无法将预期类型"[foo]"与实际类型"也许 [foo]"匹配

Question

我有两种记录类型：

data Attribute = Attr {
attName  :: Text,
attValue :: Text
} deriving (Show, Generic)
data Attributes = Attrs {
attributes :: [Attribute]
} deriving (Show, Generic)

在main中，contents值包含一个Maybe [Attribute]，我正试图将其铲入Attrs构造函数：

main :: IO ()
main = do
bytes <- readFile "data/file.json"
contents <- pure (decode bytes :: Maybe [Attribute])
let attributes = Attrs { attributes = contents }
print attributes

运行此操作会产生以下错误：

• Couldn't match expected type ‘[Attribute]’
with actual type ‘Maybe [Attribute]’
• In the ‘attributes’ field of a record
In the expression: Attrs {attributes = contents}

当something是Maybe [Attribute]而不是直接是[Attribute]时，如何应用Attrs { attributes = something }构造函数？

Answer 1

您需要处理解码失败的Nothing情况。这是一种可能的方法。

main :: IO ()
main = do
bytes <- readFile "data/file.json"
case decode bytes :: Maybe [Attribute] of
Nothing -> putStrLn "Error: can't decode the file!"
Just contents -> do
let attributes = Attrs { attributes = contents }
print attributes

:: Maybe [Attribute]部分可能是多余的，因为编译器可以从程序的其余部分推断出该类型。尽管如此，你可能还是想把它留在那里，让代码对人类更可读。

请注意，decode库函数被设计为返回Maybe [Attribute]而不是[Attribute]，这样它既可以报告错误(解码失败(，也可以强制调用方处理此类错误。以这种方式，程序员不能再"；忘记"；以检查错误，因为类型不匹配。

如果decode不这样做，decode将需要使程序崩溃(非常不方便(或报告一个伪值(例如[](，这可能会被误认为是实际输入，并允许调用方忘记检查错误。

Answer 2

如果你想危险地生活：

let Just contents = (decode bytes :: Maybe [Attribute])