                free = (`HS.member` env)
+
+-- | Returns True if the top-level expression is reducible by beta-reduction.
+betaRedex :: Expression -> Bool
+betaRedex (Application (Abstraction _ _) _) = True
+betaRedex _ = False
+
+-- | Returns True if the top-level expression is reducible by eta-reduction.
+etaRedex :: Expression -> Bool
+etaRedex (Abstraction var1 (Application ef (Variable var2)))
+  = var1 /= var2 || var1 `freeIn` ef
+etaRedex _ = False
+
+-- | In an expression in normal form, all reductions that can be applied have been applied.
+-- This is the result of applying eager evaluation.
+normal :: Expression -> Bool
+-- The expression is beta-reducible.
+normal (Application (Abstraction _ _) _) = False
+-- The expression is eta-reducible.
+normal (Abstraction var1 (Application fe (Variable var2)))
+  = var1 /= var2 || var1 `freeIn` fe
+normal (Application ef ex) = normal ef && normal ex
+normal _ = True
+
+-- | In an expression in weak head normal form, reductions to the function have been applied,
+-- but not all reductions to the parameter have been applied.
+-- This is the result of applying lazy evaluation.
+whnf :: Expression -> Bool
+whnf (Application (Abstraction _ _) _) = False
+whnf (Abstraction var1 (Application fe (Variable var2)))
+  = var1 /= var2 || var1 `freeIn` fe
+whnf (Application ef _) = whnf ef
+
+eval :: (Expression -> Expression) -> Expression -> Expression
+eval strategy = eval'
+  where eval' :: Expression -> Expression
+        eval' (Application ef ex) =
+          case ef' of
+            -- Beta-reduction
+            Abstraction var body -> eval' $ substitute var ex' body
+            _ -> Application ef' ex'
+          where ef' = eval' ef
+                ex' = strategy ex
+        eval' unmodified@(Abstraction var1 (Application ef (Variable var2)))
+          -- Eta-reduction
+          | var1 == var2 && not (var1 `freeIn` ef) = eval' ef
+          | otherwise = unmodified
+        eval' x = x
+
+-- | Reduce an expression to normal form.
+eagerEval :: Expression -> Expression
+eagerEval = eval eagerEval
+
+-- | Reduce an expression to weak head normal form.
+lazyEval :: Expression -> Expression
+lazyEval = eval id

src/LambdaCalculus/Expression.hs

@@ -1,149 +1,73 @@
 {-# LANGUAGE DeriveGeneric #-}
 module LambdaCalculus.Expression where
 
-import Data.List (elemIndex, find)
-import Data.Maybe (fromJust)
-import Data.HashSet (HashSet)
-import qualified Data.HashSet as HS
+-- The definition of Expression is in its own file because:
+-- * Expression and AbstractSyntax should not be in the same file
+-- * Expression's `show` definition depends on AbstractSyntax's show definition,
+--   which means that `ast2expr` and `expr2ast` can't be in AbstractSyntax
+--   because of mutually recursive modules
+-- * I don't want to clutter the module focusing on the actual evaluation
+--   with all of these irrelevant conversion operators.
+
+import Data.Bifunctor (first, second)
 import Data.Text (Text)
-import qualified Data.Text as T
+import Data.Text qualified as T
 import GHC.Generics (Generic)
+import LambdaCalculus.Parser.AbstractSyntax (AbstractSyntax)
+import LambdaCalculus.Parser.AbstractSyntax qualified as AST
 import TextShow
 
