Functor Fact @FunctorFact
Functional programming and category theory tweets from @JohnDCook johndcook.com/functor Houston, TX Joined June 2016-
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The category of fields has no initial or terminal objects.
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Category theory without categories johndcook.com/blog/2023/05/2…
'Tensor calculus is vitally concerned with the distinction between contravariance and covariance, and consistently employs upper indices to indicate the one and lower indices to indicate the other.' – David Kay
The Haskell language and the operation of currying were both named after logician Haskell Curry.
A tutorial on the universality and expressiveness of fold cs.nott.ac.uk/~pszgmh/fold.p… [pdf]
One diagram, two completely different meanings johndcook.com/blog/2022/01/0…
Haskell is not category theory pema.dev/2023/02/01/has…
'A comathematician is a device for turning cotheorems into ffee.'
Simply typed lambda calculus is the internal language of Cartesian closed categories (CCCs).
Given categories C and D, D^C is the category whose objects are functors from C to D and whose morphisms are natural transformations.
There are several things that have similar notation: the set Hom(A, B), the function Hom(A, g), and the functor Hom(A, -).
“As we now know, nobody truly understands macros.” -- Doug Hoyte, Let Over Lambda
Folds in 40 programming languages en.wikipedia.org/wiki/Fold_%28h…
Galois connections and Galois theory johndcook.com/blog/2022/07/2…
Category theory turns math inside-out: Definitions depend on nothing inside, but on everything outside.
The glass disk game (homological algebra) johndcook.com/blog/2025/03/2…
Haskell-Java interop blog.tweag.io/posts/2016-10-…
'Roughly speaking, homological algebra is concerned with the question of how much modules differ from being projective, injective, or flat.' -- M. Scott Osborne
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