# Blog

### natural transformation vs functor

This forms a category since for any functor $${\displaystyle F}$$ there is an identity natural transformation $${\displaystyle 1_{F}:F\to F}$$ (which assigns to every object $${\displaystyle X}$$ the identity morphism on $${\displaystyle F(X)}$$) and the composition of two natural transformations (the "vertical composition" above) is again a natural transformation. Wikipedia : natural transformations Natural transformations (25:36) by Daniel Chan (2019-06-27). Let's consider monoids. In fact, if we render C x as {x} as in "wrapped x", then the previous can be re-written as: which quite literally means that applying the transformed f to the wrapped value is the same as unwrapping, applying f to the unwrapped value and re-wrap it. A monoid homomorphism is a function between two monoids $f : M \rightarrow N$ which "respects" the two operations: $$f(0_M) = 0_M$$ Can Haskell ensure a Functor (or other typeclasses) satisfies its law? And if not, then what is the purpose of the functor laws from a "practical" point of view? Functor laws and natural transformations in Haskell, reasonablypolymorphic.com/blog/theorems-for-free/, Creating new Help Center documents for Review queues: Project overview. But here's the executive summary of what I think you want to know: First off, let's get a bit of intuition about what the parametricity theorem states. Why doesn't process substitution work with VLC playlists? It’s most likely my fault but I could draw hardly anything from this. Anyways, this should hold in particular when F is the identity functor, which if understand correctly would correspond to the aforesaid function having the type a -> G a. What are the fundamental reasons for indirect presidential vote in the US? Consider, for example, something with the same type as fmap but isn't necessarily fmap: From this you can conclude a bunch of interesting things, such as that if somethingLikeFmap id = id then fmap = somethingLikeFmap. Then there can be no natural transformation form $F$ to $G$, since there are no morphisms from $F(C)$ to $G(C)$ for any $C$ in C. have you seen this: https://mathoverflow.net/questions/35731/conditions-for-natural-transformations-to-exist. What properties do natural isomorphisms between functors preserve? Anyways, this should hold in particular when F is the identity functor, which if understand correctly would correspond to the aforesaid function having the type a -> G a. Has Trump ever explained why he, as incumbent President, is unable to stop the alleged electoral fraud? Use MathJax to format equations. Why is the “functor category” functor $(C,B)\mapsto B^{C}$ contravariant in $C$? Can I assume that groundings in electrical circuits can be split? If $${\displaystyle C}$$ is any category and $${\displaystyle I}$$ is a small category, we can form the functor category $${\displaystyle C^{I}}$$ having as objects all functors from $${\displaystyle I}$$ to $${\displaystyle C}$$ and as morphisms the natural transformations between those functors.