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/