On 4 September 2011 21:44, Mario Blažević <blama...@acanac.net> wrote: > I was recently surprised to discover that the maximum and maximumBy > functions always return the *last* maximum, while minimum and minimumBy > return the *first* minimum in the list. The following GHCi session > demonstrates this: > > $ ghci > GHCi, version 7.2.1: http://www.haskell.org/ghc/ :? for help > Loading package ghc-prim ... linking ... done. > Loading package integer-gmp ... linking ... done. > Loading package base ... linking ... done. > Loading package ffi-1.0 ... linking ... done. > Prelude> :module +Data.List Data.Ord > Prelude Data.List Data.Ord> let list = [(1, 'B'), (1, 'A')] > Prelude Data.List Data.Ord> maximumBy (comparing fst) list > (1,'A') > Prelude Data.List Data.Ord> minimumBy (comparing fst) list > (1,'B') > > I would normally consider this kind of gratuitous asymmetry a bug, but > seeing that these functions' implementations have been specified in the > Haskell 98 Library Report, I guess they are now a permanent feature of the > language. Can anybody explain the reason for this behaviour? > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >
The asymmetry is a result of EQ and LT being treated as equivalent in both the minBy and maxBy helper functions in the report's definition of the two functions, which has opposite effects for minimumBy and maximumBy. Since the documentation doesn't specify a behavior for equal values, I could guess that it was just an oversight or that it was considered unimportant. But maybe someone more knowledgeable will correct me. Alexander Dunlap _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
