isInfinite isInadequate

[monarch_dodra]

On Tuesday, 12 March 2013 at 10:01:38 UTC, deadalnix wrote:
> I want to resurrect that thread. Can someone explains the 
> benefices of isInfinite ? I fail to see how it really benefit 
> the code.

The advantage of "enum empty = false" is that algorithms gain a 
great performance boost by optimizing out any "if (r.empty)". 
This can be exploited for things like take, or anything that 
iterates as a matter of fact. I don't think anybody will argue 
that this is a bad approach.

The trait itself checks if empty can be evaluated at compile time 
(to false). The advantage for the *coder* to know if the range is 
infinite is less obvious.

One of the advantages is that an infinite range can have random 
access (meets RA requirements), even though it has no length 
member (normally, any RA range must have length).

Having "isInfinite" can also have the advantage of protecting 
users from stupid calls. For example, calling "count" on an 
infinite range is forbidden => shifting problems from runtime to 
compile time is a HUGE gain.

One of downsides to having infinite ranges is that their 
existence tends to make dealing with generic RA ranges a bit more 
difficult. A lot of our algorithms have changed requirements 
conditions from:
"if (isRandomAccessRange!R)"
to
"if (isRandomAccessRange!R && hasLength!R)"
or
"if (isRandomAccessRange!R && !isInfinite!R)"
//NOTE: Both are strictly equivalent: An RA range is either 
infinite, or has length, but not neither nor both.

Up until not so long ago, it was not possible to slice infinite 
ranges. It is now possible. Unfortunatly, because RA ranges with 
slicing don't guarantee that r[0 .. $] is legal, things are still 
complicated in template code.

The last point (IMO minor) that is raised in the thread is 
"runtime" infiniteness. EG: a range that may or may not be 
infinite at compile time, but which will be known at runtime. 
IMO, these a rare enough (arguably, inexistent) to not really 
matter. The workarounds are easy:
1) Make the range always infinite, but with a requirement that 
user must intialize it or whatnot.
2) Make a runtime fork that will call code that operates on a 
compile-time known infinite adaptor/subtype.