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# Algebraic Data Types

<https://bartoszmilewski.com/2015/01/13/simple-algebraic-data-types/> <https://codewords.recurse.com/issues/three/algebra-and-calculus-of-algebraic-data-types>

0 ≅ Void 1 ≅ () 2 ≅ Bool 3 ≅ Ordering (+) sum-type (×) product a type-var L type-ctor C data-ctor

* product: `a × b` ≅ `Pair   a b` ≅ `(a,b)` ≅ `And a b` ≅ `a ∧ b`
* sumtype: `a + b` ≅ `Either a b` ≅ `(a|b)` ≅ `Or  a b` ≅ `a ∨ b`

a -> b -> c ≡ a -> (b -> c)

a -> b ≅ bᵃ

* Pairs

`(a, b) -> c` ≅ `a -> b -> c` CURRY/UNCURRY

`a -> (b * c)` ≅ `(a -> b * a -> c)` a -> (b, c) ≅ (a -> b, a -> c)

a -> (b + c) ≅ (a -> b + a -> c)

* Arithmetics

(a *b)ˢ = aˢ* bˢ (a *b) ^ n = a ^ n* b ^ n

m1 a b n = (a ^ n) \* (b ^ n) m1 :: (Integral b, Num a) => a -> a -> b -> a

m2 a b n = (a \* b) ^ n m2 :: (Integral b, Num a) => a -> a -> b -> a