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pattern matching and records vs tuples
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Date: -- (:)
From: Yoann Padioleau <padator@w...>
Subject: Re: [Caml-list] pattern matching and records vs tuples
Goswin von Brederlow <goswin-v-b@web.de> writes:

> Yoann Padioleau <padator@wanadoo.fr> writes:
>
>> Hi, 
>>
>> I've found that while records provide advantages over tuples, 
>> they also have disadvantages when it comes to evolution issues. 
>> If I decide to evolve code using a tuple type, for instance adding
>> new information and so extend a 4-uple in a 5-uple, then the compiler
>> will tell me all the places that I need to update, which is good.
>> If I use records instead, and have 4 fields, and I want to add again
>> some new information in a new field, then the way the compiler works
>> right now will not help me at all. E.g with this code 
>>
>>   type record = { 
>>     field1: int;
>>     field2: int;
>>   }
>>   let foo = function 
>>    { field1 = v1; field2 = v2 } -> ...
>>
>>
>> If I extend record with a new field   field3, then ocaml will
>> not warn me and tell me to modify also the 'foo' function :(
>
> On the other hand that is a verry good thing.
>

I didn't say records have only disadvantages.


> let set_field1 r x = { r with field1 = x }
> let set_field2 r x = { r with field2 = x }
>
> Try doing that with tuples and then add a 3rd, 4th, 5th field.
>
> I have to say I didn't even know you could match a record partially
> or have ever wanted to match a record at all. I use record only when I
> have a collection of otherwise independent values. As such any
> matching will be done on a single component of the record but never
> pairs of them. At least I can't remember having done so before.

Ok ... 

Sometimes people want the partial match behavior, but sometimes people
prefer another behavior, so I just propose a kind of annotation
in the pattern that describes what behavior the programmer wants.

>
> MfG
>         Goswin