haskell - 映射 RankNTypes 函数

Answer 1

The reason that it does not work by default is because the type of map is (a -> b) -> [a] -> [b], but you cannot instantiate that a to a type involving forall, in this case forall n. Num n => n -> n. Such instantiation is called impredicative, which was not supported (reliably) by GHC for a long time.

Since GHC 9.2.1 there is a new reliable implementation of the ImpredicativeTypes extension, which does allow you to instantiate impredicatively:

GHCi, version 9.2.0.20210821: https://www.haskell.org/ghc/  :? for help
ghci> :set -XImpredicativeTypes
ghci> :t rankN
rankN :: (forall n. Num n => n -> n) -> (Int, Double)
ghci> :t map rankN 
map rankN :: [forall n. Num n => n -> n] -> [(Int, Double)]
ghci> :t map rankN [negate]
map rankN [negate] :: [(Int, Double)]
ghci> map rankN [negate]
[(-1,-1.0)]