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

[Alain Frisch <alain@frisch.fr>]

On 01/18/2015 03:47 PM, Jun Furuse wrote:
> Thanks,
>
> let! is not valid in the vanilla syntax therefore I guess you have
> patched the compiler.
>
> 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.

If it's a pain, it's rather for users of ppx, no?

Providing "alternative" forms of existing constructs (i.e. forms that 
look syntactically similar to these constructs) to be interpreted before 
they reach the type-checker, is indeed what extension nodes [%foo ...] 
were designed for.  For expression constructs starting with a keyword, 
the short form KEYWORD%foo ... makes the use of extension nodes very 
cheap syntactically.

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.)


With the short form, an alternative binding construct can be as short as:

  let%L p = e in ....

I'm not suggesting to reserve one-letter extension identifiers for 
specific projects, but each tool could let the user choose explicitly 
which letter to use:


   [%%Project1 alt_id = "L"]
   [%%Project2 alt_id = "A"]
   ...
   ...
     let%L p1 = e1 in ....
     let%A p2 = e2 in ....



Alain