How to remove the trivial parameter?

Question

I have a function defined as follow:

errorWhenNothing :: Maybe a -> M (Maybe a) -- M is a Monad
errorWhenNothing Nothing = throwError "is Nothing!"
errorWhenNothing m = return m

The parameter m seems trivial and removing it makes the function simpler and more compact. The problem is that I cannot rewrite the second definition as

errorWhenNothing = return

GHC complains Equations for 'errorWhenNothing' have different numbers of arguments..

I want to know is there a way to remove the m?

Answer 1

Yeah you can't have different numbers of arguments for different clauses of the same function. It's mostly to catch the common error of leaving out a parameter on one of the lines, so instead of a confusing type error it just tells you what you did wrong straight away.

If you don't want the m there, you could make the wholething pointfree with the maybe eliminator

errorWhenNothing :: Maybe a -> M a
errorWhenNothing = maybe (throwError "isNothing!") return

whether this is an improvement is a matter of opinion.

Answer 2

Why return a Maybe from errorWhenNothing? If the input were Nothing, the computation would have been aborted with a throwError anyway. Having to pattern-match on the result of the function seems like needless work, because we know at that point that it is not Nothing.

Perhaps we could rewrite the function like this:

errorWhenNothing :: Maybe a -> M a
errorWhenNothing Nothing = throwError "is Nothing!"
errorWhenNothing (Just a) = return a

Now the second clause doesn't pass the parameter unchanged.