On Wed, May 7, 2014 at 4:31 PM, Henning Thielemann <schlepptop@henning-thielemann.de> wrote:

Am 07.05.2014 13:43, schrieb Michael Snoyman:

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.

However, even if imported in the second way, runReaderT identifier can be used in record field update syntax. I guess it would be more consistent to allow only the use as a function when imported like an ordinary function.

If you think *that's* bad, you'll love this:

module Person
    ( Person
    , name
    , age
    , defaultPerson
    ) where

data Person = Person { name :: String, age :: Int}
    deriving Show

defaultPerson = Person "Alice" 25
----
module Main where

import Person

main = print defaultPerson { name = "Bob", age = 30 }
----

(Yes, I've been known to abuse this in my libraries, and in 20/20 hindsight, I wish I hadn't.)

Michael