问题描述
我想使用 servant-client ， servant-xml 和 xmlbf 库.
I want to parse an API response into a data type using servant-client, servant-xml and xmlbf libraries.
这是一个示例API响应
This is an example API response
<GoodreadsResponse>
<Request>
<authentication>true</authentication>
<key>api_key</key>
<method>search_index</method>
</Request>
<search>
<query>Ender's Game</query>
<results-start>1</results-start>
<results-end>20</results-end>
</search>
</GoodreadsResponse>
这是我想解析为的数据类型
and this is the data type I want to parse it into
data GoodreadsRequest =
GoodreadsRequest { authentication :: Text
, key :: Text
, method :: Text
}
data GoodreadsSearch =
GoodreadsSearch { query :: Text
, resultsStart :: Int
, resultsEnd :: Int
}
data GoodreadsResponse =
GoodreadsResponse { goodreadsRequest :: GoodreadsRequest
, goodreadsSearch :: GoodreadsSearch
}
这是我要用于的仆人API类型
This is the servant API type I want to use it with
type API
= "search" :> "index.xml" :> QueryParam "key" Key :> QueryParam "q" Query :> Get '[XML] GoodreadsResponse
它构建了这样的端点
https://www.goodreads.com/search/index.xml?key=api_key&q=Ender%27s+Game
并且在编写完其余的脚手架代码(clientM，baseURL，客户端环境等)之后，我得到的错误是
and after writing the rest of the scaffolding code (clientM, baseURL, client environment, etc), the error I get is
No instance for (FromXml GoodreadsResponse) arising from a use of 'client'
写作
instance FromXml GoodreadsResponse where
fromXml = undefined
抑制了错误，所以我认为我走在正确的轨道上，但是我不知道如何编写解析器.
suppresses the error so I think I'm on the right track, but I don't know how to go about writing the parser.
来自包含作品"列表的另一个端点的结果
Result from a different end-point that contains a list of 'works'
<GoodreadsResponse>
<Request>
<authentication>true</authentication>
<key>api_key</key>
<method>search_index</method>
</Request>
<search>
<query>Ender's Game</query>
<results-start>1</results-start>
<results-end>20</results-end>
<results>
<work>
<id type="integer">2422333</id>
<average_rating>4.30</average_rating>
<best_book type="Book">
<id type="integer">375802</id>
<title>Ender's Game (Ender's Saga, #1)</title>
</best_book>
</work>
<work>
<id type="integer">4892733</id>
<average_rating>2.49</average_rating>
<best_book type="Book">
<id type="integer">44687</id>
<title>Enchanters' End Game (The Belgariad, #5)</title>
</best_book>
</work>
<work>
<id type="integer">293823</id>
<average_rating>2.30</average_rating>
<best_book type="Book">
<id type="integer">6393082</id>
<title>Ender's Game, Volume 1: Battle School (Ender's Saga)</title>
</best_book>
</work>
</results>
</search>
</GoodreadsResponse>
要解析为
data GoodreadsResponse =
GoodreadsResponse { goodreadsRequest :: GoodreadsRequest
, goodreadsSearch :: GoodreadsSearch
}
data GoodreadsRequest =
GoodreadsRequest { authentication :: Text
, key :: Text
, method :: Text
}
data GoodreadsSearch =
GoodreadsSearch { query :: Text
, resultsStart :: Int
, resultsEnd :: Int
, results :: GoodreadsSearchResults
}
data GoodreadsSearchResults = GooreadsSearchResults { works :: [Work] }
data Work = Work { workID :: Int
, workAverageRating :: Double
, workBestMatchingBook :: Book
}
data Book = Book { bookID :: Int
, bookTitle :: Text
}
推荐答案
哇，在 xmlbf 中没有示例或预定义的实例，并且其文档中也存在多个错误.无论如何，在玩了一段时间之后，看起来是这样的:
Wow, there's no examples or predefined instances in xmlbf, and its documentation also has multiple mistakes. Anyway, after playing with it for a bit, it looks like this is how you do it:
{-# LANGUAGE OverloadedStrings #-}
import Data.Text.Lazy (unpack)
import Text.Read (readEither)
import Xmlbf
instance FromXml GoodreadsRequest where
