[Haskell-cafe] Nested tests [Code walking off the right edge of the screen]

[Andrew Coppin]

Sebastian Fischer wrote:
> using Control.Monad.Error:
>
>  either (hPutStrLn stderr) return =<< runErrorT $
>   do d1x <- lift $ doesDirectoryExist d1
>      unless d1x $ fail "Directory " ++ d1 ++ " not found."
>      f1x <- lift $ doesFileExist (d1 </> f1)
>      unless f1x $ fail "File " ++ f1 ++ " not found."
>      d2x <- lift $ doesDirectoryExist d2
>      unless d2x $ fail "Directory " ++ d2 ++ " not found."
>      f2x <- lift $ doesFileExist (d2 </> f2)
>      unless f2x $ fail "File " ++ f2 ++ " not found."
>      lift $ doStuff d1 d2 f1 f2

So... essentially, run the ErrorT transformer on top of IO to do the 
error handling?

That seems like a much nicer solution - if I can get it to work. (In 
general, any code involving monad transformers tends to fail with 
elaborate type checker failures. Typically it takes me hours to figure 
out why... Maybe somebody has written a tutorial on this stuff?)