haskell - 将命令行参数派生的真值表转换为数据类型

Question

我的问题源于真实世界的代码比下面的例子(由几个模拟函数组成)复杂得多。这个例子应该说明我的问题产生的现象:(或者)case/of 语句中的一个巨大的表(或者嵌套的巨大级联 ifs)。 case/of 语句包含类似于真值表的内容，它决定如何参数化结果记录的类型(称为 ComposedRecord 因为它是三个通用类型记录的组合)。模拟函数已经实现，但我对巨大的 case/of block 不满意。因此，我试图找到一个更简洁的解决方案:

-- intermediate record type 1
data SimpleRecord = SimpleRecord {
  _description :: String
}

-- intermediate record type 2
data DetailedRecord = DetailedRecord {
  _title :: String,
  _details :: String
}

-- intermediate record type 3
data RawRecord = RawRecord {
  _x :: Int,
  _y :: Int,
  _z :: Int
}

class AnyRecord r where
instance AnyRecord SimpleRecord
instance AnyRecord DetailedRecord
instance AnyRecord RawRecord

-- to-be-serialized record type (composed of intermediate record types 1, 2 or 3)
data ComposedRecord a b c = ComposedRecord {
  _a :: a,
  _b :: b,
  _c :: c
}

inputToSimpleRecord :: String -> SimpleRecord
inputToSimpleRecord input = SimpleRecord {
  _description = "description derived from input"
}

inputToDetailedRecord :: String -> DetailedRecord
inputToDetailedRecord input = DetailedRecord {
  _title = "title derived from input",
  _details = "details derived from input"
}

inputToRawRecord :: String -> RawRecord
inputToRawRecord input = RawRecord {
  _x = 1, -- number derived from input string
  _y = 2, -- number derived from input string
  _z = 3  -- number derived from input string
}

serializeToJsonString :: (AnyRecord a, AnyRecord b, AnyRecord c) => ComposedRecord a b c -> String
serializeToJsonString composedRecord = "{}"
-- this function will also contain some post processing of the records
-- thus the "AnyRecord" type constraint is needed

commandLineArguments = ["simple", "detailed", "raw"]
-- the value of commandLineArguments will be read from the command line

inputFromCommandLine = "1, 2, 3" -- some input

main = do
  let result = case commandLineArguments of
        ["simple", "simple", "simple"] -> serializeToJsonString ComposedRecord {
          _a = inputToSimpleRecord inputFromCommandLine,
          _b = inputToSimpleRecord inputFromCommandLine,
          _c = inputToSimpleRecord inputFromCommandLine
        }
        ["simple", "simple", "detailed"] -> serializeToJsonString ComposedRecord {
          _a = inputToSimpleRecord inputFromCommandLine,
          _b = inputToSimpleRecord inputFromCommandLine,
          _c = inputToDetailedRecord inputFromCommandLine
        }
        ["simple", "simple", "raw"] -> serializeToJsonString ComposedRecord {
          _a = inputToSimpleRecord inputFromCommandLine,
          _b = inputToSimpleRecord inputFromCommandLine,
          _c = inputToRawRecord inputFromCommandLine
        }
        ["simple", "detailed", "simple"] -> serializeToJsonString ComposedRecord {
          _a = inputToSimpleRecord inputFromCommandLine,
          _b = inputToDetailedRecord inputFromCommandLine,
          _c = inputToSimpleRecord inputFromCommandLine
        }
        ["simple", "detailed", "detailed"] -> serializeToJsonString ComposedRecord {
          _a = inputToSimpleRecord inputFromCommandLine,
          _b = inputToDetailedRecord inputFromCommandLine,
          _c = inputToDetailedRecord inputFromCommandLine
        }
        ["simple", "detailed", "raw"] -> serializeToJsonString ComposedRecord {
          _a = inputToSimpleRecord inputFromCommandLine,
          _b = inputToDetailedRecord inputFromCommandLine,
          _c = inputToRawRecord inputFromCommandLine
        }
        ["simple", "raw", "raw"] -> serializeToJsonString ComposedRecord {
          _a = inputToSimpleRecord inputFromCommandLine,
          _b = inputToRawRecord inputFromCommandLine,
          _c = inputToRawRecord inputFromCommandLine
        }
        ["detailed", "simple", "simple"] -> serializeToJsonString ComposedRecord {
          _a = inputToDetailedRecord inputFromCommandLine,
          _b = inputToSimpleRecord inputFromCommandLine,
          _c = inputToSimpleRecord inputFromCommandLine
        }
        -- and so on...

  putStrLn result

Answer 1

对我来说，这里的关键元素是 serializeToJsonString 的类型。该函数不明确需要Record，只要它是AnyRecord。考虑到这一点，您可以使用 ExistentialQuantification 来封装该想法。请考虑以下事项:

data AnyRecordData = forall r. AnyRecord r => AnyRecordData r

data ComposedRecord = ComposedRecord {
  _a :: AnyRecordData,
  _b :: AnyRecordData,
  _c :: AnyRecordData
}


inputToAnyRecord :: String -> String -> AnyRecordData
inputToAnyRecord "simple"   = AnyRecordData . inputToSimpleRecord
inputToAnyRecord "detailed" = AnyRecordData . inputToDetailedRecord
inputToAnyRecord "raw"      = AnyRecordData . inputToRawRecord

main = do
  let result = case commandLineArguments of
        [a,b,c] -> serializeToJsonString ComposedRecord {
          _a = inputToAnyRecord a inputFromCommandLine
        , _b = inputToAnyRecord b inputFromCommandLine
        , _c = inputToAnyRecord c inputFromCommandLine
        }

  putStrLn result