Problem

我有一个从字符串到值(使用 Tries 作为执行)的有限地图, 我从二进制/文本文件( json, xml,...) 中分析。

type Value = ...
type Attributes = Data.Trie Value 
data Object = Object Attributes

Each map has the same type of values but not the same set of keys. I group maps with the same set of keys together to be able to prevent having to type-switch all the time I have a specialised function that requires certain keys:

data T1
data T2 
...

data Object a where
    T1 :: Attributes -> Object T1
    T2 :: Attributes -> Object T2
    ...

这让我可以写一些东西比如:

f1 :: Object T1 -> ...

代替

f1 :: Object ->
f1 o | check_if_T1 o = ...

这有两种缺点:

Homogeneous lists of Object now become heterogeneous, i.e. I cannot have a list [Object] anymore.

我需要写许多锅炉板才能获得/设置属性:

get :: Object a -> Attributes
get (T1 a) = a
get (T2 a) = a
...

Question

Is there a better way to specialise functions depending on the constructor of an ADT?
How could I regain the ability to have a list [Object]? Is there a specialized version of Dynamic that only allows certain types? I thought about wrapping the Object again, but this would add a lot of boilerplate. For instance,

数据对象 = TT1 T1 TT2 T2...

我需要的是:

get :: a -> TObject -> Object a

这样我就可以得出:

collect :: a -> [TObject] -> [Object a]

我查看了HList,但我不认为它适合我的问题。特别是,因为[Object]中的种类顺序在编译时并不为人所知。

这听起来像是可以用功能依赖/类型算术来解决的但我只是还没有找到一个好办法

Answer 1

如果所有构建器都返回单态 < code> Object 类型且没有循环, 您可能想要考虑仅仅使用不同的类型。而不是

data T1
data T2

data Object a where
    T1 :: Attributes -> Object T1
    T2 :: Attributes -> Object T2

考虑考虑

data T1 = T1 Attributes
data T2 = T2 Attributes

Dynamic 是一种方法,使用上述方法,您可以添加 发送可输入 并完成。或者,您可以手工完成:

data TSomething = It s1 T1 | It s2 T2

getT1s :: [TSomething] -> [T1]
getT2s :: [TSomething] -> [T2]
getT1s xs = [t1 | It s1 t1 <- xs]
getT2s xs = [t2 | It s2 t2 <- xs]

正如你说的,这涉及一些锅炉板。 Typeable 版本看起来略好一点 :

deriving Typeable T1
deriving Typeable T2

-- can specialize at the call-site to
-- getTs :: [Dynamic] -> [T1] or
-- getTs :: [Dynamic] -> [T2]
getTs :: Typeable a => [Dynamic] -> [a]
getTs xs = [x | Just x <- map fromDynamic xs]

Problem

Question

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