Burning bridges

[Ian Lynagh]

On Wed, May 22, 2013 at 11:16:04AM -0400, Ben Gamari wrote:
> I'm another from the crowd of silence observers. It seems to me that
> we still don't have a fully specified proposal. In particular which
> functions are we discussing? Only those from Traversable and Foldable?
> map? It would be nice to make this clear with a concrete proposal.

Yes, my intention was to describe what I think a proposal should look
like, not to make one. I think a proposal ought not be under a subject
of "Burning bridges" too  :-)

> Concerning removing mapM without replacement: I'd be concerned that
> moving such a frequently used function out of the Prelude might be a bit
> confusing for beginners.

The Prelude is a bizarre snapshot of what was common (or perhaps even
what existed?) more than a decade ago. For example, it includes 'mapM'
and 'sequence', but not 'when'. Here are the usage counts for the ghc
package:

    mapM        562
    mapM_       175
    sequence    41
    sequence_   8   
    when        205
    unless      84

and while this is only one package, I would be surprised if 'sequence'
isn't generally less used than 'when'. I would also expect that most
mapM users would also use when, so would need to import Control.Monad
anyway, at which point there's no point having mapM in Prelude (although
that argument is diminished if the generalised mapM is exported
instead).

The Prelude is also bizarre due to being very broad, which also causes
headaches with where to put it when splitting base up (the IO functions
in particular are problematic).

> Given the prominence of Monads in Haskell, 
> it might be perplexing to find (f)map exported by the Prelude yet not
> mapM.

Well, why should map be exported by Prelude?

If you want to do other things with lists, like 'sort' them, then you
need to import Data.List, which makes a lot of sense. But if you want to
'scanl1' a list (when's the last time you did that?), then for
historical reasons the function is magically imported through the
Prelude.

So why shouldn't you need to tell the compiler (i.e. "import Data.List")
if you want to do /anything/ with a list, including 'scanl1' or 'map'?


Thanks
Ian
-- 
Ian Lynagh, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com/