[Haskell-cafe] Open unqualified imports

Dan Doel dan.doel at gmail.com
Fri Jan 16 18:18:53 EST 2009

On Friday 16 January 2009 9:42:46 am eyal.lotem at gmail.com wrote:
> I think currently many modules are designed to be imported
> unqualified, and this is unfortunate. Haskell' libraries can fix
> this.  For example, the various Monadic counterparts such as mapM,
> replicateM, etc could do without the "M" in their names, and one could
> use:
> import qualified Control.Monad as M
> M.for
> M.map

I don't think I can fully agree with this. The fooM functions are so named 
because their type is different in a fairly predictable way from ordinary foo 
functions. And there are, in fact, two different functions that could lay 
claim to M.map. One is named mapM, of course, but the other is liftM:

  liftM :: Monad m => (a -> b) -> m a -> m b

Which is an implementation of the ordinary map using the methods of Monad.

Similarly, Data.Traversable has two functions:

  mapM :: Monad m => (a -> m b) -> t a -> m (t b)
  fmapDefault :: Traversable t => (a -> b) -> t a -> t b

So, do we end up with T.map is a monadic map, but T.mapDefault is an ordinary 
Functor map using the Traversable methods. There are also mapAccumL and 
mapAccumR, but those don't allow monadic functions like T.map, just pure 
functions like T.mapDefault (although, secretly it's T.map specialized to the 
state monad).

Anyhow, if you have some easy way to view the types of these functions, that 
clears up their use quite a bit. However, the naming scheme is not purely due 
to "this is map from the Monad module we're going to import unqualified." 
There is some method to it.

-- Dan

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