class Maybe
    ...
end

class Left
    ...
end

class Right
    ...
end

def more_than_zero?(num)
    if num.kind_of?(Integer)
        Right[num > 0 ? true : false]
    else
        Left['only integers allowed here']
    end
end

result = more_than_zero(3)

Maybe[result].
    on(Left,  -> (res) { p "Error: #{res}" }).
    on(Right, -> (res) { p "Result: #{res}"}))