Question

I am struggling to understand a block of code which is extremely easy in imperative world.
That’s what I need to do: given an executable full path, which is a Maybe FilePath type, I need to execute it conditionally.
If the path is a Nothing – print an error, if the path is Just Path – execute it and print message that the file has been executed. Only “Hello, World” can be easier,right?
But in Haskell I dug my self into numerous layers of Maybe’s and IO’s and got stuck.
Two concrete questions arise from here:
How do I feed a Maybe FilePath into a system or rawSystem? liftM does not work for me here.
What is the correct way of doing this kind of conditional branching?
Thanks.

Answer 1

Simple pattern matching will do the job nicely.

case command of
  Just path -> system path >> putStrLn "Done"
  Nothing   -> putStrLn "None specified"