[Caml-list] default values for record field definitions

[Caml-list] default values for record field definitions

[Alexander V. Voinov]

Hi All,

It may well be that I'm not in the first hundred of those who proposed
this, if so - sorry :-).

Why not to introduce a syntax for default values of record fields:

type rec1 = { attr1: type1 = val1; attr2: type2; attr3: type3 = val3 }

It this case constructions of the form

let x = { attr2 = val2 } are possible, with other fields filled by val1,
val3.

Can it be done via camlp4?

Alexander
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Re: [Caml-list] default values for record field definitions

[Ian Zimmerman]

Alexander> Why not to introduce a syntax for default values of record
Alexander> fields:

Alexander> type rec1 = { attr1: type1 = val1; attr2: type2; attr3:
Alexander> type3 = val3 }

Alexander> It this case constructions of the form

Alexander> let x = { attr2 = val2 } are possible, with other fields
Alexander> filled by val1, val3.

type rec1 = { attr1: type1; attr2: type2; attr3: type3 };
let rec1_default = { attr1 = val1; attr2 = val2; attr3 = val3 } in
let x = { rec1_default with
        attr2 = val2'
         }
and ...

BTW, I couldn't find this in the manual!

-- 
Ian Zimmerman, Oakland, California, U.S.A.
EngSoc adopts market economy: cheap is wasteful, efficient is expensive.
GPG pub key: 433BA087 9C0F 194F 203A 63F7 B1B8 6E5A 8CA3 27DB 433B A087
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Re: [Caml-list] default values for record field definitions

[Dmitry Bely]

"Alexander V. Voinov" <avv@quasar.ipa.nw.ru> writes:

> It may well be that I'm not in the first hundred of those who proposed
> this, if so - sorry :-).
> 
> Why not to introduce a syntax for default values of record fields:
> 
> type rec1 = { attr1: type1 = val1; attr2: type2; attr3: type3 = val3 }
> 
> It this case constructions of the form
> 
> let x = { attr2 = val2 } are possible, with other fields filled by val1,
> val3.
> 
> Can it be done via camlp4?

Why don't you like

type rec1 = { attr1: type1; attr2: type2; attr3: type }
let default_rec1 = { attr1 = init_val1; attr2 = init_val2; attr = init_val3 }
...
let x = { default_rec1 with attr2 = val2 }

?

Hope to hear from you soon,
Dmitry


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Re: [Caml-list] default values for record field definitions

[David Mentre]

Ian Zimmerman <itz@speakeasy.org> writes:

> BTW, I couldn't find this in the manual!

It is burried in it: paragraph 7.5 at
http://caml.inria.fr/ocaml/htmlman/manual020.html


d.
-- 
 David.Mentre@inria.fr
 Opinions expressed here are only mine.
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Re: [Caml-list] default values for record field definitions

[Alexander V. Voinov]

Hi All,

Ian Zimmerman wrote:

> type rec1 = { attr1: type1; attr2: type2; attr3: type3 };
> let rec1_default = { attr1 = val1; attr2 = val2; attr3 = val3 } in
> let x = { rec1_default with
>         attr2 = val2'
>          }

Dmitry Bely wrote:

>
> type rec1 = { attr1: type1; attr2: type2; attr3: type }
> let default_rec1 = { attr1 = init_val1; attr2 = init_val2; attr = init_val3 }
> ...
> let x = { default_rec1 with attr2 = val2 }
>

Yes, this is nice. But it's a workaround.

Alexander

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Re: [Caml-list] default values for record field definitions

[FabienFleutot]

> > Why not to introduce a syntax for default values of record fields:
> > type rec1 = { attr1: type1 = val1; attr2: type2; attr3: type3 = val3 }
>
> Why don't you like
> type rec1 = { attr1: type1; attr2: type2; attr3: type }
> let default_rec1 = { attr1 = init_val1; attr2 = init_val2; attr =
init_val3 }
> ...
> let x = { default_rec1 with attr2 = val2 }

That seems to be the most usual workaround, but it'd be nice to allow
writing:

let x = {default_rec1 with (* Nothing! *) }

to produce a copy of default_rec1 instead of a pointer to it, as would do:

let x = default_rec1

(useful when there are some mutable fields). It doesn't seem to be difficult
to add to the compiler...
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Re: [Caml-list] default values for record field definitions

[Daniel de Rauglaudre]

Hi,

On Wed, Jul 18, 2001 at 09:13:47PM -0700, Alexander V. Voinov wrote:

> Why not to introduce a syntax for default values of record fields:
> type rec1 = { attr1: type1 = val1; attr2: type2; attr3: type3 = val3 }
> 
> It this case constructions of the form
> let x = { attr2 = val2 } are possible, with other fields filled by val1,
> val3.
> 
> Can it be done via camlp4?

No, it is not purely syntactic. The definition type has to generate a
"normal" type definition *plus* some appropriate function to set the
default values to the undefined fields. This is possible to do that
in Camlp4, but I don't see what interface give to this function.

And for the record usage, it should call this function, but I am not
sure it is possible. And we have to change that for all defined record
expressions. Simpler if there is a specific syntax for those records.
And I am not sure it would be enough.

An argument to say that it is not syntactic is the fact that the record
fields have a meaning, what is typically a work at typing time. The fact
that "attr2" is a record field (with a default value or not) is typing.
In Camlp4, at parse time, there is no link between the "attr2" of the
record definition and the "attr2" of the record expression. They can
even exist in different compilation units: in this case, syntactic analysis
can do nothing.

-- 
Daniel de RAUGLAUDRE
daniel.de_rauglaudre@inria.fr
http://cristal.inria.fr/~ddr/
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