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I've grown used to the idea that generics (data structure macros, C++ templates...) aren't that useful. If I find myself in a situation where I'm thinking of a

I've grown used to the idea that generics (data structure macros, C++ templates...) aren't that useful. If I find myself in a situation where I'm thinking of a solution that involves generics, I stop and ponder what is actually the essence of the repeated stuff. It rarely is on the syntactic level, often it runs deeper. Probably the commonalities can be distilled into a data structure. Simple example: Intrusive linking headers (e.g. Linux kernel list.h). While those can benefit from an optional very very thin generics layer on top, essentially the code for linking e.g. nodes in a tree should be the same regardless of the data structure where the headers are embedded. Getting this right simplifies the code but can also speed up compile times.

pcwalton 5y ago
How do you write a generic quicksort function without generics? There are two ways I know of: (1) throw type and memory safety out the window and use void pointers plus size/alignment like qsort(3) does; (2) require that users manually write an interface with a swap(int i, int j) function like Go before generics does. Both solutions are really bad.
- joe_guy 5y ago
  Void pointers also add an unfortunate layer of dereferencing. In C# for example, generics are important to keep values on the stack instead of the heap, not only avoiding garbage collection but improving data locality.
  - pcwalton 5y ago
    Well, a modern optimizing compiler can get rid of the indirection if it inlines qsort() and then inlines the comparison function. Of course, it needs to be able to see the source of qsort() to do so (which might be a problem if it's dynamically linked).
    - joe_guy 5y ago
      If I'm understanding correctly, you're saying a compiler will optimize out "void*" to instead be the value put into it, so theres no pointer dereference to get to it? That's what I am talking about. I don't know much about modern C optimizations.
      - jstimpfle 5y ago
        There isn't any pointer to optimize out really, I don't think - at the machine level, a pointer is a pointer is an integer number regardless of its type at the source code level. What can be measureable though is function call overhead - if you write a sort function that takes a function pointer and your sort function calls that, there is a lot of work for saving the local state, preparing all the arguments for the function, etc. That work can potentially be avoided if the compiler can inline the function call into (a copy of) your sort function.
        joe_guy 5y ago
        At the machine level there is absolutely a difference. one means your data is right here, the other means the data is elsewhere. That poor data locality can cause cache misses and always consumes extra cycles.
        jstimpfle 5y ago
        What is "right there"? If the computer should sort an array that is located in main memory, it needs to know their addresses (pointers) to even load them. There is no way around.
        joe_guy 5y ago
        It depends on how many times removed it is. Lets say it's 4 8bit ints. It can be sequentially and directly in memory like: stack: [0x01, 0x02, 0x03, 0x03] Each int is directly next to each other in memory. But it could instead be pointers to the ints, like: stack: [0x07002412, 0x0700241A, 0x07002424, 0x070036A0] heap (or stack, really): 0x07002412: 0x01 0x0700241A: 0x02 0x07002424: 0x03 0x070036A0: 0x04 The CPU needs to dereference that pointer, and the cache is getting all screwed up. That is what "void*" is doing. You're using a pointer instead of a value. In C# this is what generics allow you to avoid. // This is like my first memory layout above. struct Foo<T> { public T MyField; } vs: // This is like the second layout above. struct Foo { public object MyField; }
  - deleted 5y ago
    [deleted]
- jstimpfle 5y ago
  I rarely ever do need a sorting algorithm for what I do. I might not have done even one sort call in 2021. Usually the data I have is sorted by construction, or doesn't need to be sorted in any particular order. When I do need to sort, I just use qsort from libc. It's also easy to write my own version of qsort. I once measured qsort vs std::sort on plain ints (basically the most pessimistic case for qsort vs std::sort) and the difference was like 2x. If this becomes a bottleneck then it's worth investigating how to take advantage of additional context to speed up the sorting much more than any generic sorting algorithm could do anyway. Simple example: bucket sort. (I'm 100% confident sorting performance hasn't ever been noticeable in my career so far, but I've done simple optimizations like that once in a while for fun). > Both solutions are really bad. Due to what I described above, I'm actually in favour of qsort - no new code is generated, just a simple library call that takes a pointer to the comparison function. Really really easy to use.
  - dnautics 5y ago
    I don't know what you do, but for our codebase, we run transactions against the database in most of the integration tests. We can't generally be sure the database won't return rows in some order it likes that aren't the rows we expect -- so if we don't have a generic sorting function we will have flaky tests.
    - jstimpfle 5y ago
      If you can't bring the database to return the values in a well defined order, absolutely go ahead and sort it! No need for _generics_ though, qsort() will do just fine. (C probably isn't great for DB interop in any case).
  - tsimionescu 5y ago
    qsort is a horrible function, and infamously so. First, since it's part of the C stdlib, it is almost universally dynamically linked, precluding any chance for it to be inlined, and even worse, it calls a function pointer. Even ignoring performance, it has no support for checking the types, which in C means it can easily mess up memory badly. For example, I can call `qsort(arrayOfInts, num, sizeof(float), funcThatComparesTwoStrings)` and I'll get a really bad time, with absolutely no way for the compiler to tell me I'm doing something obviously wrong.
- mpweiher 5y ago
  Use Smalltalk?
