The free theorem for fmap - School of Haskell | School of Haskell

Interactive code snippets not yet available for SoH 2.0, see our Status of of School of Haskell 2.0 blog post When we write down the definition of Functor we carefully state two laws: fmap id = idfmap f . fmap g = fmap (f . g)These are pretty well known in the Haskell community. What is less well known is that the second actually follows from the first and parametricity, so you only need to sit do

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Functorの則はfmap id = idさえ示せればよい