transformers 0.4: change in accessor function exports?

[John Lato]

I don't really care one way or the other how this is resolved, but does it
strike anyone else that current practices must be remarkably fragile for
such a minor change (which doesn't even change exported names!) to
supposedly cause so much breakage?

John L.


On Tue, May 6, 2014 at 2:30 PM, Edward Kmett <ekmett at gmail.com> wrote:

> It may make the code in transformers a bit more confusing for a handful of
> instances, but this is breaking almost every user I've talked to.
>
> This is proving to be a major breaking change as users commonly import
> StateT(..) and go off and use runStateT, etc.
>
> I wrote the imports in mtl 2.2 in such a way that if you recanted and
> chose to switch back to the old style, it'd still work.
>
> I just know that I personally have 200+ modules to change as a result, for
> no better experience as a user.
>
> -Edward
>
>
> On Wed, May 7, 2014 at 3:18 AM, Ross Paterson <R.Paterson at city.ac.uk>wrote:
>
>> On Tue, May 06, 2014 at 07:16:40PM +0300, Michael Snoyman wrote:
>> > One of the changes[1] in transformers 0.4 is as follows:
>> >
>> > 0.3:
>> >
>> >     newtype Identity a = Identity { runIdentity :: a }
>> >
>> > 0.4:
>> >
>> >     newtype Identity a = Identity a
>> >     runIdentity (Identity x) = x
>> >
>> > While this may seem benign, I've already seen three cases where this
>> caused
>> > breakage[2][3][4].
>> >
>> > Is there a reason for this change in 0.4? If not, I'd like to request
>> moving
>> > back to the previous formulation to avoid unnecessary breakage.
>>
>> Read and Show instances were introduced in 0.4.  With the record form,
>> the default instances for those classes would be very cumbersome.
>> The alternative of defining custom instances that differ from the default
>> ones would make the interface more confusing.
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>
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