transformers 0.4: change in accessor function exports?

[Merijn Verstraaten]

Wouldn't it be simpler/less breaking to keep "newtype Identity a = Identity { runIdentity :: a }" and simply hand write Show/Read instances that function asif it was "newtype Identity a = Identity a", giving us best of both worlds?

Cheers,
Merijn

On May 6, 2014, at 23:30 , Edward Kmett 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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