Is outputting a Maybe acceptable in Haskell? [duplicate]

Question

The vote on this output option has moved here.

Haskell has a very strong type system, so there is a type called Maybe to work around some restrictions.

The Maybe type is defined as follows:

data Maybe a = Just a | Nothing
    deriving (Eq, Ord)

It allows the programmer to specify something may not be there.

^{From the Haskell wiki.}

Essentially, instead of erroring out, a function can return a Maybe that is either a Just x where x is the intended output or it can return Nothing, which indicates that there is no possible output.

One example is the second solution in one of my answers. As you can see, in the output, the answer asked for is Just prefixed.

This can be fixed by applying foldl1 seq to the result, which I think is the golfiest way to do this.

Is outputting a Maybe valid?

^{_{Maybe we can allow this...}}

Answer 1

Yes

Maybe is used to emulate failure in Haskell. The equivalent question in, say, Python, is

is it OK that a submission could, in theory, return None, even though it never does?

or

is it OK that a submission could, in theory, raise an exception, even though it never does?

The answers to these are obvious.

What the type system allows the submission to do ≠ what the submission actually does.

Also, a Maybe is equivalent to a list restricted to 0 or 1 elements, so the consensus about singleton lists could apply here.

Answer 2

As long as the function is guaranteed to not output Nothing, it is valid

The title explains it all. If the function is not supposed to return Maybe and it is guaranteed to not output Nothing, it is valid.