Haskell I can not get exception ReadFile with try - haskell

Question

I have a function "management" that checks parameters and return a Maybe (String):
If there are not parameter -> return Nothing
If my parameter is equal to "-h" -> Return a string help
My problem arrived when I get a file and check if this file exists.
Couldn't match expected type ‘Maybe String’
with actual type ‘IO (Either e0 a2)’
managagment :: [String] -> Maybe (String)
managagment [] = Nothing
managagment ["-h"] = Just (help)
managagment [file] = try $ readFile file >>= \result -> case result of
Left e -> Nothing
Right content -> Just (content)

Answer 1

There are several problems
Function application ($) is lower precedence than bind (>>=)
You said:
try $ readFile file >>= \res...
Which means
try ( readFile file >>= \res... )
But you wanted:
try ( readFile file ) >>= \res...
IO (Maybe a) and Maybe a are distinct
You have a function using IO (via readFile and try) but many of the cases do not return an IO result (Nothing and Just content).
Solution: Return via return Nothing or pure Nothing to lift a value into the IO monad.
The exception type was ambiguous
The try function can catch any exception type, just look at the signature:
try :: Exception e => IO a -> IO (Either e a)
When you totally ignore the exception you leave the type checker with no information to decide what e should be. In these situations an explicit type signature is useful, such as:
Left (e::SomeException) -> pure Nothing
managagment is partial
managagment ["a","b"] is undefined as is any input list of length over one. Consider a final equational definition of:
managagment _ = managagment ["-h"]
Or more directly:
managagment _ = pure $ Just help
Style and other notes
managagment should probably management
Just (foo) is generally Just foo
help is not a function that returns a String. It is a value of type String.
The example was not complete, missing imports and help.
Fixed Code
Consider instead:
#!/usr/bin/env cabal
{- cabal:
build-depends: base
-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE LambdaCase #-}
import Control.Exception (try, SomeException)
main :: IO ()
main = print =<< management []
help :: String
help = "so helpful"
management :: [String] -> IO (Maybe String)
management [] = pure Nothing
management ["-h"] = pure $ Just help
management [file] =
try (readFile file) >>=
\case
Left (e::SomeException) -> pure Nothing
Right content -> pure $ Just content
management _ = pure $ Just help
And test as such:
% chmod +x x.hs
% ./x.hs