fromXml = pElement "Request" $ do
a <- pElement "authentication" pText
k <- pElement "key" pText
m <- pElement "method" pText
pure GoodreadsRequest{ authentication = a, key = k, method = m }
instance FromXml GoodreadsSearch where
fromXml = pElement "search" $ do
q <- pElement "query" pText
s <- pElement "results-start" pText
s' <- either fail return . readEither $ unpack s
e <- pElement "results-end" pText
e' <- either fail return . readEither $ unpack e
pure GoodreadsSearch{ query = q, resultsStart = s', resultsEnd = e' }
instance FromXml GoodreadsResponse where
fromXml = pElement "GoodreadsResponse" $ do
r <- fromXml
s <- fromXml
pure GoodreadsResponse{ goodreadsRequest = r, goodreadsSearch = s }
这就是您的示例XML:
And here it is working with your example XML:
GHCi, version 8.8.2: https://www.haskell.org/ghc/ :? for help
Prelude> :l Main.hs
[1 of 1] Compiling Main ( Main.hs, interpreted )
Ok, one module loaded.
*Main> :set -XOverloadedStrings
*Main> import Xmlbf.Xeno
*Main Xmlbf.Xeno> fromRawXml "<GoodreadsResponse>\n <Request>\n <authentication>true</authentication>\n <key>api_key</key>\n <method>search_index</method>\n </Request>\n <search>\n <query>Ender's Game</query>\n <results-start>1</results-start>\n <results-end>20</results-end>\n </search>\n</GoodreadsResponse>" >>= runParser fromXml :: Either String GoodreadsResponse
Right (GoodreadsResponse {goodreadsRequest = GoodreadsRequest {authentication = "true", key = "api_key", method = "search_index"}, goodreadsSearch = GoodreadsSearch {query = "Ender's Game", resultsStart = 1, resultsEnd = 20}})
*Main Xmlbf.Xeno>
这是您在列表上与其他端点一起使用的方式:
Here's how you use it on lists, with your other endpoint:
{-# LANGUAGE OverloadedStrings #-}
import Control.Applicative (Alternative(many))
import Data.Text.Lazy (unpack)
import Text.Read (readEither)
import Xmlbf
instance FromXml GoodreadsResponse where
fromXml = pElement "GoodreadsResponse" $ do
r <- fromXml
s <- fromXml
pure GoodreadsResponse{ goodreadsRequest = r, goodreadsSearch = s }
instance FromXml GoodreadsRequest where
fromXml = pElement "Request" $ do
a <- pElement "authentication" pText
k <- pElement "key" pText
m <- pElement "method" pText
pure GoodreadsRequest{ authentication = a, key = k, method = m }
instance FromXml GoodreadsSearch where
fromXml = pElement "search" $ do
q <- pElement "query" pText
s <- pElement "results-start" pText
s' <- either fail return . readEither $ unpack s
e <- pElement "results-end" pText
e' <- either fail return . readEither $ unpack e
r <- fromXml
pure GoodreadsSearch{ query = q, resultsStart = s', resultsEnd = e', results = r }
instance FromXml GoodreadsSearchResults where
fromXml = pElement "results" $ do
w <- many fromXml
pure GooreadsSearchResults{ works = w }
instance FromXml Work where
fromXml = pElement "work" $ do
i <- pElement "id" pText -- the type attribute is ignored
i' <- either fail return . readEither $ unpack i
r <- pElement "average_rating" pText
r' <- either fail return . readEither $ unpack r
b <- fromXml
pure Work{ workID = i', workAverageRating = r', workBestMatchingBook = b }
instance FromXml Book where
fromXml = pElement "best_book" $ do -- the type attribute is ignored
i <- pElement "id" pText -- the type attribute is ignored
i' <- either fail return . readEither $ unpack i
t <- pElement "title" pText
pure Book{ bookID = i', bookTitle = t }
结果:
GHCi, version 8.8.2: https://www.haskell.org/ghc/ :? for help
Prelude> :l Main.hs
[1 of 1] Compiling Main ( Main.hs, interpreted )
Ok, one module loaded.
