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From: oleg@okmij.org
To: jun.furuse@gmail.com, caml-list@inria.fr
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Message-Id: <20150119073303.038ECC38BA@www1.g3.pair.com>
Date: Mon, 19 Jan 2015 02:33:03 -0500 (EST)
Subject: Re: [Caml-list] [ANN] ppx_monadic.1.0.2, ppx for monadic do,


> Current OCaml syntax has shortage of pattern binding expression and
> only usable is let%xxx p = e in which is a bit pain for ppx writers.

Indeed. One may wish that

        let rec p = e1 in e2

were treated as if it were
        let[@ocaml.let "rec"] p = e1 in e2

and likewise let module. Come to think of it, 'rec! or 'module' are
annotations on let. Then we can generalize so that

        let something p = e1 in e2
(where something is any reserved word or an operator -- something that
cannot be mistaken for a regular identifier) is parsed as if it were

        let[@ocaml.let "something"] p = e1 in e2

The type-checker will complain if it finds the ocaml.let attribute it
does not understand. (Therefore, Typedtree does not have to be changed
-- although one may wish for a specifically letrec node).  Thus we
can define let !, let >>=, let LWP, let struct, let when, let if and all other
potentially useful binding forms.