如果其中一个参数为Nothing,我想返回空列表,但我不能
data UtilsDiff = UtilsDiff {syntaxBlockOptions :: [String], optionsBlockOptions :: [String]}
deriving Show
data UtilsOrDiff = UtilsOrDiff Utils | Diff UtilsDiff
deriving Show
useBlockExpr :: Parser UtilsOrDiff
useBlockExpr = do
u1 <- syntaxExpr
opts <- optionsBlockExpr
_ <- absorbToStopWord "Description:"
utilDesc <- many anyChar
case options u1 of
Nothing -> Diff [] [] -- <- this line
Just val-> do
...
显然,我还没有弄清楚如何返回Diff类型的
warning: [-Wdeferred-type-errors]
• Couldn't match expected type ‘[a0] -> Text.Parsec.Prim.ParsecT String () Data.Functor.Identity.Identity UtilsOrDiff’ with actual type ‘UtilsOrDiff’
• The function ‘Diff’ is applied to two value arguments,but its type ‘UtilsDiff -> UtilsOrDiff’ has only one
In the expression: Diff [] [] In a case alternative: Nothing -> Diff [] []
Diff
的类型为UtilsDiff -> UtilsOrDiff
,而不是[a] -> [a] -> UtilsDiff
。您需要首先创建UtilsDiff
值,然后使用来创建Diff
值。
Nothing -> return $ Diff (UtilsDiff [] [])
由于Just
分支估计为Parser UtilsOrDiff
值,因此Nothing
分支也必须如此,因此调用return
。