15

假设我的 PureScript 代码中有以下类型的用户记录：

{ id        :: Number
, username  :: String
, email     :: Maybe String
, isActive  :: Boolean
}

CommonJS 模块派生自 PureScript 代码。导出的用户相关函数将从外部 JavaScript 代码调用。

在 JavaScript 代码中，“用户”可以表示为：

var alice = {id: 123, username: 'alice', email: 'alice@example.com', isActive: true};

email可能是null：

var alice = {id: 123, username: 'alice', email: null, isActive: true};

email可以省略：

var alice = {id: 123, username: 'alice', isActive: true};

isActive可以省略，在这种情况下假定true：

var alice = {id: 123, username: 'alice'};

id不幸的是，有时是一个数字字符串：

var alice = {id: '123', username: 'alice'};

上面的五种 JavaScript 表示是等价的，并且应该产生等价的 PureScript 记录。

如何编写一个接受 JavaScript 对象并返回用户记录的函数？它将使用 null/省略的可选字段的默认值，将字符串强制id转换为数字，如果缺少必填字段或值的类型错误，则抛出异常。

我可以看到的两种方法是在 PureScript 模块中使用 FFI 或在外部 JavaScript 代码中定义转换函数。后者似乎毛茸茸的：

function convert(user) {
  var rec = {};
  if (user.email == null) {
    rec.email = PS.Data_Maybe.Nothing.value;
  } else if (typeof user.email == 'string') {
    rec.email = PS.Data_Maybe.Just.create(user.email);
  } else {
    throw new TypeError('"email" must be a string or null');
  }
  // ...
}

我不确定 FFI 版本将如何工作。我还没有处理过效果。

很抱歉，这个问题不是很清楚。我还没有足够的了解来确切地知道我想知道什么。

4

4 回答 4

9

我已经提出了一个解决方案。我确信可以改进很多，例如更改toUserto的类型Json -> Either String User和保留错误信息。如果您能看到可以改进此代码的任何方法，请发表评论。:)

除了一些核心模块之外，该解决方案还使用了PureScript-Argonaut 。

module Main
  ( User()
  , toEmail
  , toId
  , toIsActive
  , toUser
  , toUsername
  ) where

import Control.Alt ((<|>))
import Data.Argonaut ((.?), toObject)
import Data.Argonaut.Core (JNumber(), JObject(), Json())
import Data.Either (Either(..), either)
import Data.Maybe (Maybe(..))
import Global (isNaN, readFloat)

type User = { id :: Number
            , username :: String
            , email :: Maybe String
            , isActive :: Boolean
            }

hush :: forall a b. Either a b -> Maybe b
hush = either (const Nothing) Just

toId :: JObject -> Maybe Number
toId obj = fromNumber <|> fromString
  where
    fromNumber = (hush $ obj .? "id")
    fromString = (hush $ obj .? "id") >>= \s ->
      let id = readFloat s in if isNaN id then Nothing else Just id

toUsername :: JObject -> Maybe String
toUsername obj = hush $ obj .? "username"

toEmail :: JObject -> Maybe String
toEmail obj = hush $ obj .? "email"

toIsActive :: JObject -> Maybe Boolean
toIsActive obj = (hush $ obj .? "isActive") <|> Just true

toUser :: Json -> Maybe User
toUser json = do
  obj <- toObject json
  id <- toId obj
  username <- toUsername obj
  isActive <- toIsActive obj
  return { id: id
         , username: username
         , email: toEmail obj
         , isActive: isActive
         }

更新：我根据Ben Kolera的要点对上面的代码进行了改进。

于 2015-01-20T10:02:37.543 回答
6

你看过purescript-foreign（https://github.com/purescript/purescript-foreign）吗？我想这就是你在这里寻找的。

于 2015-01-11T16:27:27.963 回答
2

作为gb。写道，这正是Foreign构建数据类型的目的。在我的头顶上：

convert :: Foreign -> F User
convert f = do
  id <- f ! "id" >>= readNumber
  name <- f ! "name" >>= readString
  email <- (f ! "email" >>= readNull >>= traverse readString) <|> pure Nothing
  isActive <- (f ! "isActive" >>= readBoolean) <|> pure true
  return { id, name, email, isActive }
于 2018-02-24T11:01:59.303 回答
1

多一点 ffi

module User where

import Data.Maybe
import Data.Function

foreign import data UserExternal :: *

type User =
  {
    id :: Number,
    username :: String,
    email :: Maybe String,
    isActive :: Boolean
  }

type MbUser =
  {
    id :: Maybe Number,
    username :: Maybe String,
    email :: Maybe String,
    isActive :: Maybe Boolean 
  }

foreign import toMbUserImpl """
function toMbUserImpl(nothing, just, user) {
  var result = {},
      properties = ['username', 'email', 'isActive'];

  var i, prop;
  for (i = 0; i < properties.length; i++) {
    prop = properties[i];
    if (user.hasOwnProperty(prop)) {
      result[prop] = just(user[prop]);
    } else {
      result[prop] = nothing;
    }
  }
  if (!user.hasOwnProperty('id') || isNaN(parseInt(user.id))) {
    result.id = nothing;
  } else {
    result.id = just(user.id);
  }
  return result;
}
""" :: forall a. Fn3 (Maybe a) (a -> Maybe a) UserExternal MbUser

toMbUser :: UserExternal -> MbUser
toMbUser ext = runFn3 toMbUserImpl Nothing Just ext

defaultId = 0
defaultName = "anonymous"
defaultActive = false

userFromMbUser :: MbUser -> User
userFromMbUser mbUser =
  {
    id: fromMaybe defaultId mbUser.id,
    username: fromMaybe defaultName mbUser.username,
    email: mbUser.email,
    isActive: fromMaybe defaultActive mbUser.isActive
  }

userFromExternal :: UserExternal -> User
userFromExternal ext = userFromMbUser $ toMbUser ext
于 2015-01-20T22:38:09.110 回答