使用 'servant-client' 和 'servant-xml' 解析 XML 响应
Parsing XML response using 'servant-client' and 'servant-xml'
我想使用 servant-client, servant-xml and xmlbf 库将 API 响应解析为数据类型。
这是一个例子API响应
<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>
这是我要将其解析成的数据类型
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
}
这是仆人 API 类型我想用它
type API
= "search" :> "index.xml" :> QueryParam "key" Key :> QueryParam "q" Query :> Get '[XML] GoodreadsResponse
它构建了这样一个端点
https://www.goodreads.com/search/index.xml?key=api_key&q=Ender%27s+Game
并且在编写了其余的脚手架代码（clientM、baseURL、客户端环境等）之后，我得到的错误是
No instance for (FromXml GoodreadsResponse) arising from a use of 'client'
写作
instance FromXml GoodreadsResponse where
fromXml = undefined
抑制错误，所以我认为我走在正确的轨道上，但我不知道如何着手编写解析器。
编辑：来自不同端点的结果包含 'works'
的列表
<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>
被解析为
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
}
哇，xmlbf 中没有示例或预定义实例，其文档也有多个错误。无论如何，在玩了一会儿之后，看起来你是这样做的：
{-# 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 }
这里正在使用您的示例 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>
编辑：以下是您如何在列表中使用它以及您的其他端点：
{-# 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>
这一篇的新关键概念是Control.Applicative.many。它保持 运行 和 Alternative 直到失败，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 直到它开始失败（希望是因为没有 <work> 剩余）。请注意，many 在这种情况下的工作方式存在一个缺陷（IMO，因为 xmlbf 的解析器接口不是很好），即格式错误的 <work> 元素只会导致一切从它到 </results> 被忽略，而不是错误冒泡。如果需要，您可以使用涉及 pChildren 的稍微复杂的代码来修复它。
我想使用 servant-client, servant-xml and xmlbf 库将 API 响应解析为数据类型。
这是一个例子API响应
<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>
这是我要将其解析成的数据类型
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
}
这是仆人 API 类型我想用它
type API
= "search" :> "index.xml" :> QueryParam "key" Key :> QueryParam "q" Query :> Get '[XML] GoodreadsResponse
它构建了这样一个端点
https://www.goodreads.com/search/index.xml?key=api_key&q=Ender%27s+Game
并且在编写了其余的脚手架代码（clientM、baseURL、客户端环境等）之后，我得到的错误是
No instance for (FromXml GoodreadsResponse) arising from a use of 'client'
写作
instance FromXml GoodreadsResponse where
fromXml = undefined
抑制错误，所以我认为我走在正确的轨道上，但我不知道如何着手编写解析器。
编辑：来自不同端点的结果包含 'works'
的列表<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>
被解析为
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
}
哇，xmlbf 中没有示例或预定义实例，其文档也有多个错误。无论如何，在玩了一会儿之后，看起来你是这样做的：
{-# 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 }
这里正在使用您的示例 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>
编辑：以下是您如何在列表中使用它以及您的其他端点：
{-# 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>
这一篇的新关键概念是Control.Applicative.many。它保持 运行 和 Alternative 直到失败，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 直到它开始失败（希望是因为没有 <work> 剩余）。请注意，many 在这种情况下的工作方式存在一个缺陷（IMO，因为 xmlbf 的解析器接口不是很好），即格式错误的 <work> 元素只会导致一切从它到 </results> 被忽略，而不是错误冒泡。如果需要，您可以使用涉及 pChildren 的稍微复杂的代码来修复它。