transformers 0.4: change in accessor function exports?

[Michael Snoyman]

On Wed, May 7, 2014 at 2:41 PM, Tom Ellis <
tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:

> On Wed, May 07, 2014 at 02:10:42PM +0300, Michael Snoyman wrote:
> > > > Btw, as I can't think of an example right now, what would be an
> example
> > > > for code that works with transformers-0.4.0.0 but wouldn't compile
> > > > anymore with transformers-0.4.1.0?
> > >
> > > Surely just the reverse of the breaking change?
> > >
> > >
> > >
> https://github.com/ibotty/streaming-commons/commit/b1137852e4f58520ad65d01b20623a968c18ae8f
> > >
> >
> > No, that code is compatible with transformers 0.3, 0.4, and 0.4.1. With
> the
> > syntax:
> >
> >     import Data.Functor.Constant (Constant (Constant), getConstant)
> >
> > We're importing the data type Constrant, the data construct Constant for
> > that data type, and the function getConstant. It doesn't matter if
> > getConstant is a normal function or a field accessor, it can still be
> > accessed by that syntax.
>
> Then I don't understand why field accessors are even *allowed* to be
> imported beneath the type.  Surely that's only necessary to disambiguate
> constructors from types?
>
>
In the case here, the syntax:

    import Control.Monad.Trans.Reader (ReaderT (..))

is very convenient. But it's true that you don't really gain much from the
distinction between these two:

    import Control.Monad.Trans.Reader (ReaderT (ReaderT, runReaderT))
    import Control.Monad.Trans.Reader (ReaderT (ReaderT), runReaderT)

Perhaps the latter should be considered a better approach, since it's more
resilient to changes in datatypes.

Michael
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