[Caml-list] Records: syntax error...

[Caml-list] Records: syntax error...

[chris.danx]

Hi,

I'm working my way through the ocaml book, and came across one exercise 
I can't get to work (sort of).  The exercise asked to define insert over 
the following type

type 'a dList =
   {l: 'a list; compare : 'a -> 'a -> bool; in_order : bool};;

I can get it to work with code similar to the solution, but since I had 
defined a merge function in the same file I decided to try using pattern 
matching on the record...


let insert item dl =
   match dl with
       {l = _; compare = _; in_order = true}  ->
               {merge dl.compare (dl.l, [item]), dl.compare, true}
     | {l = _; compare = _; in_order = false} ->
               {item::dl.l, dl.compare, false};;

but ocaml complains of a syntax error at dl.compare.  What did I do wrong?



Thanks,
Chris

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Re: [Caml-list] Records: syntax error...

[Aleksey Nogin]

On 17.11.2003 16:23, chris.danx wrote:

> let insert item dl =
>   match dl with
>       {l = _; compare = _; in_order = true}  ->

Hint: you do not have to mention the fiends that you are not interested in.

So you could just use

... match dl with
        { in_order = true } -> ...

>               {merge dl.compare (dl.l, [item]), dl.compare, true}

You have to use the explicit field names when you construct records:

{ l = merge dl.compare (dl.l, [item]);
   compare = dl.compare;
   in_order = true
}

Hint: if you want to create a new record that only slightly differs from 
an existing one, use the "with" construction:

{ dl with l = merge dl.compare (dl.l, [item]) }

>     | {l = _; compare = _; in_order = false} ->
>               {item::dl.l, dl.compare, false};;

Hint: it is IMHO a bad style to use "match" expressions simply to check 
on a boolean value - it's much more intuitive to use "if/then/else" for 
that.

P.S. Here is how I would have written the insert function above:

let insert item dl =
    { dl with l =
        if dl.in_order then
           merge dl.compare (dl.l, [item])
        else
           item::dl.l
    }

-- 
Aleksey Nogin

Home Page: http://nogin.org/
E-Mail: nogin@cs.caltech.edu (office), aleksey@nogin.org (personal)
Office: Jorgensen 70, tel: (626) 395-2907

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Re: [Caml-list] Records: syntax error...

[chris.danx]

Aleksey Nogin wrote:

> Hint: you do not have to mention the fiends that you are not interested in.

> Hint: if you want to create a new record that only slightly differs from 
> an existing one, use the "with" construction:

> Hint: it is IMHO a bad style to use "match" expressions simply to check 
> on a boolean value - it's much more intuitive to use "if/then/else" for 
> that.

Thankyou!  This is very helpful.


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