將函數映射到定義為數據結構列表的newtype

Question

我是Haskell和FP的新手，我正在通過LYAH和其他來源工作，但要“邊做邊學”，我正在嘗試編寫一個涉及JSON解析的小程序。 但是，我已將自己編入角落，無法離開。 我的代碼是從各種教程中拼湊而成的，我可以感覺到我仍然“在程序上思考”如何將它整合在一起，但我還沒有達到必要的突破才能使它工作。

首先，這是一個多級JSON文件的簡化版本，它是Weather Underground API的天氣預報，減少到三個小時。

{
    "response": {
        "version": "0.1",
        "termsofService": "http://www.wunderground.com/weather/api/d/terms.html",
        "features": {
            "hourly": 1
        }
    },
    "hourly_forecast": [{
        "FCTTIME": {
            "hour": "8",
            "epoch": "1479736800",
            "pretty": "8:00 AM CST on November 21, 2016"
        },
        "temp": {
            "english": "27",
            "metric": "-3"
        },
        "condition": "Partly Cloudy"
    }, {
        "FCTTIME": {
            "hour": "9",
            "epoch": "1479740400",
            "pretty": "9:00 AM CST on November 21, 2016"
        },
        "temp": {
            "english": "32",
            "metric": "0"
        },
        "condition": "Partly Cloudy"
    }, {
        "FCTTIME": {
            "hour": "10",
            "epoch": "1479744000",
            "pretty": "10:00 AM CST on November 21, 2016"
        },
        "temp": {
            "english": "35",
            "metric": "2"
        },
        "condition": "Clear"
    }]
}

接下來，這是我的Haskell程序。 我成功地解析JSON成newtype稱為ForecastPointCollection其定義為一個List的WeatherPoint ，這是一個data的，從JSON文件來各種事物的結構。 但是，我無法弄清楚如何將[WeatherPoint]列表退出（參見代碼注釋）。 作為對列表中“要做的事情”的測試，我想將Celcius溫度轉換為開爾文並獲得一個我可以使用的新List （輸出到JSON，做一個show ，等等）。

{-# LANGUAGE OverloadedStrings   #-}
-- {-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE ScopedTypeVariables #-}

module Main where

import           Data.Aeson
import           Data.Aeson.Types

import           Control.Applicative  ((<$>), (<*>))
import           Control.Monad        (mzero)

import qualified Data.ByteString.Lazy as BSL
import qualified Data.Text            as T
import qualified Data.Vector          as V

type MetricTemperature = Int
type KelvinTemperature = Int

newtype ForecastPointCollection = ForecastPointCollection
  {forecastpointcollection :: [WeatherPoint]} deriving Show

data WeatherPoint = WeatherPoint
  { epoch      :: T.Text
  , prettyTime :: T.Text
  , tempMetric :: MetricTemperature
  , condition  :: T.Text
  } deriving Show

instance FromJSON ForecastPointCollection where
  parseJSON (Object o) =
    ForecastPointCollection <$> o .: "hourly_forecast"
  parseJSON _ = mzero

data ProcessedWeatherPoint = ProcessedWeatherPoint
  { newEpoch      :: T.Text
  , newPrettyTime :: T.Text
  , newTempKelvin :: KelvinTemperature
  , newCondition  :: T.Text
  } deriving Show

instance FromJSON WeatherPoint where
  parseJSON =
    withObject "Root Object Arbitrary Name" $ \o -> do
    fctO <- o .: "FCTTIME"
    epoch <- fctO .: "epoch" -- contained within FCTTIME
    pretty <- fctO .: "pretty" -- contained within FCTTIME
    tempO <- o .: "temp"
    metric <- tempO .: "metric" -- contained within temp
    condition <- o .: "condition" -- at top level under hourly_forecast
    return $ WeatherPoint epoch pretty (read metric) condition
  -- parseJSON _ = mzero

kelvinizeTemp :: MetricTemperature -> KelvinTemperature
kelvinizeTemp x = x + 273 -- hey, close enough

adjustTemp :: Maybe ForecastPointCollection -> [ProcessedWeatherPoint]
adjustTemp Nothing = []
adjustTemp x = [] -- HERE IS WHERE I AM LOSING MY WAY!
                  -- HOW CAN I WALK THROUGH THE LIST INSIDE ForecastPointCollection
                  -- TO map kelvinizeTemp ACROSS THAT LIST AND
                  -- GET A [ProcessedWeatherPoint] LIST BACK TO PLAY WITH?

getSampleForecast = BSL.readFile "/home/mypath/test/forecastsubmit.json"

main = do
  textOfJson <- getSampleForecast
  let (forecasts2 :: Maybe ForecastPointCollection) = decode textOfJson
  case forecasts2 of
    Just (ForecastPointCollection forecasts2) -> do
         putStrLn ("Success!")
         putStrLn . show  $ forecasts2
    _ -> putStrLn "Could not parse ForecastPointCollection JSON correctly."
  -- So far so good, we've extracted data from the JSON and stored it in memory.
  -- But now, how can we manipulate that data and start doing stuff with it?
  -- Currently, the "adjustTemp" function returns an empty list no matter what.
  let (processed2 :: [ProcessedWeatherPoint]) = adjustTemp forecasts2
  putStrLn ("More success (OK, not really, yet)!")
  putStrLn . show  $ processed2

任何建議表示贊賞 我不應該讓ForecastPointCollection一個newtype ？ 我在哪里慣用，我在哪里愚蠢？ :-P

基於答案更新：對於后代，這里是新定義的processWeatherPoint函數的可能（工作）實現。 應該將data結構的各個部分視為一個功能！

processWeatherPoint :: WeatherPoint -> ProcessedWeatherPoint
processWeatherPoint x = ProcessedWeatherPoint
  (epoch x)
  (prettyTime x)
  (kelvinizeTemp (tempMetric x))
  (condition x)

kelvinizeTemp :: MetricTemperature -> KelvinTemperature
kelvinizeTemp x = x + 273 -- this works OK because both types are type of Int

Answer 1

它應該足以定義一個函數......

processWeatherPoint :: WeatherPoint -> ProcessedWeatherPoint

...使用[WeatherPoint]的[WeatherPoint]提取字段，並將該函數映射到列表上：

adjustTemp :: Maybe ForecastPointCollection -> [ProcessedWeatherPoint]
adjustTemp Nothing = []
adjustTemp (Just (ForecastPointCollection points)) = processWeatherPoint <$> points

ForecastPointCollection上的模式匹配的替代方法是使用該字段的記錄訪問器。 如果您不打算導出構造函數，那將特別有用：

adjustTemp :: Maybe ForecastPointCollection -> [ProcessedWeatherPoint]
adjustTemp Nothing = []
adjustTemp (Just forecast) = processWeatherPoint <$> forecastpointcollection forecast

一種可以說是更方便的編寫上面定義的方法涉及使用maybe函數而不是對Maybe進行明確的案例分析：

adjustTemp :: Maybe ForecastPointCollection -> [ProcessedWeatherPoint]
adjustTemp = maybe [] (fmap processWeatherPoint . forecastpointcollection)