grains: Change return type to Maybe

[rbasso]

Currently, the exercise grains has a function square :: Int -> Integer that returns the number of grains of wheat on sequentially numbered squares on a chessboard. The problem doesn't specify what should be returned if the number given is not in the chessboard.

This is a great opportunity to make it more idiomatic and get the users acquainted with Maybe.
I propose to to the following:

1. Change the return type of function square from an Integer to a Maybe Integer.
2. Add three test:
   • square (-1)
   • square 0
   • square 65
3. Add a stub solution.

[petertseng]

Change seems good here, 👍

Might be good to see if this should be specified in x-common?

[rbasso]

Maybe we don't need to change there:

• There is no mention in exercism/x-common/grains.md about values out of the board.
• There is no exercism/x-common/grains.json.

Should we write something in exercism/x-common/grains.md?

[petertseng]

Yeah we can see if other tracks might want to check for out of bounds values like here. Maybe they'll like it, in which case it can be standard for all tracks. Or maybe they won't, an argument against might be exercism/problem-specifications#287 (comment) , but that means it'll just be a specific characteristic of this track.

[rbasso]

I think that grains.md is good enough the way it is, but I understand that it would be nice to exposed the other tracks to out-of-bound cases. Maybe a exercism/x-common/grains.json marking the out-of-bound cases as implementation specific could be the solution.