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@minfuk

ついでにF#でparamorphism/apomorphism

前回の投稿に続いて。相変わらず応用先は不明。

// 前回の定義を前提としています。

let rec ana phi x = 
    let z : F<'c> = (x |> phi)
    Wrap(z.fmap(ana phi) z)

let fork f g x = (f x, g x)

let rec para phi x =
    let y = (x |> unIn)
    y.fmap (fork (para phi) id) y |> phi

type Sum<'a,'b> =
    | InL of 'a
    | InR of 'b

let join f g x =
    match x with
    | InL(a) -> f a
    | InR(b) -> g b

let rec apo phi x =
    let (y : F<Sum<'c, Nu<'a>>>) = x |> phi
    Wrap(y.fmap(join (apo phi) id) y)

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