Re: [Caml-list] concerning using of `deriving' syntactic plugin

[Gabriel Scherer <gabriel.scherer@gmail.com>]

You can inject the definition you want in the following way:

  module Meths = struct
    include Meths
    module Show_t = ...
  end

  type value_kind = ...
  deriving (Show)

This works because deriving, being a Camlp4 extension, does not refer
to "the" module Meths, but any module Meths that is in scope at the
place of expansion. This could be seen as a defect of hygiene (indeed
that makes camlp4 macro non-hygienic according to the Scheme notion of
hygiene) but is also a feature, rather than a bug, of the syntactic
macro system; because as a macro-writer you can give flexibility to
the user by specifying the free variables you rely on.

On Wed, Mar 7, 2012 at 12:49 PM, Matej Košík
<5764c029b688c1c0d24a2e97cd764f@gmail.com> wrote:
> Hi,
>
> I would like to print out the response of a modified Ocaml's typechecker
> to various inputs.
>
> One way to do it would be to write a pretty-printer by hand.
>
> Before I do that, I would like to apply "deriving" machinery:
> http://code.google.com/p/deriving/wiki/Introduction
> to this (chore) job.
>
> In some cases I do not know what to do.
>
> E.g., file "types/types.mli" contains the following definition:
>
>  and value_kind =
>      Val_reg
>    | Val_prim of Primitive.description
>    | Val_ivar of mutable_flag * string
>    | Val_self of
>        (Ident.t * type_expr) Meths.t ref *
>        (Ident.t * mutable_flag * virtual_flag * type_expr) Vars.t ref *
>      string * type_expr
>    | Val_anc of (string * Ident.t) list * string
>    | Val_unbound
>
> If I add
>
>  deriving (Show)
>
> at the end of the above definition, I get an error:
>
>  Error: Unbound module Meths.Show_t
>
> That is expected but I am not sure what to do. That is, I am not sure
> what is the official way to "deriving"-sify the "Meth" module which
> defined in the following way:
>
>  module Meths = Map.Make(OrderedString)
>
> without the need to modify files "map.ml{i,}".
>
> Is something like that possible?
>
> Thank you very much in advance for any help.
>
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