From: Steven Thomson <sthomson@zeta.org.au>
To: caml-list@inria.fr
Subject: Re: Syntax for label, ANOTHER NEW PROPOSAL
Date: Sat, 18 Mar 2000 10:04:22 +1000 [thread overview]
Message-ID: <38D2C806.219E784B@zeta.org.au> (raw)
In-Reply-To: <20000315145830.22463@pauillac.inria.fr>
Pierre Weis wrote: [part of long post omitted]
>
> $ ocaml
> Objective Caml version 2.99+10
>
> # let sum l = List.fold_right ( + ) l 0;;
> val sum : int list -> int = <fun>
>
> Clearly application is denoted in ML with only one character: a space.
>
> Now, consider using the so-called ``Modern'' versions of these
> functionals, obtained with the -modern option of the compiler:
>
> $ ocamlpedantic
> Objective Caml version 2.99+10
>
> # let sum l = List.fold_right ( + ) l 0;;
> ^^^^^
> This expression has type int -> int -> int but is here used with type 'a list
>
> Clearly, there is something wrong now! We may remark that the error
> message is not that clear, but this is a minor point, since error
> messages are never clear enough anyway!
>
> The real problem is that fixing the code makes no good at all to its
> readability (at least that's what I would say):
>
> # let sum l = List.fold_right fun:begin fun x acc:y -> x + y end acc:0;;
> val sum : 'a -> int list -> int = <fun>
>
> It seems that, in the ``modern'' mode, application of higher order
> functions is now denoted by a new kind of parens opening by
> ``fun:begin fun'' and ending by ``end''. This is extremely explicit
> but also a bit heavy (in my mind).
>
> For all these reasons, I would suggest to carefully use labels into
> the standard libraries:
>
> -- remove labels from higher-order functional
I don't think that merely omitting labels from higher-order functional
arguments really solves the problem. Consider the following:
$ ocaml -modern
Objective Caml version 2.99 (99/12/08)
# let add' :acc x = acc + x;;
val add' : acc:int -> int -> int = <fun>
# let rec fold_left' f accu l =
match l with
[] -> accu
| a::l -> fold_left' f (f accu a) l;;
val fold_left' : ('a -> 'b -> 'a) -> 'a -> 'b list -> 'a = <fun>
# let sum' = fold_left' add' 0;;
Characters 24-28:
This expression has type acc:int -> int -> int but is here used with
type
'a -> 'b -> 'a
If the definition of fold_left' does NOT use a label, we still get a
compile error whenever we try to use an argument that DOES have a label.
My proposal is to introduce an operator that can be used to tell the
compiler to ignore labels when type-checking a higher-order functional
argument. This could be thought of as a pragma which locally switches
the compiler from modern to classic mode.
Some possible variations:
1. A prefix operator, here shown as "~", that causes only the following
expression to be evaluated in classic mode:
let sum = List.fold_left fun:~(+) acc:0
let sum' = fold_left' ~add' 0
2. A bracketing construct, here shown as [~ ~], that causes everything
inside the brackets to be evaluated in classic mode.
let sum = List.fold_left fun:[~(+)~] acc:0
let sum' = fold_left' [~add'~] 0
option 2 is not as visually lightweight, but might be more useful if
there are several arguments that need to be altered:
let aaa = bbb ccc:~ddd eee:~fff ggg:~hhh ~iii jjj
versus
let aaa = bbb [~ ccc:ddd eee:fff ggg:hhh iii ~] jjj
next prev parent reply other threads:[~2000-03-18 17:55 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2000-03-14 16:53 Syntax for label Don Syme
2000-03-14 18:05 ` Pierre Weis
2000-03-15 3:15 ` Syntax for label, NEW PROPOSAL Jacques Garrigue
2000-03-15 6:58 ` Christophe Raffalli
2000-03-15 21:54 ` Julian Assange
2000-03-15 11:56 ` Wolfram Kahl
2000-03-15 13:58 ` Pierre Weis
2000-03-15 15:26 ` Sven LUTHER
2000-03-17 7:44 ` Pierre Weis
2000-03-15 17:04 ` John Prevost
2000-03-17 10:11 ` Jacques Garrigue
2000-03-15 17:06 ` Markus Mottl
2000-03-15 19:11 ` Remi VANICAT
2000-03-17 8:30 ` Pierre Weis
2000-03-17 14:05 ` Jacques Garrigue
2000-03-17 16:08 ` Pierre Weis
2000-03-18 10:32 ` Syntax for label, NEW SOLUTION Christophe Raffalli
2000-03-19 2:29 ` Jacques Garrigue
2000-03-20 18:25 ` Christophe Raffalli
2000-03-22 8:37 ` Claudio Sacerdoti Coen
2000-03-21 23:29 ` John Max Skaller
2000-03-29 8:42 ` Semantic of label: The best (only ?) solution to merge both mode Christophe Raffalli
2000-03-29 9:53 ` Christophe Raffalli
2000-03-30 9:49 ` John Max Skaller
2000-03-30 9:39 ` John Max Skaller
2000-03-31 4:34 ` Jacques Garrigue
2000-04-01 1:53 ` John Max Skaller
2000-04-02 19:24 ` Christophe Raffalli
2000-04-04 5:50 ` Jacques Garrigue
2000-04-03 7:57 ` backward compatibility Christophe Raffalli
2000-03-15 21:30 ` Syntax for label, NEW PROPOSAL John Max Skaller
2000-03-16 2:55 ` Jacques Garrigue
2000-03-17 15:13 ` Pierre Weis
2000-03-17 17:33 ` Wolfram Kahl
2000-03-18 11:59 ` Jacques Garrigue
2000-03-21 16:51 ` Pascal Brisset
2000-03-23 11:14 ` Nicolas barnier
2000-03-24 9:54 ` labels & ocaml 3 & co David Mentré
2000-03-24 12:19 ` David Mentré
2000-03-21 22:22 ` Unsigned integers? John Max Skaller
2000-03-22 16:22 ` Sven LUTHER
2000-03-23 2:08 ` Max Skaller
2000-03-23 7:50 ` Sven LUTHER
2000-03-24 2:50 ` Jacques Garrigue
2000-03-24 15:59 ` Xavier Leroy
2000-03-25 4:03 ` John Max Skaller
2000-03-24 14:50 ` Xavier Leroy
2000-03-22 17:05 ` Jean-Christophe Filliatre
2000-03-22 19:10 ` Markus Mottl
2000-03-23 2:41 ` Max Skaller
2000-03-22 19:47 ` Xavier Leroy
2000-03-23 12:55 ` John Max Skaller
2000-03-16 8:50 ` Syntax for label, NEW PROPOSAL Pascal Brisset
2000-03-17 11:15 ` Sven LUTHER
2000-03-18 0:04 ` Steven Thomson [this message]
2000-03-15 20:39 ` Syntax for label (and more) Xavier Leroy
2000-03-17 10:03 ` Christian RINDERKNECHT
2000-03-17 17:19 ` Christophe Raffalli
2000-03-21 1:29 ` Markus Mottl
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