Missing dependency?

[smondet]

In Utop, for OCaml 4.06.1:

ocaml# #require "ppx_sexp_conv";;
No such package: ppx_deriving - required by `ppx_sexp_conv'

 $ opam info ppx_sexp_conv

=-=- ppx_sexp_conv: information on all versions -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
name                   ppx_sexp_conv
all-installed-versions v0.10.0 [4.05.0 4.06.1]
all-versions           113.09.00  113.24.00  113.33.00  113.33.00+4.03  113.33.01+4.03  113.33.03  v0.9.0  v0.10.0  v0.11.0

=-=- Version-specific details -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
version       v0.10.0
repository    default
url.src:      https://ocaml.janestreet.com/ocaml-core/v0.10/files/ppx_sexp_conv-v0.10.0.tar.gz
url.checksum: md5=c14ca06337e21899ee9ea32cf52aa374
homepage:     https://github.com/janestreet/ppx_sexp_conv
bug-reports:  https://github.com/janestreet/ppx_sexp_conv/issues
dev-repo:     git+https://github.com/janestreet/ppx_sexp_conv.git
authors:      Jane Street Group, LLC <[email protected]>
maintainer:   [email protected]
license:      Apache-2.0
depends:      "ocaml" {>= "4.04.1"}
              "ppx_core" {>= "v0.10" & < "v0.11"}
              "ppx_driver" {>= "v0.10" & < "v0.11"}
              "ppx_metaquot" {>= "v0.10" & < "v0.11"}
              "ppx_type_conv" {>= "v0.10" & < "v0.11"}
              "sexplib" {>= "v0.10" & < "v0.11"}
              "jbuilder" {build & >= "1.0+beta12"}
              "ocaml-migrate-parsetree" {>= "0.4"}
synopsis      Generation of S-expression conversion functions from type definitions
description   Part of the Jane Street's PPX rewriters collection.

[smondet]

Manually installing ppx_deriving then works.

[ghost]

yh that's expected. Basically ppx_sexp_conv doesn't need ppx_deriving but exposes a compatibility mode that uses ppx_deriving. The toplevel relies on this compatibility mode.

[mars0i]

It's expected that people will encounter a missing lib when using ppx_sexp_conv in an obvious way? Well, OK, often one will have already installed ppx_deriving for other reasons, so maybe this won't happen very often, but perhaps at least a note about this could be added to the README.

(I worry that little things like this, encountered repeatedly in different contexts--as one does--is the sort of thing that will turn people off of OCaml before they are far enough into it to appreciate it.)

[shonfeder]

Is there a way to intercept this missing library error and print a user friendly message? Perhaps using some compilation flags at build time?

[aalekseyev]

I don't understand everything that's going on, but this sounds like a widespread issue that affects many ppxes.

For example here a user is reasonably(ish) describing a related issue as a bug, but it's in ppx_deriving_json.

Not sure what the right solution is, maybe it's just phasing out findlib.

[aalekseyev]

Actually, closing it as a duplicate of #30.