# Why error is occured when call module functions

**URL:** <https://discuss.ocaml.org/t/why-error-is-occured-when-call-module-functions/5332>\
**Category:** Learning\
**Created:** [March 17, 2020, 10:50am UTC](https://discuss.ocaml.org/t/why-error-is-occured-when-call-module-functions/5332 "2020-03-17T10:50:33Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![hss](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/hss/32/1489_2.png) [@hss](https://discuss.ocaml.org/u/hss)\
**Post date:** [March 17, 2020, 10:50am UTC](https://discuss.ocaml.org/t/why-error-is-occured-when-call-module-functions/5332/1 "2020-03-17T10:50:33Z")

</div>

Hi 🙂

I’m learning OCaml module syntax currently and confusing why this syntax is not allowed and why OCaml says “unbound value pp”.

Code:

```auto
  1 module Tree = struct
  2 type 'a tree =
  3 Br of 'a * 'a tree * 'a tree
  4 | Lf
  5 
  6 let create v =
  7 Br(v,Lf,Lf)
  8 
  9 let link_left dst src =
 10 match src with
 11 | Br(v,_,r) -> Br(v,dst,r)
 12 | _ -> Lf
 13 
 14 let link_right dst src =
 15 match src with
 16 | Br(v,l,_) -> Br(v,l,dst)
 17 | _ -> Lf
 18 
 19 let rec size = function
 20 | Br(v,l,r) -> 1 + max (size l) (size r)
 21 | Lf -> 0
 22 
 23 let rec pp f = function
 24 | Br(v,l,r) -> (f v); (pp f l); (pp f r)
 25 | Lf -> ()
 26 end
 27 
 28 
 29 (* test *)
 30 let _ =
 31 let a = create 1 in
 32 let b = create 2 in
 33 let c = create 3 in
 34 let tree = link_left b a |> link_right c in
 35 let fmt = Printf.printf "%d\n" in
 36 pp fmt tree;;
~                       

```

What I want to learn the semantics of syntax in the above code is three points:

1. `31, 32, 33 : let .. = create n` How can I call `create` function? create is inside module `Tree` but this code works. I don’t know why.

2. `31, 32, 33 : let .. = create n` If I change `create` function to `Tree.create`, generates stuff what I don’t understand. the return value is `Tree.tree` not `'a tree`. Why it has happened? questions 1 and 2 make me difficult to understand module syntax.

3. `36: pp fmt tree;;` create, link\_left, link\_right, and size are no problems when I call them but, the function `pp` occurred error when I call it.

Thank you.

---

<div class="post-metadata">

**Author:** ![octachron](https://avatars.discourse-cdn.com/v4/letter/o/49beb7/32.png) [@octachron](https://discuss.ocaml.org/u/octachron)\
**Post date:** [March 17, 2020, 11:24am UTC](https://discuss.ocaml.org/t/why-error-is-occured-when-call-module-functions/5332/2 "2020-03-17T11:24:40Z")

</div>

From your description, you most probably defined a type `tree`, a function `create` and another module `Tree` earlier in your file or REPL session. You may also be using an editor terminal and not evaluating the full buffer. Consequently, you end with left-over values, types and modules from your previous attempts.

Using `Test.create`,

```ocaml
let _ =
  let a = Test.create 1 in
  let b = Test.create 2 in
  let c = Test.create 3 in
  let tree = Test.link_left b a |> Test.link_right c in
  let fmt = Printf.printf "%d\n" in
  Test.pp fmt tree

```

or opening the `Tree` module with

```ocaml
let () =
  let open Tree in
  let a = create 1 in
  let b = create 2 in
  let c = create 3 in
  let tree = link_left b a |> link_right c in
  let fmt = Printf.printf "%d\n" in
  pp fmt tree

```

should work fine.