你能告诉我我哪里错了吗?我从单子(monad)开始,我真的不明白错误。它似乎正在工作
myforM :: (Monad m) => [a] -> (a -> m b) -> m [b]
myforM [] _ = return []
myforM (x:xs) f = (f x) >>= (\r -> myforM xs >>= (\rs -> return (r:rs)))
第一个错误:
m.hs:110:36:
Couldn't match type ‘m’ with ‘(->) (a -> [b0])’
‘m’ is a rigid type variable bound by
the type signature for
myforM :: Monad m => [a] -> (a -> m b) -> m [b]
at monady1.hs:108:11
Expected type: m [b]
Actual type: (a -> [b0]) -> [[b0]]
Relevant bindings include
f :: a -> m b (bound at monady1.hs:110:15)
xs :: [a] (bound at monady1.hs:110:11)
x :: a (bound at monady1.hs:110:9)
myforM :: [a] -> (a -> m b) -> m [b] (bound at monady1.hs:109:1)
Possible cause: ‘myforM’ is applied to too many arguments
In the first argument of ‘(>>=)’, namely ‘myforM xs’
In the expression: myforM xs >>= (\ rs -> return (r : rs))
第二个错误:
m.hs:110:36:
Couldn't match type ‘b’ with ‘[b0]’
‘b’ is a rigid type variable bound by
the type signature for
myforM :: Monad m => [a] -> (a -> m b) -> m [b]
at monady1.hs:108:11
Expected type: m [b]
Actual type: (a -> [b0]) -> [[b0]]
Relevant bindings include
r :: b (bound at monady1.hs:110:31)
f :: a -> m b (bound at monady1.hs:110:15)
myforM :: [a] -> (a -> m b) -> m [b] (bound at monady1.hs:109:1)
Possible cause: ‘myforM’ is applied to too many arguments
In the first argument of ‘(>>=)’, namely ‘myforM xs’
In the expression: myforM xs >>= (\ rs -> return (r : rs))
Failed, modules loaded: none.
最佳答案
myforM xs >>= ...
类型为
(a -> m b) -> m [b]
.你错过了f
这里。该错误有点误导:它包含问题,但不是正确的原因:Expected type: m [b]
Actual type: (a -> [b0]) -> [[b0]]
...
Possible cause: ‘myforM’ is applied to too many arguments
关于haskell - 自己实现的forM,找不到错误,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/36371683/