is type checking in D undecidable?
bcarneal at gmail.com
Fri Oct 23 04:52:01 UTC 2020
On Friday, 23 October 2020 at 04:24:09 UTC, Paul Backus wrote:
> On Friday, 23 October 2020 at 00:53:19 UTC, Bruce Carneal wrote:
>> When you write functions, the compiler helps you out with
>> fully automated constraint checking. When you write templates
>> you can write them so that they look like simple functions, in
>> which case you're on pretty solid ground. Your manual
>> constraints will probably work. Hard to screw up a four line
>> eponymous template with constraints. Hard to screw up a
>> "leaf" template with a small number of template args.
>> Possible but hard. Not so hard to screw up
>> "wanna-be-as-general-as-possible-but-special-case-performant-and-sometimes-wierdly-recursive-cuz-otherwise-the-compiler-blows-up" templates.
> This is true, but it has nothing at all to do with
> decidability--which is a term with a precise technical
> definition in computer science.
Yep. The thread started with the technical definition, as you'll
note in the wiki articles that I cited, and then moved on. I
probably should have started another thread.
> The reason writing correct generic code using templates (or any
> macro system) is so difficult is that templates (and macros in
> general) are, effectively, dynamically typed. Unlike regular
> functions, templates are not type checked when they are
> declared, but when they are "executed" (that is, instantiated).
> In that sense, writing templates in D is very similar to
> writing code in a dynamically-typed language like Python or
Yep. Good observations. Functions offer some nice benefits. I'd
like to see their use increase (type functions displacing
templates wherever appropriate).
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