From 6aa6c9f07683e743e8c2e8bd525ee1ed7ebab573 Mon Sep 17 00:00:00 2001
From: Yuri Tatishchev <itatishch@gmail.com>
Date: Wed, 22 Apr 2026 11:00:06 -0700
Subject: [PATCH] lab16: init

---
 lab16/stlc.hs | 69 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)
 create mode 100644 lab16/stlc.hs

diff --git a/lab16/stlc.hs b/lab16/stlc.hs
new file mode 100644
index 0000000..08be30c
--- /dev/null
+++ b/lab16/stlc.hs
@@ -0,0 +1,69 @@
+import Data.Map (Map)
+import qualified Data.Map as Map
+
+-- We represent variables as strings.
+type Variable = String
+
+data Expression =
+    ETrue
+  | EFalse
+  | EInt Int
+  | ESucc Expression
+  | EPred Expression
+  | EIsZero Expression
+  | EIf Expression Expression Expression
+  | ELambda Variable StlcType Expression -- (\x:T.e)
+  | EApp Expression Expression
+  | EVar Variable
+  deriving (Show)
+
+data Value =
+    VTrue
+  | VFalse
+  | VNum Int
+  deriving (Show)
+
+data StlcType =
+    TBool
+  | TInt
+  | TFun StlcType StlcType  --  T1 -> T2
+  deriving (Show,Eq)
+
+type TypingEnv = Map Variable StlcType
+
+--I'm using zero a lot, so I am making a shortcut
+zero = EInt 0
+
+typecheckFail e = error $ "Expression " ++ (show e) ++ " does not typecheck"
+
+typecheck :: Expression -> TypingEnv -> StlcType
+typecheck ETrue _ = TBool
+typecheck EFalse _ = TBool
+typecheck (EInt _) _ = TInt
+typecheck e@(ESucc e1) env = case typecheck e1 env of
+  TInt  -> TInt
+  _     -> typecheckFail e
+typecheck e@(EPred e1) env = case typecheck e1 env of
+  TInt  -> TInt
+  _     -> typecheckFail e
+typecheck e@(EIf e1 e2 e3) env =
+  let t1 = typecheck e1 env
+      t2 = typecheck e2 env
+      t3 = typecheck e3 env
+  in if t1 == TBool && t2 == t3 then t2 else typecheckFail e
+typecheck e@(EIsZero e1) env = error "TBD"
+typecheck e@(EVar x) env = error "TBD"
+typecheck e@(ELambda x tin e') env = error "TBD"
+typecheck e@(EApp e1 e2) env = error "TBD"
+
+
+--Some sample cases
+test1 = typecheck (ESucc zero) Map.empty
+test2 = typecheck (EPred (ESucc zero)) Map.empty
+test3 = typecheck (EIf ETrue zero (ESucc (ESucc zero))) Map.empty
+test4 = typecheck (ELambda "x" TInt ETrue) Map.empty
+test5 = typecheck (EApp (ELambda "x" TInt (EIf (EIsZero (EVar "x")) (ESucc zero) zero)) (ESucc zero)) Map.empty
+
+bad1 = typecheck (ESucc EFalse) Map.empty
+bad2 = typecheck (EApp (ELambda "x" TInt (EIsZero (EVar "x"))) ETrue) Map.empty
+