Elegant Haskell case/error handling

Question

I\'m trying to better understand how to handle error cases in haskell and made wrote some code to help me with that.

Is there a better way (more elegant, sho

Answer 1

Nested Maybes is indeed messy.

Suggestion 1: roll a custom error type and use Either

data MyError = ReadError | TooBig Int

explain :: MyError -> String
explain ReadError = "Error: the requested Int could not be found"
explain TooBig i = "Error: the supplied Int should be at most 10, but it was " ++ show i

Now use Either to mix Ok values (Right) and error values (Left):

type MyType = Either MyError Int

Now lots of handy functions like either and the Applicative and Monad instances for Either a make it easy to write nice code:

myAdd :: MyType -> MyType -> MyType
myAdd i1 i2 = (+) <$> i1 <*> i2

is nicely applicative, or

myMult i1 i2 = do
    a <- i1
    b <- i2
    return $ a * b

if you prefer monadic notation. We can use either in a crash-the-program way

myShow :: MyType -> String
myShow = either (error.explain) show 

or a tell-me-anyway way:

process4 :: MyType -> String
process4 = either explain show

Suggestion 2: roll a custom type

data MyType' = OK Int | ReadError | TooBig Int

and use pattern matching. This isn't as nice as suggestion 1 in my veiw, because you lose the higher order function reuse, but it's better than Maybe (Maybe Int)

Suggestion 3: Use the Error monad

Read up about Control.Monad.Error and use the supplied functions or the ErrorT monad transformer.

Answer 2

I think the most readable way is the maybe function you already tried, just a little bit prettified by avoiding lambdas (pointfree style) and using maybe even for the innermost check (by replacing if with mfilter):

import Control.Monad(mfilter)

process2 :: MyType -> String
process2 =  
  maybe "error"  $ 
  maybe "error2" $ 
  maybe "error3" 
  show . mfilter (<10) . Just