Haskell - 类型包装器统一化

Question

我有一堆函数，例如：

f1 :: String -> String -> ... -> String -> ()
f1 a b ... z = g [("a", a), ("b", b), ... ("z", z)]
...
fn :: String -> Int -> String -> ... -> String -> ()
fn a b ... z = g [("a", a), ("b", show b), ... ("z", z)]

所以用户可以像这样调用它们：f1 "abc" "def"。我不希望他这样做，因为他可能会错误地交换“abc”和“def”，而且在调试时浪费的时间不可估量。我希望他像这样传递参数：fk (A "abc") (B "def")。 据我所见，有两个选项：

1. Massive data construction and massive unpack function:

   data Value = A String
              | B String
              | C Int
              | D String
              ...
   
    unpack :: Value -> String
    unpack (A a) = a
    unpack (B b) = b
    unpack (C c) = show c
    unpack (D c) = d
   

   Lots of code.

2. Common typeclass and newtypes:
   EDIT: Okay then, we can use GeneralizedNewtypeDeriving in such simple case.

     {-# LANGUAGE GeneralizedNewtypeDeriving #-}
   
     class Value a where
       unpack :: a -> String
     instance Value String where
       unpack = id
     instance Value Int where
       unpack = show
   
     newtype A = A String deriving Value
     newtype B = B String deriving Value
     newtype C = C Int deriving Value
     newtype D = D String deriving Value
   
     ...
   

   Looks much better but all fk would look like

      fk a b ... z = g [("a", unpack a), ("b", unpack b), ... ("z", unpack z)]
   

   Lots of code and duplication.

我想要的是一些魔术技巧，它可以让我做到以下几点：

1. fk a b ... z = g [("a", a), ("b", b), ... ("z", z)]
2. g = h . map (second unpack)

- Matvey Aksenov

Answer 1

我认为问题归结为：列表只能有相同类型的元素；这意味着你要么在f中将其合并成单一类型，要么就不能依赖于Haskell的类型检查。例如，以下代码可以正常工作，但类型检查是运行时进行的：

{-# LANGUAGE GADTs #-}

import Control.Arrow (second)

data Item where
    A :: String -> Item
    B :: Int -> Item

unpack (A s) = s
unpack (B i) = show i

myf a@(A {}) b@(B {}) c@(B {}) = 
    let g = [("a", a), ("b", b), ("c", c)]
    in map (second unpack) g
myf _ _ _ = error "Bad types"

main = do
    putStrLn $ show $ myf (A "test") (B 13) (B 14)
    putStrLn $ show $ myf (A "test") (B 13) (A "xxx")

当你想进行编译时类型检查时，可以像这样做；但是，您仍然需要将参数重新输入为相同的类型，因此从某种意义上讲，在解包方面并没有太大的区别，只是它可能会更少出现错误。一个好的技巧来自json包 - 它们重新定义了一些运算符（例如= :)来创建类型，因此您将有：

{-# LANGUAGE ExistentialQuantification #-}
import Control.Arrow (second)

class Value a where
    unpack :: a -> String
newtype A = A String
newtype B = B Int

instance Value A where
    unpack (A a) = a

instance Value B where
    unpack (B b) = show b

data Item = forall b. Value b => Item b
a =: b = (a, Item b)

myf :: A -> B -> B -> [(String, String)]
myf a b c = 
    let g = ["a" =: a, "b" =: b, "c" =: c]
    in map (second (\(Item x) -> unpack x)) g

main = do
    putStrLn $ show $ myf (A "test") (B 13) (B 14)

这和只定义a =: b = (a, unpack b)其实没太大区别。

- ondra