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+-- | Abstract syntax tree for the untyped lambda calculus, plus some helpers.
+module AST (
+  Expr (..)
+) where
+
+-- | Lambda Expressions
+data Expr
+  = Var String
+  | Lam String Expr
+  | App Expr Expr
+  deriving (Eq, Ord)
+
+
+-- https://www.haskellforall.com/2020/11/pretty-print-syntax-trees-with-this-one.html
+showLam, showApp, showVar :: Expr -> String
+showLam (Lam i e) = "\\" ++ i ++ " . " ++ showLam e
+showLam e = showApp e
+
+showApp (App e1 e2) = showApp e1 ++ " " ++ showVar e2
+showApp e = showVar e
+
+showVar (Var i) = i
+showVar e = "(" ++ showLam e ++ ")"
+
+instance Show Expr where
+  show e = showLam e
+
+