OCaml error with types - functional-programming

Question

I have the following error from OCaml and I don't understand why. I'm trying to define an interpreter in OCaml. I have some types and functions to evaluate these types. I paste the relevant code.
I have these types:
type ide = string
type exp = Eint of int
| Ebool of bool
| Var of ide
| Prod of exp * exp
| Sum of exp * exp
| Diff of exp * exp
| Eq of exp * exp
| Minus of exp
| Iszero of exp
| Or of exp * exp
| And of exp * exp
| Not of exp
| Ifthenelse of exp * exp * exp
| Let of ide * exp * exp
| Fun of ide list * exp
| Funval of exp * exp env
| Appl of exp * exp list
| Dot of ide * field_name
|Field of ide * exp
| Record of ide * exp list;;
type 'a env = Env of (ide * 'a) list;;
I have a function eval used to eval exp. It works correctly.
let rec eval ((e: exp), (r: exp env)) =
match e with
| Eint(n) -> Eint(n)
| Ebool(b) -> Ebool(b)
| Var(i) -> lookup r i
| Iszero(a) -> iszero(eval(a, r))
| Eq(a, b) -> equ(eval(a, r),eval(b, r))
| Prod(a, b) -> mult(eval(a, r), eval(b, r))
| Sum(a, b) -> plus(eval(a, r), eval(b, r))
| Diff(a, b) -> diff(eval(a, r), eval(b, r))
| Minus(a) -> minus(eval(a, r))
| And(a, b) -> et(eval(a, r), eval(b, r))
| Or(a, b) -> vel(eval(a, r), eval(b, r))
| Not(a) -> non(eval(a, r))
| Ifthenelse(a, b, c) -> let g = eval(a, r) in
if typecheck("bool", g) then
(if g = Ebool(true) then eval(b, r) else eval(c, r))
else failwith ("nonboolean guard")
| Let(i, e1, e2) ->
eval(e2, bind (r, i, eval(e1, r)))
| Fun(x, a) -> Funval(e, r)
| Appl(e1, e2) -> match eval(e1, r) with
| Funval(Fun(x, a), r1) ->
eval(a, bind_list r1 x e2)
| _ -> failwith("no funct in apply")
let eval_field (field:exp) (r: exp env)= match field with
| Field (id, e) -> Field (id, (eval e r))
| _ -> failwith ("Not a Field");;
And finally I have a function to evaluate fields of record:
let eval_field (field:exp) (r: exp env)= match field with
| Field (id, e) -> Field (id, (eval e r))
| _ -> failwith ("Not a Field");;
The problem is with eval_field: OCaml signals me ths error:
Characters 22-24:
let f1 = Field ("f1", e1);;
^^
Error: This expression has type exp/1542
but an expression was expected of type exp/2350
What could be wrong?
Thank you very much for your help.

Answer 1

The compiler is trying to tell you that you have two different types named exp and that you have one of them where the other is expected.
I only see one definition for the type exp in this code. I suspect your environment isn't clean. You might try loading the code up in a new OCaml interpreter. Or perhaps the problem is with some code you're not showing.
Here's a session showing how the error is produced:
$ ocaml
OCaml version 4.02.1
# type abc = A | B | C;;
type abc = A | B | C
# let f (x: abc) = x = A;;
val f : abc -> bool = <fun>
# type abc = A | B | C;;
type abc = A | B | C
# f (C: abc);;
Error: This expression has type abc/1024
but an expression was expected of type abc/1018
My guess is that you have a function (like f here) that was defined using an old definition of your exp type. But you're calling it with a value from the new definition of the type (as here).