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I have discovered generics with the Ada language (yes, I am old). When learning C++ template, my first feelings were that it was an insane inconsistent hack. Ma

by webreac 10y ago

I have discovered generics with the Ada language (yes, I am old). When learning C++ template, my first feelings were that it was an insane inconsistent hack. Many C++ compilers struggle during many years before standard was reasonably well supported. Standards have also evolved and consistency was guaranteed. C++ template is an incredibly complex but also flexible and powerful beast that has no equivalent apart maybe the macro system of scheme. C++ templates allows to change the semantic of the program as if the language was changed. Genericity is just a special case of usage of C++ template. Its true nature is to extend the C++ language.

qwertyuiop924 10y ago
...The difference is, Scheme's macro system (syntax-rules: there are others) is elegant and simple, whereas C++ templates are still an insane inconsistant hack. They're just a standard insane, inconsistant hack. But the other thing about the Scheme macro system is that while its (relative) simplicity, elegance, safety, and power lead to its adoption by other languages (most famously, JS (in the form of an external preprocessor) and rust), it's actually quite underpowered by Lisp standards, and thus most implementations provide a nonstandard system for hygenic imperative macros, usually either er/ir macros, sc/rsc macros, or syntax-case. Of the three, I'd say syntax-case is an overly complex mess, and that the other two both have merits. So since TMP is doing similar substitutions (AFAICT: I haven't done the deep dive) to syntax-rules, it's got very real, and relatively common, limits. And as C++ slowly expands in scope, it will have to deal with problems that syntax-rules has already solved (what if a function creates a lambda or takes a function pointer argument called ioctl, and your template expansion calls ioctl?), and TMP hasn't AFAICT. And even when they fix all that, they'll still have C++, which is about as regular as the Linux Kernel relase schedule combined with The DevTeam's.
- gpderetta 10y ago
  Templates are quite complex for sure, but certainly not an hack. They are also unlike scheme macro system which works at the AST level; C++ templates work at the type level, which scheme lacks entirely[1]. > what if a function creates a lambda or takes a function pointer argument called ioctl, and your template expansion calls ioctl? Are you talking about macro hygiene? C++ templates are hygienic, except for type dependent lookup which intentionally isn't.
  - qwertyuiop924 10y ago
    Ah, so there is hygene. I couldn't tell, so I assumed not.
fiddlerwoaroof 10y ago
Recently, I was looking into Ada and the way its generics worked was quite eye-opening.

by webreac 10y ago

qwertyuiop924 10y ago

...The difference is, Scheme's macro system (syntax-rules: there are others) is elegant and simple, whereas C++ templates are still an insane inconsistant hack. They're just a standard insane, inconsistant hack. But the other thing about the Scheme macro system is that while its (relative) simplicity, elegance, safety, and power lead to its adoption by other languages (most famously, JS (in the form of an external preprocessor) and rust), it's actually quite underpowered by Lisp standards, and thus most implementations provide a nonstandard system for hygenic imperative macros, usually either er/ir macros, sc/rsc macros, or syntax-case. Of the three, I'd say syntax-case is an overly complex mess, and that the other two both have merits. So since TMP is doing similar substitutions (AFAICT: I haven't done the deep dive) to syntax-rules, it's got very real, and relatively common, limits. And as C++ slowly expands in scope, it will have to deal with problems that syntax-rules has already solved (what if a function creates a lambda or takes a function pointer argument called ioctl, and your template expansion calls ioctl?), and TMP hasn't AFAICT. And even when they fix all that, they'll still have C++, which is about as regular as the Linux Kernel relase schedule combined with The DevTeam's.

gpderetta 10y ago
Templates are quite complex for sure, but certainly not an hack. They are also unlike scheme macro system which works at the AST level; C++ templates work at the type level, which scheme lacks entirely[1]. > what if a function creates a lambda or takes a function pointer argument called ioctl, and your template expansion calls ioctl? Are you talking about macro hygiene? C++ templates are hygienic, except for type dependent lookup which intentionally isn't.
- qwertyuiop924 10y ago
  Ah, so there is hygene. I couldn't tell, so I assumed not.

fiddlerwoaroof 10y ago

Recently, I was looking into Ada and the way its generics worked was quite eye-opening.