更合理的instance Monad []

使

join $ map (\y->map (\x->(x,y)) [1..]) [1..]

的结果更合理： https://github.com/zaoqi/zaoqilc/blob/master/featuring/MonadList.hs

Trac metadata

Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Core Libraries
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