* [Caml-list] Default values for structures (feature request)
@ 2004-06-02 12:12 Richard Jones
2004-06-02 12:56 ` Jacques GARRIGUE
0 siblings, 1 reply; 6+ messages in thread
From: Richard Jones @ 2004-06-02 12:12 UTC (permalink / raw)
To: caml-list
Could have a syntax for default values for structures? Something
like:
type t = { field1 : int = 0;
field2 : string; (* no default *)
}
It's sort of possible to do this currently for non-mutable structures
by having a default_value declared, and writing:
let new_value = { default_value with field2 = "new string" }
but this doesn't allow the module to force the user to declare certain
fields in a new value.
The reason for having default fields is so that I can export struct-
ures from my module interface, but be able to add fields with default
values later without breaking dependent code. (But in some circum-
stances I would deliberately add fields without default values in
order to break code).
Rich.
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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Caml-list] Default values for structures (feature request)
2004-06-02 12:12 [Caml-list] Default values for structures (feature request) Richard Jones
@ 2004-06-02 12:56 ` Jacques GARRIGUE
2004-06-03 9:30 ` Sven Luther
2004-06-03 9:31 ` Richard Jones
0 siblings, 2 replies; 6+ messages in thread
From: Jacques GARRIGUE @ 2004-06-02 12:56 UTC (permalink / raw)
To: rich; +Cc: caml-list
From: Richard Jones <rich@annexia.org>
> Could have a syntax for default values for structures? Something
> like:
>
> type t = { field1 : int = 0;
> field2 : string; (* no default *)
> }
The difficulty should be apparent in your example: this supposes
writing values in interfaces, which would be a major (huge) change in
language design...
Note that labeled arguments let you do that.
let new_t ?(field1 = 0) ~field2 () = {field1=field1; field2=field2}
> The reason for having default fields is so that I can export struct-
> ures from my module interface, but be able to add fields with default
> values later without breaking dependent code. (But in some circum-
> stances I would deliberately add fields without default values in
> order to break code).
This is the point with optional/labeled arguments.
Jacques Garrigue
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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Caml-list] Default values for structures (feature request)
2004-06-02 12:56 ` Jacques GARRIGUE
@ 2004-06-03 9:30 ` Sven Luther
2004-06-03 16:33 ` Christopher Dutchyn
2004-06-03 9:31 ` Richard Jones
1 sibling, 1 reply; 6+ messages in thread
From: Sven Luther @ 2004-06-03 9:30 UTC (permalink / raw)
To: Jacques GARRIGUE; +Cc: rich, caml-list
On Wed, Jun 02, 2004 at 09:56:55PM +0900, Jacques GARRIGUE wrote:
> From: Richard Jones <rich@annexia.org>
>
> > Could have a syntax for default values for structures? Something
> > like:
> >
> > type t = { field1 : int = 0;
> > field2 : string; (* no default *)
> > }
>
> The difficulty should be apparent in your example: this supposes
> writing values in interfaces, which would be a major (huge) change in
> language design...
>
> Note that labeled arguments let you do that.
>
> let new_t ?(field1 = 0) ~field2 () = {field1=field1; field2=field2}
>
> > The reason for having default fields is so that I can export struct-
> > ures from my module interface, but be able to add fields with default
> > values later without breaking dependent code. (But in some circum-
> > stances I would deliberately add fields without default values in
> > order to break code).
>
> This is the point with optional/labeled arguments.
> Jacques Garrigue
Well, what about adding a syntaxic wrapper over those to let you
automatically do it using a syntax similar to what Richard Jones
proposed ?
Friendly,
Sven luther
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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Caml-list] Default values for structures (feature request)
2004-06-02 12:56 ` Jacques GARRIGUE
2004-06-03 9:30 ` Sven Luther
@ 2004-06-03 9:31 ` Richard Jones
2004-06-03 16:34 ` Christopher Dutchyn
1 sibling, 1 reply; 6+ messages in thread
From: Richard Jones @ 2004-06-03 9:31 UTC (permalink / raw)
Cc: caml-list
On Wed, Jun 02, 2004 at 09:56:55PM +0900, Jacques GARRIGUE wrote:
> From: Richard Jones <rich@annexia.org>
>
> > Could have a syntax for default values for structures? Something
> > like:
> >
> > type t = { field1 : int = 0;
> > field2 : string; (* no default *)
> > }
>
> The difficulty should be apparent in your example: this supposes
> writing values in interfaces, which would be a major (huge) change in
> language design...
Out of interest, is there a theoretical reason why types shouldn't
contain values, or is it just because it's a lot of work to change the
compiler?
Rich.
--
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Merjis Ltd. http://www.merjis.com/ - improving website return on investment
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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Caml-list] Default values for structures (feature request)
2004-06-03 9:30 ` Sven Luther
@ 2004-06-03 16:33 ` Christopher Dutchyn
0 siblings, 0 replies; 6+ messages in thread
From: Christopher Dutchyn @ 2004-06-03 16:33 UTC (permalink / raw)
To: Sven Luther; +Cc: Jacques GARRIGUE, rich, caml-list
On Thu, 3 Jun 2004, Sven Luther wrote:
> Well, what about adding a syntaxic wrapper over those to let you
> automatically do it using a syntax similar to what Richard Jones
> proposed ?
Sounds like a job for ... camlp4!
Chris Dutchyn
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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Caml-list] Default values for structures (feature request)
2004-06-03 9:31 ` Richard Jones
@ 2004-06-03 16:34 ` Christopher Dutchyn
0 siblings, 0 replies; 6+ messages in thread
From: Christopher Dutchyn @ 2004-06-03 16:34 UTC (permalink / raw)
To: Richard Jones; +Cc: caml-list
On Thu, 3 Jun 2004, Richard Jones wrote:
> Out of interest, is there a theoretical reason why types shouldn't
> contain values, or is it just because it's a lot of work to change the
> compiler?
This leads to dependent types, and pure type systems, and other esoterica
that are still active research areas -- (read: there are important
questions lacking goood answers).
Chris Dutchyn
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^ permalink raw reply [flat|nested] 6+ messages in thread
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2004-06-02 12:12 [Caml-list] Default values for structures (feature request) Richard Jones
2004-06-02 12:56 ` Jacques GARRIGUE
2004-06-03 9:30 ` Sven Luther
2004-06-03 16:33 ` Christopher Dutchyn
2004-06-03 9:31 ` Richard Jones
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