Question

loadTexture :: String -> IO (Either String GL.GLuint)
loadTexture filename = do
    p <- PNG.loadPNGFile filename
    oglLoadImg p
    where
        oglLoadImg :: (Either String PNG.PNGImage) -> IO (Either String GL.GLuint)
        oglLoadImg (Left e) = return $ Left e 
        oglLoadImg (Right png) = do
            ... I need todo IO stuff in here

The code above seems really bloated and nasty. What can I do to make it simpler?

Answer 1

You essentially want a combination of the Either e monad and the IO monad. That’s what monad transformers are for!

In this case, you can use the ErrorT monad transformer which adds error handling using Either to an underlying monad, in this case IO.

import Control.Monad.Error

loadTexture :: String -> IO (Either String GL.GLuint)
loadTexture filename = runErrorT $ ErrorT (PNG.loadPNGFile filename) >>= oglLoadImg
    where
        oglLoadImg :: PNG.PNGImage -> ErrorT String IO GL.GLuint
        oglLoadImg png = do
            -- [...]

This keeps the old interface, although it would probably be even nicer to use ErrorT for your function as well, and have the call to runErrorT in your main function.

loadTexture :: String -> ErrorT String IO GL.GLuint
loadTexture filename = ErrorT (PNG.loadPNGFile filename) >>= oglLoadImg
    where
        oglLoadImg :: PNG.PNGImage -> ErrorT String IO GL.GLuint
        oglLoadImg png = do
            -- [...]

Monad transformers can take some getting used to, but they are very useful.