Question

我有数据类型

data Constants = Constants { x1 :: Int, x2 :: Int, ... , x100 :: Int }

名称x1，x2，...，x100可能有不规则的命名。

是否有一种优雅的方法可以从100个Int值的列表中创建Constants对象？

mkConstants :: [Int] -> Constants
mkConstants [x1, x2, ..., x100] = Constants x1 x2 ... x100 -- The bad way

逆向任务怎么样？

extractInts :: Constants -> [Int]
extractInts (Constants x1 x2 ... x100) = [x1, x2, ..., x100] -- The bad way

Answer 1

编辑：当记录字段属于不同类型（即并非所有整数）时，请参阅Reading long data structure in Haskell了解修改后的版本。

一种可能性是使用类型类：

{-# LANGUAGE FlexibleInstances #-}

data Constants = Constants { a :: Int, b :: Int, c :: Int, d :: Int, e :: Int }
    deriving Show

class Cons a where
    cons :: a -> [Int] -> Maybe Constants

instance Cons Constants where
    cons c [] = Just c
    cons _ _  = Nothing

instance (Cons a) => Cons (Int -> a) where
    cons f (x:xs) = cons (f x) xs
    cons _ _      = Nothing

然后，如果列表大小合适：

\> cons Constants [1..5]
Just (Constants {a = 1, b = 2, c = 3, d = 4, e = 5})

否则你什么也得不到：

\> cons Constants [1..4]
Nothing
\> cons Constants [1..6]
Nothing

Answer 2

这是通过泛型编程（在这种情况下，通过generics-sop）实现此目的的概念验证方法。这是否是解决实际问题的合适方法取决于我目前无法判断的很多因素：

{-# LANGUAGE DeriveGeneric, ScopedTypeVariables, DataKinds, TypeFamilies, FlexibleContexts, TypeOperators, PolyKinds #-}
{-# OPTIONS_GHC -fcontext-stack=200 #-}

module Constants where

import Data.Maybe
import Generics.SOP
import qualified GHC.Generics as G

data Constants =
  Constants
    { x00 :: Int, x01 :: Int, x02 :: Int, x03 :: Int, x04 :: Int, x05 :: Int, x06 :: Int, x07 :: Int, x08 :: Int, x09 :: Int
    , x10 :: Int, x11 :: Int, x12 :: Int, x13 :: Int, x14 :: Int, x15 :: Int, x16 :: Int, x17 :: Int, x18 :: Int, x19 :: Int
    , x20 :: Int, x21 :: Int, x22 :: Int, x23 :: Int, x24 :: Int, x25 :: Int, x26 :: Int, x27 :: Int, x28 :: Int, x29 :: Int
    , x30 :: Int, x31 :: Int, x32 :: Int, x33 :: Int, x34 :: Int, x35 :: Int, x36 :: Int, x37 :: Int, x38 :: Int, x39 :: Int
    , x40 :: Int, x41 :: Int, x42 :: Int, x43 :: Int, x44 :: Int, x45 :: Int, x46 :: Int, x47 :: Int, x48 :: Int, x49 :: Int
    , x50 :: Int, x51 :: Int, x52 :: Int, x53 :: Int, x54 :: Int, x55 :: Int, x56 :: Int, x57 :: Int, x58 :: Int, x59 :: Int
    , x60 :: Int, x61 :: Int, x62 :: Int, x63 :: Int, x64 :: Int, x65 :: Int, x66 :: Int, x67 :: Int, x68 :: Int, x69 :: Int
    , x70 :: Int, x71 :: Int, x72 :: Int, x73 :: Int, x74 :: Int, x75 :: Int, x76 :: Int, x77 :: Int, x78 :: Int, x79 :: Int
    , x80 :: Int, x81 :: Int, x82 :: Int, x83 :: Int, x84 :: Int, x85 :: Int, x86 :: Int, x87 :: Int, x88 :: Int, x89 :: Int
    , x90 :: Int, x91 :: Int, x92 :: Int, x93 :: Int, x94 :: Int, x95 :: Int, x96 :: Int, x97 :: Int, x98 :: Int, x99 :: Int
    }
  deriving (Show, G.Generic)

instance Generic Constants

fromConstantList ::
  forall a c xs . (Generic a, Code a ~ '[ xs ], All ((~) c) xs) =>
  [c] -> a
fromConstantList =
  to . SOP . Z . hcmap (Proxy :: Proxy ((~) c)) (I . unK) . fromJust . fromList

toConstantList ::
  forall a c xs . (Generic a, Code a ~ '[ xs ], All ((~) c) xs) =>
  a -> [c]
toConstantList =
  hcollapse . hcmap (Proxy :: Proxy ((~) c)) (K . unI) . unZ . unSOP . from

unZ :: NS f (x ': xs) -> f x
unZ (Z x) = x

test1 :: Constants
test1 = fromConstantList [1..100]

test2 :: [Int]
test2 = toConstantList test1

在GHCi中（这是版本7.10.3，见下文）：

*Constants> test1
Constants {x00 = 1, x01 = 2, x02 = 3, x03 = 4, x04 = 5, x05 = 6, x06 = 7, x07 = 8, x08 = 9, x09 = 10, x10 = 11, x11 = 12, x12 = 13, x13 = 14, x14 = 15, x15 = 16, x16 = 17, x17 = 18, x18 = 19, x19 = 20, x20 = 21, x21 = 22, x22 = 23, x23 = 24, x24 = 25, x25 = 26, x26 = 27, x27 = 28, x28 = 29, x29 = 30, x30 = 31, x31 = 32, x32 = 33, x33 = 34, x34 = 35, x35 = 36, x36 = 37, x37 = 38, x38 = 39, x39 = 40, x40 = 41, x41 = 42, x42 = 43, x43 = 44, x44 = 45, x45 = 46, x46 = 47, x47 = 48, x48 = 49, x49 = 50, x50 = 51, x51 = 52, x52 = 53, x53 = 54, x54 = 55, x55 = 56, x56 = 57, x57 = 58, x58 = 59, x59 = 60, x60 = 61, x61 = 62, x62 = 63, x63 = 64, x64 = 65, x65 = 66, x66 = 67, x67 = 68, x68 = 69, x69 = 70, x70 = 71, x71 = 72, x72 = 73, x73 = 74, x74 = 75, x75 = 76, x76 = 77, x77 = 78, x78 = 79, x79 = 80, x80 = 81, x81 = 82, x82 = 83, x83 = 84, x84 = 85, x85 = 86, x86 = 87, x87 = 88, x88 = 89, x89 = 90, x90 = 91, x91 = 92, x92 = 93, x93 = 94, x94 = 95, x95 = 96, x96 = 97, x97 = 98, x98 = 99, x99 = 100}
*Constants> test2
[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100]

有趣的是，当我尝试用ghc-8.0.1编译它时（那时你也必须用-fcontext-stack替换-freduction-depth选项），我得到了一个意外的内部GHC错误，我有进一步调查......

构造具有许多字段的Haskell数据类型

2 个答案: