[Haskell-cafe] record update

[Jonathan Geddes]

On Sat, Sep 11, 2010 at 11:53 AM, Henning Thielemann
<schlepptop at henning-thielemann.de> wrote:
> Jonathan Geddes schrieb:
>> I know that record updates is a topic that has become a bit of a dead
>> horse, but here I go anyway:
>>
>> I find that most of the record updates I read and write take the form
>>
>>> someUpdate :: MyRecord -> MyRecord
>>> someUpdate myRecord = myRecord
>>>     { field1 = f $ field1 myRecord
>>>     , field2 = g $ field2 myRecord
>>>     , field3 = h $ filed3 myRecord
>>>     }
>>
>> I find myself wishing I could write something more like
>>
>>> someUpdate :: MyRecord -> MyRecord
>>> someUpdate myRecord = myRecord
>>>     { field1 => f
>>>     , field2 => g
>>>     , field3 => h
>>>     }
>
> data-accessor and similar packages may become your friends.
>
> data-accessor allows you to write:
>
> someUpdate =
>   (field1 ^: f) .
>   (field2 ^: g) .
>   (field3 ^: h)
>

data-accessor is a pretty cool package, but if I understand correctly,
your "fields" are not the same as the straight functions you get from
defining a record, and cant' be used as regular functions. So you have
to create these Data.Accessor.Accessors.

Is defining accessors really any better than just writing update
functions like the following?

> updateField1 :: (Field1Type -> Field1Type) -> MyRecord -> MyRecord
> updateField1 f x = x{field1 = f $ field1 x}
> someUpdate = (updateField1 f) . j(updateField2 g) . (updateField3 h)

I understand that there is a package data-accessor-template for
generating accessors, but couldn't you use TH for generating updater
functions as well?

It seems like something as fundamental as record update should have a
clean, build-in syntax. Or am I thinking too imperatively?

--Jonathan