you define stack as a type and use it in "compile" as a variable.
in the function "compile", the variable "stack" was not declared.
you probably meant this:
#let rec compile e =
 match e with
  Num  x -> [PUSH x]
  |Opn  (e1, o, e2) -> (compile e1) @ (compile e2) 
                    @ [match o with Add -> IADD | Sub -> ISUB];;
-: val compile : exp -> ins list = <fun>
# compile (Opn (Num 5., Add, Num 2.));;
- : ins list = [PUSH 5.000000; PUSH 2.000000; IADD]

the problem is that this function is O(n^3).
this could be done in O(n), which is left as an exercise to 
the student :)

hope this helps despite my obvious lack of pedagogy


On Tue, 13 Feb 2001, maulin shah wrote:

> hi all,
> I am a student and learning ocaml. need help for one
> problem of Unbound value stack
> here is my code.
> 
> type ins =PUSH of float| IADD|ISUB;;
> type stack = ins list;;
> type opn = Add|Sub;;
> type exp = Num of float | Opn of exp* opn *exp;;
> 
> let rec compile exp=
> match exp with 
> Num x-> (PUSH x)::stack
> |Opn e1,Add,e2 -> compile e2,
>                   compile e1,
>                   (IADD)::stack;;
> 
> gives error : unbound value stack????
>  could any one clrify this???
> 
> thanks
> maulin
> 
> 
> 
> 
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