Haskell语言学习笔记（65）Lens

Posted 2020-10-17 zwvista

手动计算 set mapped 5 [1,2,3]

set l b = runIdentity . l (\_ -> Identity b)
mapped = sets fmap
sets f g = taintedDot (f (untaintedDot g))

instance Settable Identity where
  untainted = runIdentity
  untaintedDot = (runIdentity #.)
  taintedDot = (Identity #.)

set mapped 5 [1,2,3]
= runIdentity . (sets fmap) (\_ -> Identity 5) $ [1,2,3]
= runIdentity $ sets fmap (\_ -> Identity 5) $ [1,2,3]
= runIdentity $ taintedDot (fmap (untaintedDot (\_ -> Identity 5))) $ [1,2,3]
= runIdentity $ (Identity .) (fmap ((runIdentity .) (\_ -> Identity 5))) $ [1,2,3]
= runIdentity $ (Identity .) (fmap (fmap runIdentity (\_ -> Identity 5))) $ [1,2,3]
= runIdentity $ Identity . (fmap (\_ -> 5)) $ [1,2,3]
= runIdentity $ Identity $ fmap (\_ -> 5) [1,2,3]
= runIdentity $ Identity $ [5,5,5]
= [5,5,5]