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.
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 Javascript.
