root/cobra/trunk/HowTo/210-UseNilAndNilableTypes.cobra

"""
Languages have a special value to indicate "no object":

    Name     Languages
    ----     ---------
    nil      Cobra, Smalltalk, Objective-C
    null     C#, Java
    NULL     C, C++
    None     Python
    Nothing  Visual Basic

But Cobra's nil can only be applied when a type is "nilable" as indicated by a
question mark suffix such as "String?". Cobra then enforces how nilable types
are used with the upshot that run-time "null reference exceptions" almost
never happen in Cobra programs.
"""

class Thing

    var _name as String  # must be initialized
    var _alternateName as String?  # can be left nil
    
    cue init(name as String)
        base.init
        _name = name
    
    pro name from var
    
    pro alternateName from var


class Limits
    """
    Tracks min/max limits which default to 0 and 10 respectively.
    """

    shared
        var _defaultMin = 0
        var _defaultMax = 10
    
    var _min as int?  # start life as nil
    var _max as int?
    
    cue init
        base.init
    
    get min as int
        if _min  # checks for non-nil
            return _min
        else
            return _defaultMin

    get max as int
        # can use if-expression instead of if-statement
        return _max ? _defaultMax


class ExampleCalls

    def printValue(v as Object)
        # note that this method won't take nil because the arg is "Object" not "Object?"
        print v

    def main
        th = Thing('two')
        th.alternateName = '2'
        th.alternateName = nil  # no problem since .alternateName is a String?
        # th.name = nil  # compiler error: cannot assign nil to non-nilable type

        .printValue(th.name)

        altName = th.alternateName  # type inference determines that `altName` type is String?
        # .printValue(altName)  # compiler error: cannot pass nilable type where non-nilable expected
        # two solutions:
        if altName  # checks for non-nil
            .printValue(altName)  # compiler understands this is okay
        else
            print 'alternate name is nil'

        # or typecast to non-nil ("x to !") for those times when you know the value will not be nil
        altName = 'Two'
        .printValue(altName to !)


class MoreInfo

    def foo
        # you can also affect type inference by casting to nilable with "x to ?"
        value = 0 to ?   # type is "int?"

        # you can return nil in an if-expression which makes the type nilable
        name = if(value and value <> 0, 'value', nil)  # type is "String?"
        if name, print name

        # you can coalesce with ? as a binary operator
        displayName = name ? '(no name)'
        print displayName