*Main> :set -XOverloadedStrings
*Main> import Xmlbf.Xeno
*Main Xmlbf.Xeno> fromRawXml "<GoodreadsResponse>\n <Request>\n <authentication>true</authentication>\n <key>api_key</key>\n <method>search_index</method>\n </Request>\n <search>\n <query>Ender's Game</query>\n <results-start>1</results-start>\n <results-end>20</results-end>\n <results>\n <work>\n <id type=\"integer\">2422333</id>\n <average_rating>4.30</average_rating>\n <best_book type=\"Book\">\n <id type=\"integer\">375802</id>\n <title>Ender's Game (Ender's Saga, #1)</title>\n </best_book>\n </work>\n <work>\n <id type=\"integer\">4892733</id>\n <average_rating>2.49</average_rating>\n <best_book type=\"Book\">\n <id type=\"integer\">44687</id>\n <title>Enchanters' End Game (The Belgariad, #5)</title>\n </best_book>\n </work>\n <work>\n <id type=\"integer\">293823</id>\n <average_rating>2.30</average_rating>\n <best_book type=\"Book\">\n <id type=\"integer\">6393082</id>\n <title>Ender's Game, Volume 1: Battle School (Ender's Saga)</title>\n </best_book>\n </work>\n </results>\n </search>\n</GoodreadsResponse>" >>= runParser fromXml :: Either String GoodreadsResponse
Right (GoodreadsResponse {goodreadsRequest = GoodreadsRequest {authentication = "true", key = "api_key", method = "search_index"}, goodreadsSearch = GoodreadsSearch {query = "Ender's Game", resultsStart = 1, resultsEnd = 20, results = GooreadsSearchResults {works = [Work {workID = 2422333, workAverageRating = 4.3, workBestMatchingBook = Book {bookID = 375802, bookTitle = "Ender's Game (Ender's Saga, #1)"}},Work {workID = 4892733, workAverageRating = 2.49, workBestMatchingBook = Book {bookID = 44687, bookTitle = "Enchanters' End Game (The Belgariad, #5)"}},Work {workID = 293823, workAverageRating = 2.3, workBestMatchingBook = Book {bookID = 6393082, bookTitle = "Ender's Game, Volume 1: Battle School (Ender's Saga)"}}]}}})
*Main Xmlbf.Xeno>
此中的新关键概念是 Control.Applicative.many .它会一直运行 Alternative ，直到失败为止，然后将所有成功的结果放入列表中.在这种情况下，这意味着重复 fromXml :: Parser Work 直到开始失败(希望是因为没有< work> 了).请注意，许多在这种情况下的工作方式存在一个缺陷(IMO，因为 xmlbf 的解析器界面不是很好)，即格式错误的< work> 元素只会使通过</results> 的所有内容都被忽略，而不会冒出错误.如果需要，可以使用涉及 pChildren 的稍微复杂些的代码来解决该问题.
The new key concept in this one is Control.Applicative.many. It keeps running an Alternative until it fails, and then puts all of the successful results into a list. In this case, that means repeating fromXml :: Parser Work until it starts to fail (hopefully because there's no <work>s left). Note that there's one flaw in how many works in this context (IMO, because xmlbf's parser interface isn't very good), namely that a malformed <work> element will just cause everything from it through </results> to be ignored, instead of the error bubbling up. You could use slightly more complicated code involving pChildren to fix that if you want.
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