cata.cs

gistfile1.cs

using System;

namespace Catamorphisms.RecursiveTypes;

record Book(string Title, decimal Price);

abstract record ChocolateType;
sealed record Black : ChocolateType;
sealed record Milk : ChocolateType;
sealed record SeventyPercent : ChocolateType;

record Chocolate(ChocolateType ChocolateType, decimal Price);

abstract record WrappingPaperStyle;
sealed record HappyBirthday : WrappingPaperStyle;
sealed record HappyHolidays : WrappingPaperStyle;
sealed record SolidColor : WrappingPaperStyle;

abstract record Gift;
sealed record BookGift(Book Book) : Gift;
sealed record ChocolateGift(Chocolate Chocolate) : Gift;
sealed record Wrapped(Gift Gift, WrappingPaperStyle WrappingPaperStyle) : Gift;
sealed record Boxed(Gift Gift) : Gift;
sealed record WithACard(Gift Gift, string Message) : Gift;

static class Prices
{
    const decimal WrappedPrice = 0.5m;
    const decimal BoxedPrice = 1m;
    const decimal WithACardPrice = 2m;
}


static TResult Cata<TResult>(
    Func<Book, TResult> fBook,
    Func<Chocolate, TResult> fChocolate,
    Func<TResult, WrappingPaperStyle, TResult> fWrapped,
    Func<TResult, TResult> fBoxed,
    Func<TResult, string, TResult> fWithACard,
    Gift gift)
{
    // let recur = cata fBook fChocolate fWrapped fBoxed fWithACard
    TResult Recur(Gift g) => Cata(fBook, fChocolate, fWrapped, fBoxed, fWithACard, g);

    return gift switch
    {
        BookGift b      => fBook(b.Book),
        ChocolateGift c => fChocolate(c.Chocolate),
        Wrapped w       => fWrapped(Recur(w.Gift), w.WrappingPaperStyle),
        Boxed b         => fBoxed(Recur(b.Gift)),
        WithACard w     => fWithACard(Recur(w.Gift), w.Message),
        _               => throw new ArgumentOutOfRangeException(nameof(gift))
    };
}


static decimal TotalCost(Gift gift) => Gifts.Cata<decimal>(
    fBook:      b =>            b.Price,
    fChocolate: c =>            c.Price,
    fWrapped:   (inner, _)   => inner + Prices.WrappedPrice,
    fBoxed:     inner        => inner + Prices.BoxedPrice,
    fWithACard: (inner, _)   => inner + Prices.WithACardPrice,
    gift);