Re: Burning bridges

On 22.05.2013 18:41, Ben Gamari wrote:

Ian Lynagh writes:

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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).

I agree that the subset of functions exposed by the Prelude is a bit strange. Perhaps when should be added; perhaps sequence should be dropped. Either way, both of these discussions are I think mostly orthogonal to the decision of making what is currently in the Prelude polymorphic.

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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).

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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'?

In the case of these monomorphic functions on lists you may have a point. That being said, I believe that fmap has a place in the Prelude and my original point holds in this case as well.

I would argue that mapping over functors with pure functions and monadic actions are both needed often enough that requiring an import for either would be unduly onerous. I would be interested to hear if others disagree with this premise.

When I use ghci as a calculator, I need the Prelude, including map. When I do serious programming, then it is mostly monadic, then I am fine with importing stuff. I do not need mapM in the Prelude. [But since it is there, and used in existing code, it should probably stay (in a generalized form).] -- Andreas Abel <>< Du bist der geliebte Mensch. Theoretical Computer Science, University of Munich Oettingenstr. 67, D-80538 Munich, GERMANY andreas.abel@ifi.lmu.de http://www2.tcs.ifi.lmu.de/~abel/