Hello, I have started to use the show PPX extension available from https://github.com/thierry-martinez/ppx_show and found it very useful. But i’m now facing facing a problem when trying to use it in coordination with functors. I’ll try to illustrate it using a small example (which actually is a he…

You need to make the value printer, pp_value, visible in scope at the definition of t: open Value (* or *) let pp_value = Value.pp_value type t = Const of value | Var of string [@@deriving show] Note that you can expand the pp.ml file into source format by using ocamlopt -dsource file.pp.ml

Another solution might be to use the original name of value in the type definition, which should cause the PPX to expect the correct code: module type VALUE = sig type t [@@deriving show] (* ... *) end module Make (Value: VALUE) = struct type t = | Const of Valu…

@octachron Thanks for your quick and useful answer. I should still be missing sth, howver :confused: (* expr.ml *) ... module type VALUE = sig type t val from_int: int -> t end module Make (Value: VALUE) = struct open Value type t = Const of Value.t | Var of st…

@CraigFe Thanks also for your help. Using the original type name in the functor definitions makes things clearer, indeed. I’ve used this reformulation in my previous answer. I don’t understand how adding [@@deriving show] to the type VALUE.t can work without adding somewhere a reference to the ac…

You can derive the Int printer too: module Int = struct type t = int [@@deriving show] ... end

Of course ! Everything’s ok now :slight_smile: Here’s the final code, in case s/o encounters the same problem. Thanks ! (* expr.mli *) module type T = sig type value type t = Const of value | Var of string [@@deriving show] val zero: t end module type VALUE = sig …

As a small refinement, you can add the deriving annotation to the type t in the module type VALUE as suggested by @CraigFe : module type VALUE = sig type t [@@deriving show] val from_int: int -> t end With this annotation, ppx_deriving add the corresponding generated function to the inte…

Deriving show ppx and functors

Learning

jserot February 23, 2021, 2:15pm 9

Ok.
In this case, you can (and have to, to suppress warning) remove the line

  val pp: Format.formatter -> t -> unit

in the definition of VALUE signature.

Thanks again for your helpful suggestions

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Deriving show ppx and functors

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