4.2

pfx/basic/ast.ml

 type command = Push of int | Pop | Swap | Add | Sub | Mul | Div | Rem
-  DefineMe (* Question 4.1 *)
 
 type program = int * command list
 

pfx/basic/eval.ml

@@ -13,8 +13,30 @@ let string_of_state (cmds,stack) =
 let step state =
   match state with
   | [], _ -> Error("Nothing to step",state)
+  | Pop::_, [] -> Error("Nothing to pop",state)
+  | Swap::_, _::[] -> Error("Nothing to swap",state)
+  | Swap::_, [] -> Error("Nothing to swap",state)
+  | Add::_, _::[] -> Error("Nothing to add",state)
+  | Add::_, [] -> Error("Nothing to add",state)
+  | Sub::_, _::[] -> Error("Nothing to substract",state)
+  | Sub::_, [] -> Error("Nothing to substract",state)
+  | Mul::_, _::[] -> Error("Nothing to multiply",state)
+  | Mul::_, [] -> Error("Nothing to multiply",state)
+  | Div::_, _::[] -> Error("Nothing to divide",state)
+  | Div::_, [] -> Error("Nothing to divide",state)
+  | Div::_, _::0::_ -> Error("Forbidden operation",state)
+  | Rem::_, _::[] -> Error("Nothing to divide",state)
+  | Rem::_, [] -> Error("Nothing to divide",state)
+  | Rem::_, _::0::_ -> Error("Forbidden operation",state)
   (* Valid configurations *)
-  | DefineMe :: q , stack          -> Ok (q, stack)
+  | (Push x)::t, stack -> Ok (q, x::stack)
+  | Pop::q, x::stack -> Ok (q, stack)
+  | Swap::q, x::y::stack -> Ok (q, y::x::stack)
+  | Add::q, x::y::stack -> Ok (q, (x + y)::stack)
+  | Sub::q, x::y::stack -> Ok (q, (x - y)::stack)
+  | Mul::q, x::y::stack -> Ok (q, (x * y)::stack)
+  | Div::q, x::y::stack -> Ok (q, (x / y)::stack)
+  | Rem::q, x::y::stack -> Ok (q, (x mod y)::stack)
 
 let eval_program (numargs, cmds) args =
   let rec execute = function