[cwn] Attn: Development Editor, Latest Caml Weekly News

[Alan Schmitt]

Hello,

Here is the latest Caml Weekly News, for the week of March 27 to  
April 03, 2007.

1) camlp4: let binding

========================================================================
1) camlp4: let binding
Archive: <http://groups.google.com/group/ocaml-developer/browse_frm/ 
thread/60e0ea039394bff6>
------------------------------------------------------------------------
** Hugo Ferreira asked, Martin Jambon said, and Nicolas Pouillard  
answered:

 > > I would like to use a reserved word and substitute that by a  
function
 > > call. For example, the following:
 > >
 > > let  _ = HEAP in
 > > let h1 = HEAP in
 > >
 > > would become
 > >
 > > let __heap = new_heap () in
 > > let     h1 = new_heap () in


As always it will be a lot simpler to do that kind of thing using a
filter in camlp4 3.10.
...
match (* do things bottom-up *) super#expr e with
| <:expr at loc< let _ = HEAP in $e$ >> ->
      <:expr at loc< let __heap = new_heap () in $e$ >>
| <:expr at loc< let $p$ = HEAP in $e$ >> ->
      <:expr at loc< let $p$ = new_heap () in $e$ >>
| e -> e
...

 > You shouldn't try to do this because the parser looks only one  
token ahead
 > to make its decision. If you add a rule that starts from "let" (it  
has
 > to), the token which enables the parser to select this rule is in  
position
 > 3, so it comes too late. Camlp4 will not warn you about the  
conflict but
 > fail during preprocessing because it will choose either the  
predefined
 > "let" rule or yours without knowing if it's the right one.

In fact it's wrong camlp4 can takes more than one token of look ahead.
It will try to match the input with all the firsts terminals of a rule.
However you're right to discourage him to try that kind of thing.
Indeed it's highly dependent on the left factorization mechanism
performed by camlp4.
		
** Hugo Ferreira asked and Nicolas Pouillard answered:

 > Just a comment: my greatest difficulty with camlp4 is twofold:
 >
 > - knowing how the AST maps to the actual Ocaml syntax.


You can ask to camlp4 itself!
Using the old one...

$ cat test.ml
(* here is some expression that you want to understand *)
fun e -> <:expr< let x= HEAP in $e$ >>

$ camlp4o q_MLast.cmo pr_o.cmo test.ml
let _ =
   fun e ->
     MLast.ExLet
       (_loc, false, [MLast.PaLid (_loc, "x"), MLast.ExUid (_loc,  
"HEAP")], e)

 > - knowing when/where the camlp4 parser does its transformations so  
that
 > I may alter these transformations (match and replace expressions).

That's not really transformations of the AST.
However there is some grammar tranformations:
When you add a rule to a grammar entry at a specific level camlp4
insert it in a tree to factor out the prefix or rules as much as
possible.

e1 = SELF; "+"; e2 = SELF -> ...
e1 = SELF; "*"; e2 = SELF -> ...

is in fact something like:

e1 = SELF; [ "+"; e2 = SELF -> ... | "*";  e2 = SELF -> ... ]

So if you add something make it as much as possible a prefix of  
existing ones.

[...]

 > In effect I write Ocaml "expression" which is automatically  
matched and
 > I only need write an Ocaml expression to introduce the changes I  
want.
 > All this using simply using the quotation system. The rules and  
the AST
 > structure itself should, in my humble opinion, be avoided.

Indeed, extension that are highly dependent on the actual grammar
should be avoided.

 > This will also allow the underlying camlp4 system to change  
without unduly
 > affecting user's code.

Exactly!
		
========================================================================
Using folding to read the cwn in vim 6+
------------------------------------------------------------------------
Here is a quick trick to help you read this CWN if you are viewing it  
using
vim (version 6 or greater).

:set foldmethod=expr
:set foldexpr=getline(v:lnum)=~'^=\\{78}$'?'<1':1
zM
If you know of a better way, please let me know.

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========================================================================


-- 
Alan Schmitt <http://alan.petitepomme.net/>

The hacker: someone who figured things out and made something cool  
happen.
  .O.
  ..O
  OOO


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