# Most idiomatic way to reference module in signature?
**URL:** https://discuss.ocaml.org/t/most-idiomatic-way-to-reference-module-in-signature/17526
**Category:** Learning
**Created:** [November 25, 2025, 11:05am UTC](https://discuss.ocaml.org/t/most-idiomatic-way-to-reference-module-in-signature/17526 "2025-11-25T11:05:48Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@giltho](https://discuss.ocaml.org/u/giltho)
#### Post date: [November 25, 2025, 11:05am UTC](https://discuss.ocaml.org/t/most-idiomatic-way-to-reference-module-in-signature/17526/1 "2025-11-25T11:05:48Z")
Hello,
In my codebase I have a few modules that are parametric on a monad. Some of them are also functors, and receive other parameters that are parametric on this monad, and all arguments must match.
I see two solutions to enforce that.
First solution:
```ocaml
module type Param = sig
module MyMonad
...
end
module F
(M: MyMonad)
(P: Param with module MyMonad = MyMonad) =
struct ... end
```
Second solution:
```ocaml
module Param (M: MyMonad) = struct
module type S = sig ... end
end
module F
(M: MyMonad)
(P: Param(MyMonad).S) =
struct ... end
```
I prefer the second one, it’s easier to instantiate.
But 1) is this some footgun I can’t see coming and 2) is there a more idiomatic way of doing this?
---
### Author:  [@giltho](https://discuss.ocaml.org/u/giltho)
#### Post date: [November 27, 2025, 3:46pm UTC](https://discuss.ocaml.org/t/most-idiomatic-way-to-reference-module-in-signature/17526/2 "2025-11-27T15:46:37Z")
The lack of answers makes me more worried that I’m doing something entirely wrong and unusual here 😅
---
### Author:  [@octachron](https://discuss.ocaml.org/u/octachron)
#### Post date: [November 27, 2025, 4:43pm UTC](https://discuss.ocaml.org/t/most-idiomatic-way-to-reference-module-in-signature/17526/3 "2025-11-27T16:43:38Z")
I have used both approaches, and they do work.
As far as I can see, the first variant is quite more common.
However I would recommend to use a destructive substitution in this case
```ocaml
module F
(M: MyMonad)
(P: Param with module MyMonad := MyMonad) = ...
```
to avoid carrying around copies of modules (which can interfere with applicative functors).
Personally, I tend to switch to the functor variant once the module type dependencies start to get complicated.
---
### Author:  [@giltho](https://discuss.ocaml.org/u/giltho)
#### Post date: [November 28, 2025, 9:57am UTC](https://discuss.ocaml.org/t/most-idiomatic-way-to-reference-module-in-signature/17526/4 "2025-11-28T09:57:31Z")
Great thank you very much!
Didn’t think about the destructive subst for some reason 🤔