Question

I have the following snippet of code:

srcaddr <- getIfaceAddr iface >>= inet_ntoa . fromJust 
dstaddr <- getDestAddr iface >>= inet_ntoa . fromJust 
-- I want to perform actions only if neither getIfaceAddr 
-- nor getDestAddr returned Nothing
action1 srcaddr dstaddr
action2 srcaddr dstaddr
action3 srcaddr dstaddr

getIfaceAddr :: String -> IO (Maybe HostAddress)
getDestAddr :: String -> IO (Maybe HostAddress)

How to write this code in ‘nice Haskell’? I was thinking about the MaybeT monad but somehow wasn’t able make it work. I was trying to do some ‘lifting’, but wasn’t able to stich the types together. I can change the signature of the getIfaceAddr/getDestAddr.

As a sidenote: why is inet_ntoa ‘HostAddress -> IO String’? I don’t think there are any side effects, are they?

Answer 1

Oh my, what is that fromJust? If getIfaceAddr returns Nothing, this code will crash your program.

The MaybeT solution looks like this:

srcaddr <- lift . inet_ntoa =<< MaybeT (getIfaceAddr iface)
dstaddr <- lift . inet_ntoa =<< MaybeT (getDestAddr iface)
lift $ do
    action1 srcaddr dstaddr
    ...

The types for the first line fit together like this:

getIfaceAddr iface          :: IO (Maybe HostAddress)
MaybeT (getIfaceAddr iface) :: MaybeT IO HostAddress
inet_ntoa                   :: HostAddress -> IO String
lift . inet_ntoa            :: HostAddress -> MaybeT IO String
lift . inet_ntoa =<< MaybeT (getIfaceAddr iface)
                            :: MaybeT IO String

Remember that your code is going to have type MaybeT IO something, so you have to runMaybeT to get it back into IO before binding it to main.