could someone explain this type error "Couldn't match expected type `IO ()' with actual type `a0 -> c0' "
Here's the type error I am getting, followed by the code in question. This may be due to an incorrect use of function composition, if so I'd like an explanation as to how I can fix that. If it's something else, I'd like to talk about that too.
frameworkDaemon.lhs:125:5:
Couldn't match expected type `IO ()' with actual type `a0 -> c0'
In the expression: popJobState . popProcessConfig . readJobFile
In an equation for `makeJobState':
makeJobState world
= popJobState . popProcessConfig . readJobFile
where
readJobFile
= do { let ...;
.... }
where
开发者_开发百科 getFileContents (x : xs) = readFile (ixiaRoot ++ "/" ++ (show x))
popProcessConfig = undefined
popJobState = undefined
Failed, modules loaded: none.
makeJobState :: JobState -> IO ()
makeJobState world =
popJobState . popProcessConfig . readJobFile -- f .g . h -> f(g(h))
where readJobFile = do
let directoryPath = ixiaRoot ++ "/jobs"
processedPath = map (directoryPath </>) . filter (`notElem` [".",".."])
jobFileNames <- processedPath <$> getDirectoryContents directoryPath
let numStrings = map (last . splitOn "/") jobFileNames
sortJobNumbers = map read numStrings :: [Int]
getFileContents $ sort $ sortJobNumbers
where getFileContents (x:xs) = readFile (ixiaRoot ++ "/" ++ (show x))
popProcessConfig = undefined
popJobState = undefined
-- f .g . h -> f(g(h))
f . g . h
is not equivalent to f(g(h))
, it's equivalent to \x -> f(g(h x))
, so h
needs to be a function, but readJobFile
(which is the h
in your example) is an IO ()
. That's why the error message complains about expecting a function where you gave it an IO ()
.
As sepp2k said, f.g.h
is a function, \x -> f $ g $ h x
. It seems that readJobFile
is IO String
, though, not IO ()
.
But it doesn't complain about expecting a function. It explains about having inferred a function, and having expected an IO ()
. It's inferred a function because your chain of compositions is just that---a function, so the result of applying makeJobState
to a JobState
value is (or would be) a function, not an IO action.
It seems as if what you want isn't function composition but monadic composition, something like makeJobState world = readJobFile >>= popProcessConfig >>= popJobState
(with the world
parameter actually being used at some point, though it's not really obvious to me where it should go. And that might not be the preferred order; it's hard to tell since the other two variables are undefined.) Since readJobFile
is IO String
, you'd want popProcessConfig
to be String -> IO a
and popJobState
to be a -> IO ()
. That would result in an overall type of JobState -> IO ()
.
精彩评论