basic substitution implemented

src/Evaluation.hs

@@ -1,6 +1,18 @@
 module Evaluation where
 
-import Parser (Expr (Variable))
+import Parser (Expr (Abstraction, Application, Variable))
+
+-- | substitute all variables matching string in first expression with second expression
+subst :: Expr -> String -> Expr -> Expr
+subst v@(Variable vname) name content = if vname == name then content else v
+subst (Abstraction vname body) name content = Abstraction vname (subst body name content)
+subst (Application f arg) name content = Application (subst f name content) (subst arg name content)
+
+bind :: Expr -> Expr -> Either String Expr
+bind (Abstraction v b) arg = Left "Unimplemented"
+
+beta :: Expr -> Maybe Expr
+beta _ = Nothing
 
 run :: Expr -> Expr
 run _ = Variable "a"

test/EvaluationSpec.hs

@@ -1,8 +1,19 @@
 module EvaluationSpec (spec) where
+
+import Evaluation (subst)
+import Parser (Expr (Abstraction, Application, Variable))
 import Test.Hspec
 
 spec :: Spec
 spec = do
-  describe "Evaluation" $ do
-    it "can evaluation expressions" $
-      True `shouldBe` True
+  describe "subst" $ do
+    it "cannot substitute mismatched variables" $
+      subst (Variable "x") "y" (Variable "z") `shouldBe` Variable "x"
+    it "can substitute matched variables" $
+      subst (Variable "x") "x" (Variable "z") `shouldBe` Variable "z"
+    it "can substitute nested variables" $
+      subst absWithZ "z" absI `shouldNotBe` absWithZMaker absI
+  where
+    absWithZMaker z = Abstraction "x" (Application z (Variable "x"))
+    absWithZ = absWithZMaker (Variable "z")
+    absI = Abstraction "i" (Variable "i")