I’ve just started learning OCaml, and the most pronounced pain point so far has been having to repeat myself between interface and implementation files. Don’t get me wrong, I think there are a lot of places where it makes sense to separate interface from implementation, and specifically to weaken t…

Yeah people keep hitting this: https://stackoverflow.com/q/22348341/20371 https://stackoverflow.com/q/3238509/20371 Avoiding duplicated definitions in module implementation and interface There are a couple of tricks you can use to work around it in some situatuons.You already discovered the ‘top-…

See also [image] What is the reason of separation of module implementation and signatures in OCaml? Learning What is the reason of the separation of module implementation and signatures? Wouldn’t it be better to describe signatures in function definitions? For example: l…

I am not bothered by the need to create the explicit interface file, but then again, I loved using Modula-2 back in the day. My usual trick to avoid typing too much is to use the compiler itself to generate the first draft of the .mli file; if you use ocamlc with the -i flag, it will dump out the d…

I’m very much behind requiring everything specified in the implementation file to be repeated in the interface; it’s just not clear to me why the opposite is also required. If I write type foo = ... in the implementation file, it’s not clear that I intended foo to be visible outside of the implemen…

OUsing the expression type foo = ... seems to hide you the problem. AFAIK, you should respect the best practice of using an abstract type (already discussed in this website) as follows: (* foo.ml *) module type FOO = sig type t val bar : t -> int (* or anything you want *) end module Foo:FOO = s…

I’m not disputing that abstract types are highly useful (and that there are many places where choosing them is the right answer). But there are other places (e.g. module types that are accepted by functors exposed as part of the API, or variants which you consciously decide expose as part of your AP…

[image] mylincode: I’m only asking why type definitions from the interface aren’t copied into the implementation, and not the other way around. Let me try to answer this way: Type definitions in a signature can be abstract or concrete. You already know the pros and cons, especially for the u…

[image] mylincode: I’m only asking why type definitions from the interface aren’t copied into the implementation, and not the other way around. I think in the general case there’s no way to guarantee that you’d do it correctly. For example, you could have a private type abbreviation –this lets…

Quick Edit: This document may be useful to understand the OCaml module system: https://www.di.ens.fr/~zappa/teaching/stt05/ocaml_modules.pdf See page 11 Multiple views of the same module.

I’m only asking why type definitions from the interface aren’t copied into the implementation, and not the other way around. I think that doing this properly in the general case requires at least higher-order unification. So it’s probably possible but it is not easy. There are some similar requir…

Why do I need to repeat type declarations between interfaces and implementations (or how do I get around this)?

Learning

jonathandoyle February 16, 2019, 2:53pm 3

Topic		Replies	Views
De-duplicating module signatures that depend on abstract data types Learning module	11	3394	May 24, 2019
Avoiding duplicated definitions in module implementation and interface Learning	4	2827	February 9, 2018
What is the reason of separation of module implementation and signatures in OCaml? Learning	34	10103	March 20, 2018
Repeated 'module type' declarations in .mli and .ml Learning	9	392	November 3, 2025
What's the OCaml way to do multiple interfaces? Learning	30	2460	September 27, 2023

Why do I need to repeat type declarations between interfaces and implementations (or how do I get around this)?

Related topics