 data Expression
-  = Variable Text
-  | Application Expression Expression
-  | Abstraction Text Expression
-  deriving (Eq, Generic)
+    = Variable Text
+    | Application Expression Expression
+    | Abstraction Text Expression
+    deriving (Eq, Generic)
+
+-- | A naive implementation of 'show', which does not take advantage of any syntactic sugar
+-- and always emits optional parentheses.
+basicShow :: Expression -> Builder
+basicShow (Variable var) = fromText var
+basicShow (Application ef ex) = "(" <> showb ef <> " " <> showb ex <> ")"
+basicShow (Abstraction var body) = "(λ" <> fromText var <> ". " <> showb body <> ")"
+
+-- | Convert from an abstract syntax tree to an expression.
+ast2expr :: AbstractSyntax -> Expression
+ast2expr (AST.Variable name) = Variable name
+ast2expr (AST.Application []) = Abstraction "x" (Variable "x")
+ast2expr (AST.Application [x]) = ast2expr x
+ast2expr (AST.Application xs) = foldl1 Application $ map ast2expr xs
+ast2expr (AST.Abstraction [] body) = ast2expr body
+ast2expr (AST.Abstraction names body) = foldr Abstraction (ast2expr body) names
+ast2expr (AST.Let defs body) = foldr (uncurry letExpr . second ast2expr) (ast2expr body) defs
+  where letExpr :: Text -> Expression -> Expression -> Expression
+        letExpr name val body = Application (Abstraction name body) val
+
+-- | View nested applications of abstractions as a list.
+viewLet :: Expression -> ([(Text, Expression)], Expression)
+viewLet (Application (Abstraction var body) x) = first ((var, x) :) (viewLet body)
+viewLet x = ([], x)
+
+-- | View nested abstractions as a list.
+viewAbstraction :: Expression -> ([Text], Expression)
+viewAbstraction (Abstraction name body) = first (name :) (viewAbstraction body)
+viewAbstraction x = ([], x)
+
+-- | View left-nested applications as a list.
+viewApplication :: Expression -> [Expression]
+viewApplication (Application ef ex) = ex : viewApplication ef
+viewApplication x = [x]
+
+-- | Convert from an expression to an abstract syntax tree.
+--
+-- This function will use let, and applications and abstractions of multiple values when possible.
+expr2ast :: Expression -> AbstractSyntax
+expr2ast (viewLet -> (defs@(_:_), body)) = AST.Let (map (second expr2ast) defs) $ expr2ast body
+expr2ast (viewAbstraction -> (names@(_:_), body)) = AST.Abstraction names $ expr2ast body
+expr2ast (viewApplication -> es@(_:_:_)) = AST.Application $ map expr2ast $ reverse es
+expr2ast (Variable name) = AST.Variable name
 
 instance TextShow Expression where
-  showb (Variable var) = fromText var
-  showb (Application ef ex) = "(" <> showb ef <> " " <> showb ex <> ")"
-  showb (Abstraction var body) = "(λ" <> fromText var <> ". " <> showb body <> ")"
+  showb = showb . expr2ast
 
