diff --git a/lib/interp.ml b/lib/interp.ml
index 01450b8..70ba0b8 100644
--- a/lib/interp.ml
+++ b/lib/interp.ml
@@ -214,7 +214,13 @@ let rec eval : type a. a Expr.t -> value =
   | Seq (e1, e2) ->
       eval e1 |> ignore;
       eval e2
-  | Bin_op { op; left; right } -> (
+  | Uop { op; arg } -> (
+      let v = eval arg in
+      match (op, v) with
+      | Not, Bool b -> Bool (not b)
+      | Uminus, Int i -> Int ~-i
+      | _, _ -> raise Type_error)
+  | Bop { op; left; right } -> (
       let v1 = eval left in
       let v2 = eval right in
       match (op, v1, v2) with
diff --git a/lib/parser.mly b/lib/parser.mly
index 2dd7366..9393709 100644
--- a/lib/parser.mly
+++ b/lib/parser.mly
@@ -27,6 +27,7 @@ open Expr
 %right    AND
 %left     PLUS MINUS
 %left     TIMES
+%nonassoc prec_unary
 
 
 %start <Prog.t> prog
@@ -39,7 +40,7 @@ comp_lst:
 comp_expr:
     | LET; name = var; param = var; EQ; body = expr { { name; param; body = hook_full body } }
 expr:
-    | bop_expr { $1 }
+    | atom { $1 }
     | FUN; param = var; RARROW; body = expr { Ex (Fn { param; body = hook_free_exn body }) }
     | LET; id = var; EQ; bound = expr; IN; body = expr
       { let Ex body = body in
@@ -59,11 +60,13 @@ expr:
         | Some e1, Some e2 -> Ex (Seq (e1, e2))
         | _, _ -> Ex (Seq (hook_full e1, hook_full e2))
       }
-bop_expr:
-    | atom { $1 }
-    | left = bop_expr; op = op; right = bop_expr
-      { Ex (Bin_op { op; left = hook_free_exn left; right = hook_free_exn right }) }
-%inline op:
+    | op = uop; expr = expr %prec prec_unary { Ex (Uop { op; arg = hook_free_exn expr }) }
+    | left = expr; op = bop; right = expr
+      { Ex (Bop { op; left = hook_free_exn left; right = hook_free_exn right }) }
+%inline uop:
+    | PLUS { Uplus }
+    | MINUS { Uminus }
+%inline bop:
     | AND { And }
     | OR { Or }
     | PLUS { Plus }
diff --git a/lib/syntax.ml b/lib/syntax.ml
index 9eb0928..ced0505 100644
--- a/lib/syntax.ml
+++ b/lib/syntax.ml
@@ -40,10 +40,12 @@ module Expr = struct
         -> hook_full t
     | Eff : hook_free t -> hook_full t
     | Seq : 'a t * 'a t -> 'a t
-    | Bin_op : { op : bin_op; left : hook_free t; right : hook_free t } -> _ t
+    | Uop : { op : uop; arg : hook_free t } -> _ t
+    | Bop : { op : bop; left : hook_free t; right : hook_free t } -> _ t
 
   and const = Unit | Bool of bool | Int of int
-  and bin_op = And | Or | Plus | Minus | Times
+  and uop = Not | Uplus | Uminus
+  and bop = And | Or | Plus | Minus | Times
 
   type some_expr = Ex : 'a t -> some_expr [@@unboxed]
 
@@ -51,46 +53,51 @@ module Expr = struct
     let (Ex expr) = expr in
     let ( let* ) = Stdlib.Option.bind in
     match expr with
-    | Const _ as e -> Some e
-    | Var _ as e -> Some e
-    | View _ as e -> Some e
-    | Cond _ as e -> Some e
-    | Fn _ as e -> Some e
-    | App _ as e -> Some e
     | Let ({ body; _ } as e) ->
         let* body = hook_free (Ex body) in
         Some (Let { e with body })
-    | Stt _ -> None
-    | Eff _ -> None
+    | Stt _ | Eff _ -> None
     | Seq (e1, e2) ->
         let* e1 = hook_free (Ex e1) in
         let* e2 = hook_free (Ex e2) in
         Some (Seq (e1, e2))
-    | Bin_op _ as e -> Some e
+    | (Const _ as e)
+    | (Var _ as e)
+    | (View _ as e)
+    | (Cond _ as e)
+    | (Fn _ as e)
+    | (App _ as e)
+    | (Bop _ as e)
+    | (Uop _ as e) ->
+        Some e
 
   let hook_free_exn e = Option.value_exn (hook_free e)
 
   let rec hook_full (expr : some_expr) : hook_full t =
     let (Ex expr) = expr in
     match expr with
-    | Const _ as e -> e
-    | Var _ as e -> e
-    | View _ as e -> e
-    | Cond _ as e -> e
-    | Fn _ as e -> e
-    | App _ as e -> e
     | Let ({ body; _ } as e) ->
         let body = hook_full (Ex body) in
         Let { e with body }
-    | Stt _ as e -> e
-    | Eff _ as e -> e
     | Seq (e1, e2) ->
         let e1 = hook_full (Ex e1) in
         let e2 = hook_full (Ex e2) in
         Seq (e1, e2)
-    | Bin_op _ as e -> e
-
-  let string_of_bin_op = function
+    | (Const _ as e)
+    | (Var _ as e)
+    | (View _ as e)
+    | (Cond _ as e)
+    | (Fn _ as e)
+    | (App _ as e)
+    | (Stt _ as e)
+    | (Eff _ as e)
+    | (Uop _ as e)
+    | (Bop _ as e) ->
+        e
+
+  let string_of_uop = function Not -> "not" | Uplus -> "+" | Uminus -> "-"
+
+  let string_of_bop = function
     | And -> "&&"
     | Or -> "||"
     | Plus -> "+"
@@ -123,11 +130,9 @@ module Expr = struct
           ]
     | Eff e -> l [ a "Eff"; sexp_of_t e ]
     | Seq (e1, e2) -> l [ a "Seq"; sexp_of_t e1; sexp_of_t e2 ]
-    | Bin_op { op; left; right } ->
-        l
-          [
-            a "Bin_op"; a (string_of_bin_op op); sexp_of_t left; sexp_of_t right;
-          ]
+    | Uop { op; arg } -> l [ a "Uop"; a (string_of_uop op); sexp_of_t arg ]
+    | Bop { op; left; right } ->
+        l [ a "Bop"; a (string_of_bop op); sexp_of_t left; sexp_of_t right ]
 end
 
 module Prog = struct
diff --git a/samples/simple.ml b/samples/simple.ml
index 7958474..ac3202c 100644
--- a/samples/simple.ml
+++ b/samples/simple.ml
@@ -1,6 +1,6 @@
 let C x =
   stt s, setS = 42 in
-  eff (setS (fun s -> 43));
+  eff (setS (fun s -> -43));
   view [()]
 ;;
 view [C ()]
diff --git a/test/test_react_trace.ml b/test/test_react_trace.ml
index ebe142f..9abdb7c 100644
--- a/test/test_react_trace.ml
+++ b/test/test_react_trace.ml
@@ -95,7 +95,7 @@ let set_in_effect_step_indefinitely () =
                                      {
                                        param = "s";
                                        body =
-                                         Bin_op
+                                         Bop
                                            {
                                              op = Plus;
                                              left = Var "s";