Hello, I have read another thread about this. The following declaration is repeated in the .mli and .ml. The line module RADIXOp : RADIXOperator is only in the .mli. open Types module type RADIXOperator = sig type 'a radix_node = 'a include module type of MakeRadixNode ( struct type 'a t = 'a r…

[image] Mohan_Radhakrishnan: Should this be duplicated ? Is my code not adhering to the standards ? There’s nothing wrong with your code. If you want to avoid repeating the module type declaration, you can put it in a separate module without interface, eg: radix_operator_intf.ml. This is a c…

To add a bit on top of Nicolas’ answer, I would also recommend this article about the “intf trick” that I found still relevant five years after its writing date.

[image] maiste: I would also recommend this article about the “intf trick” Appendix B of that blog post (which is quite nice, thank you for linking) includes this snippet: include (val (failwith "TODO") : Stack_intf.Intf) which just about breaks my brain. Can someone walk me through how thi…

(val EXPR : MTY) casts an expression EXPR as a module of type MTY, then include includes the module in the current module. This passes the build and fails at runtime, exactly what is desired during the initial rapid development phase.

It worth to note that it works because failwith (like exit) can return any type: val failwith : string -> 'a because it breaks the flow and the returned value has no sense. So the compiler accepts to cast (I’m not sure which term to use here) the result of failwith into a new module.

Thanks. I didn’t realize 'a could be unified with module types as well. Clever trick!

More precisely, first class module types are a normal type of values. In other words, in module type T = sig end type u = (module T) let f (x: (module T)) (y:u) z = [x; y; z] (module T) is a type not a module type, and the type of the value z is inferred to be (module T).

Alternatively, you can create an interface without a module if you’re just defining module types and types. Simply create an .mli file without an .ml and add (modules_without_implementation my_module) to your dune file. I find it a bit cleaner than a standalone .ml since its clear it contains only …

Repeated 'module type' declarations in .mli and .ml

Learning

Mohan_Radhakrishnan October 29, 2025, 10:33am 4

Both references helped and I have a simple .mli now.

open Types

module type Intf = sig
module RADIXOp : RADIXOperator
end

And the emacs trick is useful.

Topic		Replies	Views
Avoiding duplicated definitions in module implementation and interface Learning	4	2826	February 9, 2018
De-duplicating module signatures that depend on abstract data types Learning module	11	3390	May 24, 2019
Why do I need to repeat type declarations between interfaces and implementations (or how do I get around this)? Learning	11	6248	February 18, 2019
Reducing duplicated code in recursive module definitions Learning	7	517	November 3, 2022
How to abstract module types? Learning	7	1019	February 6, 2021

Repeated 'module type' declarations in .mli and .ml

Related topics