Fusion Theorem

在看 Jeremy Gibbons 的 paper 之前,先來看看想得對不對,基本上是 fold 的 university property 的一個應用,有一類的函數可以用 fold 函數表示(在 F-algebra上?),亦即為 initial object 在某個 category 上(或許是也是某個 limit object),那將這類函數再作 composition of arrows 會不會仍然是在這個 category 裡頭,當然,但問題是什麼 arrow?

list 版的是這樣,

f \circ fold(g, a) = foldr(h, b) \\ {} \mbox{where} f(a) = b, f( g( x, y)) = h(x, f(y))

好,晚點來看看 paper 怎麼寫 …

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