roman-numerals: Rewrite to return a Maybe.

[rbasso]

Considering what was discussed in #194 (comment), we should probably rewrite roman-numerals to make it more idiomatic.

Currently, the reference json file only suggests test for values between 1 and 3000.

Should we really add new tests for:

• Negative values?
• Zero?
• Numbers greater than 3000?

Or should we just change the return type to a Maybe, leaving to the user to choose how it will handle those cases?

[petertseng]

All the description.md says is "There is no need to be able to convert numbers larger than about 3000.", has not mentioned lower limits. I assume we could test it.

Questions to ask:

• If we change to Maybe and add a test for 0, would we expect it to be Nothing or Just ""?
• If we change to Maybe and add a test for -1, would we expect it to be Nothing or Just "-I"?

I don't know if there is historical basis for the proposed Just answers so really I am making things up. If you asked me what I would pick, I'd pick Nothing for both of those, and also a Nothing for 4000.

Or should we just change the return type to a Maybe, leaving to the user to choose how it will handle those cases?

Interesting, we haven't done it before. It could be a worthy experiment. You may recall at #292 I wondered whether people would get confused by that, but maybe if we told them "it's up to you!" then people need not be confused.

[rbasso]

If we change to Maybe and add a test for 0, would we expect it to be Nothing or Just ""?

For historical reasons, I would prefer to use Nothing. Nulla would probably be too much for the sake of correctness. 😄

Interesting, we haven't done it before. It could be a worthy experiment. You may recall at #292 I wondered whether people would get confused by that, but maybe if we told them "it's up to you!" then people need not be confused.

I would favor this approach, using a note in HINTS.md.