 instance Show Expression where
   show = T.unpack . showt
-
--- | Free variables are variables which are present in an expression but not bound by any abstraction.
-freeVariables :: Expression -> HashSet Text
-freeVariables (Variable variable) = HS.singleton variable
-freeVariables (Application ef ex) = freeVariables ef `HS.union` freeVariables ex
-freeVariables (Abstraction variable body) = HS.delete variable $ freeVariables body
-
--- | Return True if the given variable is free in the given expression.
-freeIn :: Text -> Expression -> Bool
-freeIn var1 (Variable var2) = var1 == var2
-freeIn var (Application ef ex) = var `freeIn` ef && var `freeIn` ex
-freeIn var1 (Abstraction var2 body) = var1 == var2 || var1 `freeIn` body
-
--- | Bound variables are variables which are bound by any abstraction in an expression.
-boundVariables :: Expression -> HashSet Text
-boundVariables (Variable _) = HS.empty
-boundVariables (Application ef ex) = boundVariables ef `HS.union` boundVariables ex
-boundVariables (Abstraction variable body) = HS.insert variable $ boundVariables body
-
--- | A closed expression is an expression with no free variables.
--- Closed expressions are also known as combinators and are equivalent to terms in combinatory logic.
-closed :: Expression -> Bool
-closed = HS.null . freeVariables
-
--- | Alpha-equivalent terms differ only by the names of bound variables,
--- i.e. one can be converted to the other using only alpha-conversion.
-alphaEquivalent :: Expression -> Expression -> Bool
-alphaEquivalent = alphaEquivalent' [] []
-  where alphaEquivalent' :: [Text] -> [Text] -> Expression -> Expression -> Bool
-        alphaEquivalent' ctx1 ctx2 (Variable v1) (Variable v2)
-          -- Two variables are alpha-equivalent if they are bound in the same location.
-          = bindingSite ctx1 v1 == bindingSite ctx2 v2
-        alphaEquivalent' ctx1 ctx2 (Application ef1 ex1) (Application ef2 ex2)
-          -- Two applications are alpha-equivalent if their components are alpha-equivalent.
-          = alphaEquivalent' ctx1 ctx2 ef1 ef2
-          && alphaEquivalent' ctx1 ctx2 ex1 ex2
-        alphaEquivalent' ctx1 ctx2 (Abstraction v1 b1) (Abstraction v2 b2)
-          -- Two abstractions are alpha-equivalent if their bodies are alpha-equivalent.
-          = alphaEquivalent' (v1 : ctx1) (v2 : ctx2) b1 b2
-
-        -- | The binding site of a variable is either the index of its binder
-        -- or, if it is unbound, the name of the free variable.
-        bindingSite :: [Text] -> Text -> Either Text Int
-        bindingSite ctx var = maybeToRight var $ var `elemIndex` ctx
-          where maybeToRight :: b -> Maybe a -> Either b a
-                maybeToRight default_ = maybe (Left default_) Right
-
--- | Substitution is the process of replacing all free occurrences of a variable in one expression with another expression.
-substitute :: Text -> Expression -> Expression -> Expression
-substitute var1 value unmodified@(Variable var2)
-  | var1 == var2 = value
-  | otherwise = unmodified
-substitute var value (Application ef ex)
-  = Application (substitute var value ef) (substitute var value ex)
-substitute var1 value unmodified@(Abstraction var2 body)
-  | var1 == var2 = unmodified
-  | otherwise = Abstraction var2' $ substitute var1 value $ alphaConvert var2 var2' body
-  where var2' :: Text
-        var2' = escapeName (freeVariables value) var2
-
-        alphaConvert :: Text -> Text -> Expression -> Expression
-        alphaConvert oldName newName expr = substitute oldName (Variable newName) expr
-        -- | Generate a new name which isn't present in the set, based on the old name.
-        escapeName :: HashSet Text -> Text -> Text
-        escapeName env name = fromJust $ find (not . free) names
-          where names :: [Text]
-                names = name : map (`T.snoc` '\'') names
-                free :: Text -> Bool
-                free = (`HS.member` env)
-
--- | Returns True if the top-level expression is reducible by beta-reduction.
-betaRedex :: Expression -> Bool
-betaRedex (Application (Abstraction _ _) _) = True
-betaRedex _ = False
-
--- | Returns True if the top-level expression is reducible by eta-reduction.
-etaRedex :: Expression -> Bool
-etaRedex (Abstraction var1 (Application ef (Variable var2)))
-  = var1 /= var2 || var1 `freeIn` ef
-etaRedex _ = False
-
--- | In an expression in normal form, all reductions that can be applied have been applied.
--- This is the result of applying eager evaluation.
-normal :: Expression -> Bool
--- The expression is beta-reducible.
-normal (Application (Abstraction _ _) _) = False
--- The expression is eta-reducible.
-normal (Abstraction var1 (Application fe (Variable var2)))
-  = var1 /= var2 || var1 `freeIn` fe
-normal (Application ef ex) = normal ef && normal ex
-normal _ = True
-
--- | In an expression in weak head normal form, reductions to the function have been applied,
--- but not all reductions to the parameter have been applied.
--- This is the result of applying lazy evaluation.
-whnf :: Expression -> Bool
-whnf (Application (Abstraction _ _) _) = False
-whnf (Abstraction var1 (Application fe (Variable var2)))
-  = var1 /= var2 || var1 `freeIn` fe
-whnf (Application ef _) = whnf ef
-
-eval :: (Expression -> Expression) -> Expression -> Expression
-eval strategy = eval'
-  where eval' :: Expression -> Expression
-        eval' (Application ef ex) =
-          case ef' of
-            -- Beta-reduction
-            Abstraction var body -> eval' $ substitute var ex' body
-            _ -> Application ef' ex'
-          where ef' = eval' ef
-                ex' = strategy ex
-        eval' unmodified@(Abstraction var1 (Application ef (Variable var2)))
-          -- Eta-reduction
-          | var1 == var2 && not (var1 `freeIn` ef) = eval' ef
-          | otherwise = unmodified
-        eval' x = x
-
--- | Reduce an expression to normal form.
-eagerEval :: Expression -> Expression
-eagerEval = eval eagerEval
-
--- | Reduce an expression to weak head normal form.
-lazyEval :: Expression -> Expression
-lazyEval = eval id

