[Caml-list] "changing" immutable record elements

[Caml-list] "changing" immutable record elements

[David Fox]

Unless I am missing some core language feature, it seems to me that the 
most awkward part of the language is constructing an expression that 
represents a record with a single field modified:

let set_field3 rec value =
   match rec with
       {field1=field1; field2=field2; field3=field3; field4=field4; 
field5=field5} ->
          {field1=field1; field2=field2; field3=value; field4=field4; 
field5=field5}

is there any prettier way of doing this?


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Re: [Caml-list] "changing" immutable record elements

[Martin Jambon]

On Fri, 14 May 2004, David Fox wrote:

> Unless I am missing some core language feature, it seems to me that the
> most awkward part of the language is constructing an expression that
> represents a record with a single field modified:
>
> let set_field3 rec value =
>    match rec with
>        {field1=field1; field2=field2; field3=field3; field4=field4;
> field5=field5} ->
>           {field1=field1; field2=field2; field3=value; field4=field4;
> field5=field5}
>
> is there any prettier way of doing this?

Yes!

let new_record = { old_record with field3 = value }

NB: you must specify at least one field:
let copy r = { r with some_field = r.some_field }


Martin


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Re: [Caml-list] "changing" immutable record elements

[Virgile Prevosto]

Le vendredi 14 mai, à  8h50 -0700,
David Fox a écrit:

> Unless I am missing some core language feature, it seems to me that
> the most awkward part of the language is constructing an expression
> that represents a record with a single field modified:
> 
> let set_field3 rec value =
>    match rec with
>        {field1=field1; field2=field2; field3=field3; field4=field4; 
> field5=field5} ->
>           {field1=field1; field2=field2; field3=value; field4=field4; 
> field5=field5}
> 
> is there any prettier way of doing this?
Yes,
let set_field_3 my_rec v = {my_rec with field3=v} 
(cf http://pauillac.inria.fr/ocaml/htmlman/manual015.html, section
6.7.3)

By the way, rec and value are keywords in ocaml. you might run
into trouble if you try to use them as identifiers ;-)


-- 
E tutto per oggi, a la prossima volta
Virgile

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Re: [Caml-list] "changing" immutable record elements

[Matt Gushee]

On Fri, May 14, 2004 at 06:22:01PM +0200, Virgile Prevosto wrote:
> 
> By the way, rec and value are keywords in ocaml. you might run
> into trouble if you try to use them as identifiers ;-)

No, 'value' is safe to use. The one you need to avoid is 'val'.

-- 
Matt Gushee                 When a nation follows the Way,
Englewood, Colorado, USA    Horses bear manure through
mgushee@havenrock.com           its fields;
http://www.havenrock.com/   When a nation ignores the Way,
                            Horses bear soldiers through
                                its streets.
                                
                            --Lao Tzu (Peter Merel, trans.)

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Re: [Caml-list] "changing" immutable record elements

[Virgile Prevosto]

Le vendredi 14 mai, à 11h02 -0600,
Matt Gushee a écrit:

> On Fri, May 14, 2004 at 06:22:01PM +0200, Virgile Prevosto wrote:
> > 
> > By the way, rec and value are keywords in ocaml. you might run
> > into trouble if you try to use them as identifiers ;-)
> 
> No, 'value' is safe to use. The one you need to avoid is 'val'.
> 
Oops, sorry about that. I guess I've played too much with camlp4 and its
revised syntax lately...

-- 
E tutto per oggi, a la prossima volta
Virgile

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Re: [Caml-list] "changing" immutable record elements

[Kenneth Knowles]

On Fri, May 14, 2004 at 08:50:32AM -0700, David Fox wrote:
> Unless I am missing some core language feature, it seems to me that the 
> most awkward part of the language is constructing an expression that 
> represents a record with a single field modified:
> 
> let set_field3 rec value =
>   match rec with
>       {field1=field1; field2=field2; field3=field3; field4=field4; 
> field5=field5} ->
>          {field1=field1; field2=field2; field3=value; field4=field4; 
> field5=field5}
> 
> is there any prettier way of doing this?

Yes

let set_field3 rec value = {rec with field3 = value}

Kenn

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Re: [Caml-list] "changing" immutable record elements

[Andrew Lenharth]

let set_field3 r value = {r with field3=falue}

Have fun

Andrew Lenharth

On Fri, May 14, 2004 at 08:50:32AM -0700, David Fox wrote:
> Unless I am missing some core language feature, it seems to me that the 
> most awkward part of the language is constructing an expression that 
> represents a record with a single field modified:
> 
> let set_field3 rec value =
>   match rec with
>       {field1=field1; field2=field2; field3=field3; field4=field4; 
> field5=field5} ->
>          {field1=field1; field2=field2; field3=value; field4=field4; 
> field5=field5}
> 
> is there any prettier way of doing this?
> 
> 
> -------------------
> To unsubscribe, mail caml-list-request@inria.fr Archives: 
> http://caml.inria.fr
> Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: 
> http://caml.inria.fr/FAQ/
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-- 
"It will work in practice, yes. But will it work in theory?"
--- a french diplomat's comment, recalled by Madeleine  Albright

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