Re: [Caml-list] [ANN] ppx_monadic.1.0.2, ppx for monadic do, pattern

[Jun Furuse <jun.furuse@gmail.com>]

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I know what you wrote, since I always asked myself "isn't it too bizarre?"
when I was writing ppx_monadic. Here are some justifications I made:

The first intension of ppx_monadic is to port my code with pa_monad without
pain. Unfortunately ppx_monad required too many small fixes due to the
following reasons:

·       "perform" needed to be replaced by "begin%monad .. end" or "[%monad
.. ]" which requires closing, which is really pain. Ppx_monad provides
"fun%monad", "match%monad" without closing but they were not always helpful
for me.
·       The pattern "p" of "p <- e" is limited to a variable, since it
overrides the syntax of object member mutation. In addition, I use lots of
monadic binds inside my class methods. Therefore this override is not
acceptable for me.

Secondary, I see the monadic syntax sugar is to reduce the number of key
types, and I accepted some alien syntax weirdness for the purpose. If the
number of key types would not matter, I would be happy with the good old
bind (>>=) chains and would not use "do" at all. People think bind chains
are hard to read but my experience with Jane Street Async tells me it it
not quite. Ppx_monad does not really gain in this point unfortunately: I
was often forced to write "let%monad (x,y) = e in" just for "(x,y) <-- e;".
I write Haskell do-notations daily and wanted to have something comparable
in OCaml.

Anyway, the current OCaml syntax is limited to have do-notation which makes
everyone happy. If there would be a way in OCaml to write "foo e" for some
keyword "foo" which is like "begin" but does not require "end", I would be
pretty happy to change the weird "Option.do_; e" to "foo%Option e". Before
implementing "do_; e", I tried a bit of "[%do] e" but it did not work well
since "[%do] p <-- e" is parsed as "([%do] p) <-- e", not "[%do] (p <-- e)".

Best,
Jun
On 20 Jan, 2015 5:53 am, "Drup" <drupyog+caml@zoho.com> wrote:

>
>  I can appreciate that authors of tools that requires special syntactic
>> support would love to have their new forms look completely native to users,
>> but the counter-argument can be made that keeping an explicit syntax
>> (through the '%' character) for features that are not part of the official
>> language is a good property. (Camlp4/campl5 are still available for people
>> who want to play with the concrete syntax.)
>>
>
> I personally like the explicitness of the syntax a lot. The only issue in
> OCaml currently is that, given the need for retro compatibility, it goes
> sometimes against the terseness. For example the impossibility to do " x@foo"
> instead of "x[@foo]". That's unavoidable, though.
>
> This is, by the way, a point I dislike a lot with ppx_monadic. It abuses
> the native syntax in completely alien ways and without annotations.
>
> I like ppx_monad's syntax quite better due to the fact that it's always
> explicitly a ppx (thanks to %monad) and do not overload the "do_"
> identifier.
>
>

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