ability to run code and check output in tests

tests/Zoo1.newt

@@ -0,0 +1,105 @@
+module Zoo1
+
+-- I'm starting to translate ezoo 01-eval-closures-debruijn as a test cases.
+
+
+
+
+------- Prelude stuff
+
+data Nat : U where
+  Z : Nat
+  S : Nat -> Nat
+
+data Unit : U where
+  MkUnit : Unit
+
+data List : U -> U where
+  Nil : {a : U} -> List a
+  Cons : {a : U} -> a -> List a -> List a
+
+data Maybe : U -> U where
+  Just : {a : U} -> a -> Maybe a
+  Nothing : {a : U} -> Maybe a
+
+
+----------------------------------
+
+-- forward declaration
+Val : U
+
+data Tm : U where
+  Var : Nat -> Tm
+  Lam : Tm -> Tm -- lam (x.t)
+  App : Tm -> Tm -> Tm
+  Let : Tm -> Tm -> Tm -- let t (x.u)
+
+data Env : U where
+  ENil : Env
+  Define : Env -> Val -> Env
+
+data Closure : U where
+  MkClosure : Env -> Tm -> Closure
+
+data Val : U where
+  VVar : Nat -> Val
+  VApp : Val -> Val -> Val
+  VLam : Closure -> Val
+
+length : Env -> Nat
+length ENil = Z
+length (Define env _) = S (length env)
+
+lookup : Env -> Nat -> Maybe Val
+lookup (Define env v) Z = Just v
+-- If I write "Just (lookup env k)" on RHS, it's wrong, but the error message is unusable (mainly due to FC)
+-- The FC is fine if I write lookup {Val} env k
+lookup (Define env _) (S k) = lookup env k
+lookup (ENil) x = Nothing
+
+eval : Env -> Tm -> Val
+
+cApp : Closure -> Val -> Val
+-- If I put Closure instead of MkClosure, it reports missing case, fix that (should be bad constructor or something)
+cApp (MkClosure env t) u = eval (Define env u) t
+
+hole : Val
+
+-- TODO need to inline solved metas somewhere, as error messages are unreadable
+-- NEXT fix FC for missing case if we remove _
+
+eval env tm = case tm of
+  (Var x) =>
+    case lookup env x of
+    -- case doesn't use the new code.  We've got a wildcard here that
+    -- is forced to {Val}, but we don't have forcing/dotting
+    -- I guess we see what Jesper says about dotting
+    Just x => x
+    Nothing => VVar x
+  -- TODO no tupls yet
+  App t u => case eval env t of
+    VLam t => cApp t (eval env u)
+    t      => VApp t (eval env u)
+  Lam t => VLam (MkClosure env t)
+  Let t u => eval (Define env (eval env t)) u
+  -- NEXT this is unreachable, find out how to warn about it
+  -- _ => hole
+
+lvl2ix : Nat -> Nat -> Nat
+lvl2ix (S k) (S j) = lvl2ix k j
+lvl2ix Z (S j) = j
+lvl2ix _ Z = Z -- shouldn't happen
+
+quote : Nat -> Val -> Tm
+quote l v = case v of
+  VVar x => Var (lvl2ix l x)
+  VApp t u => App (quote l t) (quote l u)
+  VLam t => Lam (quote (S l) (cApp t (VVar l)))
+
+nf : Env -> Tm -> Tm
+nf env t = quote (length env) (eval env t)
+
+-- and then a parser / example
+-- Are we ready to try building a parser in newt?
+
+