Aeson解码非穷举模式错误：Scotty应用失败但Repl正常

架构老林

2026-7-27

Aeson解码不一致问题：Scotty应用报错但REPL测试成功

问题概述

在Scotty服务应用中使用Aeson解码Cloudflare上传响应时，触发非穷尽模式错误：

src/CFUpload.hs:(66,6)-(71,27): Non-exhaustive patterns in function parseJSON

错误指向CFUploadResponse的FromJSON实例代码段，但将相同响应数据拿到REPL中测试解码器，却能成功完成解码。

代码场景

Scotty服务核心流程：

接收POST请求中的图片数据
下载图片资源
获取Cloudflare上传URL
将图片上传至Cloudflare
接收Cloudflare返回的响应
尝试用自定义解码器解析响应（此处触发报错）
返回处理结果

将解析步骤替换为直接返回响应内容，得到的JSON字符串（记为testResponse）：

"{\n  \"result\": {\n    \"id\": \"673996fb-4d26-4332-6e6b-e8bf7b608500\",\n    \"filename\": \"bird2.jpg\",\n    \"uploaded\": \"2023-03-18T22:53:56.705Z\",\n    \"requireSignedURLs\": false,\n    \"variants\": [\n      \"https://imagedelivery.net/9U-0Y4sEzXlO6BXzTnQnYQ/673996fb-4d26-4332-6e6b-e8bf7b608500/public\"\n    ]\n  },\n  \"success\": true,\n  \"errors\": [],\n  \"messages\": []\n}"

格式化后结构：

{
  "result": {
    "id": "673996fb-4d26-4332-6e6b-e8bf7b608500",
    "filename": "bird2.jpg",
    "uploaded": "2023-03-18T22:53:56.705Z",
    "requireSignedURLs": false,
    "variants": [
         "https://imagedelivery.net/9U-0Y4sEzXlO6BXzTnQnYQ/673996fb-4d26-4332-6e6b-e8bf7b608500/public"
        ]
      },
    "success": true,
    "errors": [],
    "messages": []
}

类型定义

对应JSON结构的Haskell类型：

data CFUploadResponse = CFUploadResponse
  {
    result   :: CFUploadResult,
    success  :: Bool,
    errors   :: [String],
    messages :: [String]
  } deriving Show

data CFUploadResult = CFUploadResult {
    id                :: String,
    filename          :: String,
    uploaded          :: String,
    requireSignedURLs :: Bool,
    variants          :: [String]
  } deriving Show

解码器实现

instance FromJSON CFUploadResponse where
     parseJSON (Object v) =           -- (66)
        CFUploadResponse     <$>      -- (67) 
            v .: "result"    <*>      -- (68)
            v .: "success"   <*>      -- (69)
            v .: "errors"    <*>      -- (70)
            v .: "messages"           -- (71)

instance FromJSON CFUploadResult where
    parseJSON = withObject "CFUploadResult" $ \o -> do
      id <- o .: Data.Text.pack  "id"
      filename <- o .: Data.Text.pack "filename"
      uploaded <- o .:  Data.Text.pack "uploaded"
      requireSignedURLs <- o .: Data.Text.pack "requireSignedURLs"
      variants <- o .: Data.Text.pack  "variants"
      return (CFUploadResult id filename uploaded requireSignedURLs variants)

REPL测试结果

定义解码函数：

myDecode str = Data.Aeson.eitherDecode $ BL.pack str :: Either String (Maybe CFUpload.CFUploadResponse)

在stack repl中执行myDecode testResponse，得到成功解码结果：

Right (Just (CFUploadResponse {result = CFUploadResult {id = "49660d63-a43f-4011-1a7a-ff6435305d00", filename = "bird2.jpg", uploaded = "2023-03-16T23:08:22.768Z", requireSignedURLs = False, variants = ["https://imagedelivery.net/9U-0Y4sEzXlO6BXzTnQnYQ/49660d63-a43f-4011-1a7a-ff6435305d00/public"]}, success = True, errors = [], messages = []}))

排查方向

补全非穷尽模式处理：CFUploadResponse的parseJSON仅处理了Object类型的JSON值，未覆盖Null、String等其他可能类型。Scotty中可能接收到的响应并非直接的JSON对象（比如响应被包裹为字符串），导致触发非穷尽错误。修改解码器添加错误分支：
```
instance FromJSON CFUploadResponse where
  parseJSON (Object v) = CFUploadResponse <$> v .: "result" <*> v .: "success" <*> v .: "errors" <*> v .: "messages"
  parseJSON invalid = fail $ "Expected Object for CFUploadResponse but got: " ++ show invalid
```
此修改可输出实际接收到的JSON值类型，定位问题根源。
检查响应格式差异：Scotty中实际接收的响应可能与testResponse存在细微差异——比如Cloudflare在异常场景下返回不同结构的JSON，或响应未正确解压（如gzip压缩未处理）导致解析出错误的JSON值。
修正键值类型匹配：CFUploadResult解码器中手动使用Data.Text.pack包裹键名是冗余操作，.:操作符默认接受Text类型键名。若未启用OverloadedStrings扩展，字符串字面量"id"是String类型，与.:要求的Text不匹配，可能导致隐藏错误（REPL环境可能默认启用了该扩展），直接使用"id"即可：
```
instance FromJSON CFUploadResult where
    parseJSON = withObject "CFUploadResult" $ \o -> do
      id <- o .: "id"
      filename <- o .: "filename"
      uploaded <- o .: "uploaded"
      requireSignedURLs <- o .: "requireSignedURLs"
      variants <- o .: "variants"
      return (CFUploadResult id filename uploaded requireSignedURLs variants) 
```
验证响应读取逻辑：确认Scotty中读取Cloudflare响应的方式是否正确——若响应体被读取为字符串，需先将字符串解码为JSON值，再解析为CFUploadResponse，而非直接对字符串执行解码操作。

内容的提问来源于stack exchange，提问作者jxxcarlson

Aeson解码非穷举模式错误：Scotty应用失败但Repl正常

问题概述 #

代码场景 #

类型定义 #

解码器实现 #

REPL测试结果 #