src/LambdaCalculus/Parser.hs

@@ -1,12 +1,17 @@
-module LambdaCalculus.Parser (parseExpression) where
+module LambdaCalculus.Parser
+    ( parseAST, parseExpression
+    ) where
 
 import Control.Applicative ((*>))
 import Control.Monad (void)
 import Data.Text (Text)
 import qualified Data.Text as T
-import LambdaCalculus.Expression
+import LambdaCalculus.Expression (Expression, ast2expr)
+import qualified LambdaCalculus.Expression as Expr
+import LambdaCalculus.Parser.AbstractSyntax
 import Text.Parsec hiding (spaces)
 import Text.Parsec.Text
+import TextShow
 
 keywords :: [Text]
 keywords = ["let", "in"]
@@ -18,51 +23,50 @@ keyword kwd = do
     notFollowedBy letter
 
 -- | An identifier is a sequence of letters which is not a keyword.
-identifier :: Parser Text
+identifier :: Parser Identifier
 identifier = do
     notFollowedBy anyKeyword
     T.pack <$> many1 letter
   where anyKeyword = choice $ map (try . keyword) keywords
 
-variable :: Parser Expression
+variable :: Parser AbstractSyntax
 variable = Variable <$> identifier
 
 spaces :: Parser ()
 spaces = skipMany1 space
 
-application :: Parser Expression
-application = foldl1 Application <$> sepEndBy1 applicationTerm spaces
-  where applicationTerm :: Parser Expression
+application :: Parser AbstractSyntax
+application = Application <$> sepEndBy1 applicationTerm spaces
+  where applicationTerm :: Parser AbstractSyntax
         applicationTerm = abstraction <|> grouping <|> let_ <|> variable
-          where grouping :: Parser Expression
+          where grouping :: Parser AbstractSyntax
                 grouping = between (char '(') (char ')') expression
 
-abstraction :: Parser Expression
+abstraction :: Parser AbstractSyntax
 abstraction = do
     char '\\' <|> char 'λ' ; optional spaces
     names <- sepEndBy1 identifier spaces
     char '.'
-    body <- expression
-    pure $ foldr Abstraction body names
+    Abstraction names <$> expression
 
-let_ :: Parser Expression
+let_ :: Parser AbstractSyntax
 let_ = do
     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)
+    Let defs <$> expression
+  where definition :: Parser Definition
         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 :: Parser AbstractSyntax
 expression = optional spaces *> (abstraction <|> let_ <|> application <|> variable) <* optional spaces
 
