Option!T

[Idan Arye]

On Wednesday, 11 December 2013 at 00:08:49 UTC, Idan Arye wrote:
> On Tuesday, 10 December 2013 at 22:10:49 UTC, Jesse Phillips 
> wrote:
>> On Tuesday, 10 December 2013 at 17:28:26 UTC, Andrei 
>> Alexandrescu wrote:
>>> I talked to a programmer who knows Scala (among others) and 
>>> he mentioned the usefulness of the Option type - a zero or 
>>> one element collection (range in D terminology). Here's an 
>>> article discussing it: 
>>> http://danielwestheide.com/blog/2012/12/19/the-neophytes-guide-to-scala-part-5-the-option-type.html
>>>
>>> We have only(x) 
>>> (http://dlang.org/phobos/std_range.html#.only) to be a 
>>> collection of exactly one value, but not a type for "a value 
>>> of type T or nothing at all". Should we follow Scala's 
>>> example and add it?
>>>
>>>
>>> Andrei
>>
>> Having a easy way to return a single element range which may 
>> be empty is probably a good idea. It could make a good change 
>> to style, but I don't use Option in other languages. For that 
>> reason I'm not sure Option range probably doesn't have the 
>> same semantics as expected by users of other languages.
>
> That's a good point. `Option` is usually used with pattern 
> matching, so the value is used in a clause that is only 
> executed if it has a 
> value(http://en.wikipedia.org/wiki/Option_type#Examples). This 
> can easily be implemented in D: 
> https://gist.github.com/someboddy/7902770

Actually, now that I think about it, many range functions from 
`std.algorithm` can also be useful for (the 
functional-languages') `Option`, provided they return an 
`Option`. Functions like `map` and `filter` can not return a 
collection with more entries than what they receive, so it might 
be worthwhile to specialize them to return `Option` instead of 
regular ranges. That way we can use them to manipulate the value 
inside the option(only if it exists!) and in the end use the 
`Option` API to safely treat it as a single value(that might not 
exist) instead of a range that as far as the compile care can 
contain any number of values, and we only happen to know it can't 
contain more than one.