Package ppx_show seems breaking in UTop

[sheepduke]

Description

First of all, everything works when building with dune.

The package ppx_show (version 0.2.0) does not work in UTop. Also, when it is loaded, ppx_compare will stop working.

How to reproduce

Start utop and run the following commands in it:

utop # #require "ppx_show";;
utop # type 'a binary_tree =
  | Empty
  | Node of 'a * 'a binary_tree * 'a binary_tree
[@@deriving show];;
Error: Error while running external preprocessor
Command line: /home/sheep/.opam/default/lib/ppx_show/./ppx.exe --as-ppx '/tmp/camlppx21011f' '/tmp/camlppxb4b1ef'

Then restart utop and run:

utop # #require "ppx_show";;
utop # #require "ppx_compare";;
utop # type 'a binary_tree =
  | Empty
  | Node of 'a * 'a binary_tree * 'a binary_tree
[@@deriving show];;
Error: Error while running external preprocessor
Command line: /home/sheep/.opam/default/lib/ppx_show/./ppx.exe --as-ppx '/tmp/camlppx137bfb' '/tmp/camlppx93ab35'

However, ppx_compare works without loading ppx_show:

utop # #require "ppx_compare";;
utop # type 'a binary_tree =
  | Empty
  | Node of 'a * 'a binary_tree * 'a binary_tree
[@@deriving equal,compare];;
type 'a binary_tree = Empty | Node of 'a * 'a binary_tree * 'a binary_tree
val equal_binary_tree :
  ('a -> 'a -> bool) -> 'a binary_tree -> 'a binary_tree -> bool = <fun>
val compare_binary_tree :
  ('a -> 'a -> int) -> 'a binary_tree -> 'a binary_tree -> int = <fun>

In addition, the following dune file simply works:

(library
 (name mylib)
 (libraries ppx_show.runtime)
 (preprocess (pps ppx_compare ppx_show)))

The current workaround is to use ppx_deriving.show in order to use the [@@deriving show] ppx.

Am I missing anything, or it is a bug of ppx_show library?

[thierry-martinez]

This should be fixed now! Thank you very much for the report. Another workaround would have been (the problem was to give a name to the current module in the toplevel for the path):

type 'a binary_tree =
  | Empty
  | Node of 'a * 'a binary_tree * 'a binary_tree
[@@deriving show {with_path=false}];;

Note that you need to #require "ppx_show.runtime" as well.