Can I have separate functions for reading and writing to a txt file in Haskell, without using a 'main' function?

Question

I'm making a program using Haskell that requires simple save and load functions. When I call the save function, I need to put a string into a text file. When I call load, I need to pull the string out of the text file.

I'm aware of the complexities surrounding IO in Haskell. From some reading around online I have discovered that it is possible through a 'main' function. However, I seem to only be able to implement either save, or load... not both.

For example, I have the following function at the moment for reading from the file.

main = do  
 contents <- readFile "Test.txt"  
 putStrLn contents  

How can I also implement a write function? Does it have to be within the same function? Or can I separate it? Also, is there a way of me being able to name the functions load/save? Having to call 'main' when I actually want to call 'load' or 'save' is rather annoying.

I can't find any examples online of someone implementing both, and any implementations I've found of either always go through a main function.

Any advice will be greatly appreciated.

Answer 1

I'm aware of the complexities surrounding IO in Haskell.

It's actually not that complex. It might seem a little intimidating at first but you'll quickly get the hang of it.

How can I also implement a write function?

The same way

Or can I separate it?

Yes

Also, is there a way of me being able to name the functions load/save?

Yes, for example you could do your loading like this:

load :: IO String
load = readFile "Test.txt"

All Haskell programs start inside main, but they don't have to stay there, so you can use it like this:

main :: IO ()
main = do
  contents <- load -- notice we're using the thing we just defined above
  putStrLn contents

Note the main is always what your program does; But your main doesn't only have to do a single thing. It could just as well do many things, including for instance reading a value and then deciding what to do; Here's a more complicated (complete) example - I expect you'll not understand all parts of it right off the bat, but it at least should give you something to play around with:

data Choice = Save | Load

pickSaveOrLoad :: IO Choice
pickSaveOrLoad = do
   putStr "Do you want to save or load? "
   answer <- getLine
   case answer of
     "save" -> return Save
     "load" -> return Load
     _      -> do
       putStrLn "Invalid choice (must pick 'save' or 'load')"
       pickSaveOrLoad

save :: IO ()
save = do
  putStrLn "You picked save"
  putStrLn "<put your saving stuff here>"

load :: IO ()
load = do
  putStrLn "You picked load"
  putStrLn "<put your loading stuff here>"

main :: IO ()
main = do
  choice <- pickSaveOrLoad
  case choice of
    Save -> save
    Load -> load

Of course it's a bit odd to want to do either save or load, most programs that can do these things want to do both, but I don't know what exactly you're going for so I kept it generic.