27.05.2013 15:36, Diggory пишет:
I gather this has been discussed before and even a potential solution
submitted (https://github.com/D-Programming-Language/phobos/pull/780)
However it was dismissed due to too much existing code being broken.
I'd like to suggest a slightly less severe change which should still fix
the issues with TypeTuple:
https://github.com/D-Programming-Language/phobos/pull/1309
It introduces a new template, StaticTuple which can store any template
parameters. The two templates are exactly equivalent except that
TypeTuple checks that its parameters are actually types, so
StaticTuple!(int, float) == TypeTuple!(int, float).
Reasons for the change:
- Tuples seem to confuse everyone trying to learn D, the inconsistency
in TypeTuples is a big part of that. Naming the new type "StaticTuple"
makes it abundantly clear that the built in "Tuple" type is for storing
multiple values together at runtime while a "StaticTuple" is a similar
construction but for compile time. It then follows directly that
"TypeTuple" is a particular type of "StaticTuple" for dealing with types.
- The current functionality of using TypeTuples with non-types is
extremely useful and yet completely undocumented. This change means that
if some code expects a StaticTuple we can be safe to assume that passing
it non-types will be fine, and it also opens of the doors for other
specialized versions of StaticTuple such as ExpressionTuple. I have
avoided doing anything other than the most basic addition of StaticTuple
in this pull request as further improvements are a separate issue that
can be dealt with later.
- Unless we plan to stick to the current absurd and confusing naming for
TypeTuple forever, it's better to make the change sooner rather than later.
Reasons why this change is not detrimental:
- The result of the change is zero existing code actually failing to
compile. TypeTuple will simply show a deprecation warning if used with
non-types.
- If even a deprecation warning is too much an arbitrarily large
existing code-base can be fixed using a one off find and replace.
- The new template is also still in the "std.typetuple" module. This is
not ideal but it is also not a problem - it's common for a module to
contain related types in addition to the one it's named after.
- Any code that does use non-types with TypeTuple is using undocumented
behaviour. Making undocumented behaviour deprecated is a very reasonable
change even in the most stable of languages, so arguing that D is
supposed to be stable and that this breaks too much is not a very
convincing argument.
- Almost all uses of TypeTuple are for dealing with types and so will be
completely unaffected by the change.
- There's no necessity to ever actually completely remove the deprecated
behaviour, the deprecation warning is enough. It's not like code only
has X amount of time to change its behaviour to the new system.
Good luck from the heart as I've failed with almost every proposal to
change/add something in Phobos or in a language. For generic tuple
lovers (like me) I'd suggest just to use my `unstd.generictuple` [1] as
it has rather good code quality and I will keep it up-to-date with
language changes.
[1] http://denis-sh.github.io/phobos-additions/unstd.generictuple.html
--
Денис В. Шеломовский
Denis V. Shelomovskij
