Re: [Caml-list] PPX is harmful to our community in the long term

[Jeremie Dimino <jdimino@janestreet.com>]

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Coming back to this thread. I had a simple idea recently for a ppx that
makes it easy to do pattern matching on  abstract types. I wrote some
experiments here [1]. This essentially allows to make the AST fully
abstract while still being able to deconstruct it conveniently. In fact the
patterns are even nicer than ones matching the raw data type directly since
you can build your own helper patterns.

Making the AST abstract will allow to make the API evolve in a backward
compatible way even though the underlying AST keeps changing.

I just did some experiments for now. I think we'll eventually implement
this solution properly and use it in our ppx code.

[1] https://github.com/diml/ppx_view_pattern

On Fri, Apr 28, 2017 at 3:55 PM, Jacques Carette <carette@mcmaster.ca>
wrote:

> As co-author of the now-obsolete pa_monad, I emphatically agree!
> Jacques
>
>
> On 2017-04-28 10:50 AM, Yaron Minsky wrote:
>
> We're very similar, except that we now do use a monadic syntax pretty
> pervasively. I wrote about this here:
>
> https://blogs.janestreet.com/let-syntax-and-why-you-should-use-it/
>
> and am if anything more convinced that it's a worthwhile idiom. Monads and
> Applicatives are useful in so many places (beyond Async and Lwt) that
> having syntactic support for them is really nice, especially for the let ..
> and constructs.
>
> y
>
> On Fri, Apr 28, 2017 at 9:04 AM, Anil Madhavapeddy <anil@recoil.org>
> wrote:
>
>> On 28 Apr 2017, at 12:07, Olaf Hering <olaf@aepfle.de> wrote:
>> >
>> > On Fri, Apr 21, Fabrice Le Fessant wrote:
>> >
>> >> A lot of people use `autoconf` to generate `./configure` scripts, and
>> the
>> >> standard practice is to keep the `./configure` script so that people
>> don't need
>> >> to run `autoconf` to just compile and install the software. Maybe
>> projects
>> >
>> > This is and was a huge mistake to promote 'configure&&make' instead of
>> > autogen.sh&&configure&&make. Having a set of uptodate autotools
>> > installed is easy and cheap, they are not runtime dependencies. The
>> > result of that wrongdoing is a huge pile of broken and/or incomplete
>> configure.ac.
>>
>> Indeed -- we spent years in OpenBSD dealing with patching broken versions
>> of libtool scripts that embedded incorrect behaviour on our particular
>> platforms,
>> and were stubbornly included in upstream distributions without being
>> regenerated.
>>
>> > Do not repeat that mistake, whatever it means in the OCaml world.
>>
>> A similar analogue in the OCaml world may be the inclusion of
>> autogenerated
>> files from _oasis.  The inclusion of the autogenerated files like
>> myocamlbuild.ml
>> was a holdover from a pre-opam world when it was painful to install all
>> the
>> build dependencies of OASIS.
>>
>> Nowadays, it's quite easy to install oasis and run `oasis setup` in a
>> project
>> to get the latest build rules, and the checked in autogenerated files only
>> get in the way and/or are increasingly complex due to having to deal with
>> multiple OCaml versions (e.g. for String.lowercase vs
>> String.lowercase_ascii).
>>
>> Bundling pre-evaluated ppx output in a release tarball runs the same
>> risk...
>>
>> Our experience in Mirage with PPX has been to find a balance -- we do not
>> let
>> it proliferate beyond type_conv usage or ppx_cstruct for binary formats.
>> Some
>> libraries (such as the TLS stack) do not use it all. One of the heaviest
>> uses
>> of camlp4 in the past for us was the pa_lwt syntax extension, and most
>> libraries
>> have gone towards explicit Lwt.bind() calls instead of using the ppx
>> alternative.
>>
>> I'm hopeful that ocaml-migrate-parsetree will make it easier for us to
>> test
>> common libraries on dev versions of OCaml.  In practise with 4.05, it has
>> been
>> non-ppx changes that have been blocking testing -- for instance the
>> close-on-exec
>> flag addition to the Unix module caused rippling breakage through Lwt and
>> other
>> platform libraries.  That's not to say that PPX isn't a problem, but it
>> has
>> gotten significantly easier to deal with since Fred's work on
>> migrate-parsetree.
>>
>> regards,
>> Anil
>>
>>
>
>


-- 
Jeremie

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