+parseAST :: Text -> Either ParseError AbstractSyntax
+parseAST = parse (expression <* eof) "input"
+
 parseExpression :: Text -> Either ParseError Expression
-parseExpression = parse (expression <* eof) "input"
+parseExpression = fmap ast2expr . parseAST

src/LambdaCalculus/Parser/AbstractSyntax.hs

@@ -0,0 +1,55 @@
+module LambdaCalculus.Parser.AbstractSyntax
+    ( AbstractSyntax (..), Definition, Identifier
+    ) where
+
+import Data.Bifunctor (first)
+import Data.Text (Text)
+import Data.Text qualified as T
+import TextShow
+
+-- | The abstract syntax tree reflects the structure of the externally-visible syntax.
+--
+-- This contains a lot of syntactic sugar when compared with 'LambdaCalculus.Expression'.
+-- If this syntactic sugar were used in Expression, then operations like evaluation
+-- would become unnecessarily complicated, because the same expression
+-- can be represented in terms of multiple abstract syntax trees.
+data AbstractSyntax
+    = Variable Identifier
+    | Application [AbstractSyntax]
+    | Abstraction [Identifier] AbstractSyntax
+    | Let [Definition] AbstractSyntax
+type Definition = (Identifier, AbstractSyntax)
+type Identifier = Text
+
+-- I'm surprised this isn't in base somewhere.
+unsnoc :: [a] -> ([a], a)
+unsnoc [x] = ([], x)
+unsnoc (x : xs) = first (x :) (unsnoc xs)
+
+instance TextShow AbstractSyntax where
+    showb = unambiguous
+      where
+        unambiguous, ambiguous :: AbstractSyntax -> Builder
+        unambiguous (Variable name) = fromText name
+        -- There's no technical reason for the AST to forbid nullary applications and so forth.
+        -- However the parser rejects them to avoid confusing edge cases like `let x=in`,
+        -- so they're forbidden here too so that `show` will never print anything the parser would refuse to accept.
+        unambiguous (Application []) = error "Empty applications are currently disallowed."
+        unambiguous (Application (unsnoc -> (es, final))) = foldr (\e es' -> ambiguous e <> " " <> es') final' es
+            where final' = case final of
+                    Application _ -> ambiguous final
+                    _             -> unambiguous final
+        unambiguous (Abstraction [] _) = error "Empty lambdas are currently disallowed."
+        unambiguous (Abstraction names body) = "λ" <> fromText (T.intercalate " " names) <> ". " <> unambiguous body
+        unambiguous (Let [] body) = error "Empty lets are currently disallowed."
+        unambiguous (Let defs body) = "let " <> fromText (T.intercalate "; " $ map (toText . showDef) defs) <> " in " <> unambiguous body
+          where showDef :: Definition -> Builder
+                showDef (name, val) = fromText name <> " = " <> unambiguous val
+
+        -- | Adds a grouper if omitting it could result in ambiguous syntax.
+        -- (Which is to say, the parser would parse it wrong because a different parse has a higher priority.)
+        ambiguous e@(Variable _) = unambiguous e
+        ambiguous e = "(" <> unambiguous e <> ")"
+
+instance Show AbstractSyntax where
+    show = T.unpack . showt

stack.yaml

@@ -1,3 +1,5 @@
-resolver: lts-16.20
+# Nightly is required for the ImportQualifiedPost extension from GHC 8.10.
+# An LTS resolver will be used once GHC 8.10 is in an LTS.
+resolver: nightly-2020-11-03
 packages:
 - .

stack.yaml.lock

@@ -6,7 +6,7 @@
 packages: []
 snapshots:
 - completed:
-    size: 532177
-    url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/16/20.yaml
-    sha256: 0e14ba5603f01e8496e8984fd84b545a012ca723f51a098c6c9d3694e404dc6d
-  original: lts-16.20
+    size: 544004
+    url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/nightly/2020/11/3.yaml
+    sha256: f6988e9a2c92219dc8ff0ebd2d420ede3425fa08cb6613ba47f1bc97c9925aa8
+  original: nightly-2020-11-03

test/Spec.hs

@@ -1,7 +1,7 @@
 import Data.Char (isAlpha)
 import qualified Data.Text as T
 import Generic.Random (genericArbitraryRec, uniform)
-import LambdaCalculus.Expression
+import LambdaCalculus
 import LambdaCalculus.Parser
 import Test.QuickCheck
 import Test.Tasty