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# 20.5 Some basic derived operations
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Why does `fmap length Just [1, 2, 3]` return 1?
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```haskell
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fmap :: Functor f => (a -> b) -> f a -> f b
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length :: Foldable t => t a -> Int
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Just :: a -> Maybe a
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fmap length :: (Foldable t, Functor f) => f (t a) -> f Int
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-- notice that the next argument given is an f (t a) and since
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-- (->) is also a functor f is ((->) a) and (t a) is Maybe a
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-- this also makes f Int an (a -> Int)
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fmap length Just :: a -> Int
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```
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## Exercises: Library Functions
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see src/functions.h
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