Can one store a value indexed by a string in the IO monad?

Question

I want to use custom ManagerSettings to build a custom Network.Wreq.Session. I modify from the defaultManagerSettings both managerRawConnection and managerModifyRequest.

I want managerModifyRequest to use a configuration value known at runtime from a file. As I do a lot of requests, I would rather not make a lot of syscalls to get the configuration value from the file.

I find the type managerModifyRequest :: Request -> IO Request to be problematic. How can I use a configuration value if it is not possible to get it from the parameters?

I thought about IORefs and MVars to store the value in memory, but I should be able to pass said IORef or MVar to the function somehow...

It isn't clear what you mean by "is not possible to get it from the parameters". In what sense it is not possible? Also, why reading a value from e.g. an `IORef` is more problematic than reading one from a file? — n. m. could be an AI, Feb 18 '22 at 10:16
For the first question, because there is only a `Request` parameter to the `managerModifyRequest` function. For the second one, because I can use the filename to get the configuration value. — fyusuf-a, Feb 18 '22 at 11:29
Your function that reads from a file returns `IO Something`. Your function that reads from an IORef or `MVar` also returns `IO Something`. It isn't clear why one is more problematic than the other. Can you show some code that works, even inefficiently, with values stored in a file? What exactly stops you from modifying this code to use an IORef instead? — n. m. could be an AI, Feb 18 '22 at 11:36
Reading each time from a file is costly. @chi showed a working example. I did not realize I could use `>>=` to use a pure function with a custom IO value. — fyusuf-a, Feb 18 '22 at 11:43
I was wondering what doesn't let you come up with this exact kind of example. There isn't anything particularly custom about any IO value. You use >>= (or do notation) when reading from a file or reading from an IORef in exactly the same way. — n. m. could be an AI, Feb 18 '22 at 11:54

chi · Accepted Answer · 2022-02-18T12:22:33.890

I am not familiar with the library, but I guess you need to write something like this:

-- pseudocode
foo = do
   -- create the IORef here
   r <- newIORef "hello"
   let settings = defaultManagerSettings{
          ...
          managerModifyRequest = \req -> do
             -- use the IORef here
             s <- readIORef r
             putStrLn s
             writeIORef r (s ++ "!!")
             return req
          }
   use settings

You don't need to pass the IORef as an additional argument to managerModifyRequest , you need to define that Request -> IO Request function in the scope where the IORef is available.

Alternatively, use an helper function with the additional argument, and then partially apply it with the IORef:

-- pseudocode
foo = do
   -- create the IORef here
   r <- newIORef "hello"
   let settings = defaultManagerSettings{
          ...
          managerModifyRequest = myManager r
          }
   use settings

myManager :: IORef String -> Request -> IO Request
myManager r req = do
   -- use the IORef here
   s <- readIORef r
   putStrLn s
   writeIORef r (s ++ "!!")
   return req

Can one store a value indexed by a string in the IO monad?

1 Answers1