  - travisgriggs 5y ago
    Don't you mean "If you use a late bound language, you don't need to chase down the type rabbit hole and encounter the nasty teeth that are generics along the way"? An example of which is Smalltalk, but could also include Dart 1, Python, Elixir, CLOS/Lisp, JavaScript, etc. Smalltalk is an interesting exhibition of a similar principle though. If you did add types to Smalltalk, you'd likely solve this containment parameterization problem the same way you do nearly everything in Smalltalk: by binding some new behavior (adding a method) to some data (a class object). You'd add a message to the MetaClass types that could create flyweight class types parameterized on the fly. And when you compiled, you'd send messages to the instance of the class itself to manufacture that synthetic on-the-fly type. Hand wave, hand wave, hand wave. The "let's see where naval gazing on a unifying principle goes?" that is employed in Zig is to use the same language itself to recurse on itself when defining itself, types included. What happens in more traditional/mainstream/Algol derived languages, is that each part of the system gets a sort of sub department of the language. You have a set of keywords/syntax for expressing run time behavior. And then someone says, but how do actually bring data to life--a sort of "Creatio ex nihilo" problem--and the answer is we need a new department in the language, so we get keywords/syntax for constructors. When behavior/construction needs to be parameterized across compilation paths, we get another set of keywords/syntax. And another for type definition. And yet another for scope management. And then again with meta things. And maybe a preprocessor. I do think there's a trick to finding a sweet spot in these "turtles all the way down" systems. In Smalltalk, I was happy with how the "don't know what's going on? just follow the message sends", but at the same time still providing enough structure/composition to have to be reflecting/introspecting all the time. Forth is event simpler, but I found I had to constant recompile things in my head to resolve. I'm relatively new on the Elixir curve, and am beginning to worry this happens with macros there. Figuring out how Plug works really messes with my brain. I haven't done enough with Zig to know where it's @comptime ends up in this balance.
- deleted 5y ago
  [deleted]
ncmncm 5y ago
Generics are the way that useful semantics can find their way into useful libraries. A collection of useful libraries is what is needed to make a language useful. This is why C++ usage is still growing fast: almost every new feature makes it possible to write more powerful libraries that get, thereby, easier to use.
- jstimpfle 5y ago
  This is a blanket assertion. Why should useful semantics require generics? Why can't they come with simple fixed data structures? If you can provide a nice counterexample to the example that I gave, this would be more convincing. Absent the absolute necessity of generics to support essential semantics, I'm probably in favour of a simpler generics-less version. I don't see why "more powerful libraries" get automatically easier to use. It could often be the opposite. There is a tradition in some C++ subcultures where they try to cram in as many invariants as possible into types, concepts, templates, etc. at all costs. I tend to think that if all this heavy machinery is needed, the functionality might be too complicated from the start.
  - ncmncm 5y ago
    If you were writing a library, and a language feature made it possible to make your library easier to use safely, why would you not? You might as well say, "I don't see why a more expensive dinner has to be tastier." It is always easy to make libraries that are hard to use, in any language, but those do not become popular if there are better alternatives. We need generics because users have types that they need libraries to work with. Your own example, of an intrusively-linked list, illustrates this: without generics, you could not write a linked-list library component that would be usable for that. This is why C programs are crammed with so many custom one-off hash tables: You cannot code a useful hash table library in C. Libraries start simple, and accumulate your "heavy machinery" as they are made more useful and usable for their growing family of users. A library that lacks users does not.
    - jstimpfle 5y ago
      It's more like, "I don't see why a tastier dinner has to be more expensive". It's a tradeoff situation, and while I might be willing to buy a great $50+ meal from time to time instead of just a $20 one, I don't have any inclination to pay $1000 even if the meal is a tiny little bit better than the just-great one. It's a matter of tradeoffs. Library design is a balancing act. > We need generics because users have types that they need libraries to work with. This is right where I become sceptical. Most libraries shouldn't care for the user's types at all. They should expose their own types so you can work the library - not the other way around. Please provide an actual use case where the library has to "know" about the user's types (and please don't mention std::sort. It's as common an example as is "Dog :: Animal" examples to argue for class inheritance, and is just as irrelevant). > Your own example, of an intrusively-linked list, illustrates this: without generics, you could not write a linked-list library component that would be usable for that. As I mentioned, linked list can tangentially benefit from a very very thing generics layer on top of a fixed data structure implementation. The layer does nothing more than "instanciate" the types, but there is no code generated. But then again, the added convenience / safety is minimal, I once wrote a C++ implementation that I was quite happy with, and never used it. > This is why C programs are crammed with so many custom one-off hash tables There are probably not huge issues with a design like "HashTable ht = { .hash = &foo_key_hash, .equals = &foo_key_equals }" (if quick results are what you're after), but yes it is nice to have a few lines of type-safe wrap generated over some generic container interface. An alternative reason why you'll find a good amount of custom hashtables in C code bases is, I suppose, that there are a lot of different ways to implement hash tables. Also writing a little code here might not be so bad. I've had 1 use case for a "hash" table in 2021 (glyph hash table for a new iteration of my font rendering code) and I've gotten away without even implementing it - the for-loop I've written has never shown up in any performance profile. Considering that container data structures are probably the highest profile application for generics, I'm still not convinced that generics are needed in a systems programming language...
tsimionescu 5y ago
> the code for linking e.g. nodes in a tree should be the same regardless of the data structure where the headers are embedded. Why should that be so? It's particularly NOT true for one of the simplest and most efficient data structures in any program: the array. The code that needs to work with an array needs to know exactly the size of each element of the array, so you basically can't have an efficient language that doesn't support generic arrays. Haskell maybe sometimes gets by with a very smart compiler and heavy use of laziness, but even C has support for generic arrays.
- deleted 5y ago
  [deleted]
- jstimpfle 5y ago
  You can totally make array iteration generic, just pass the element size. Not saying it is necessarily a good idea. I agree that there are cases where the "there should be a single central piece of code" argument is not a strong one, because the generic code is so simple that it should be inlined into the usage code. Arrays iteration is one example - but note that for example sorting code is, while the sorting algorithm might be generic, more than just generic array iteration. Doubly linked lists already profit from shared code in some ways, and definitely I'd find it a bad idea to replicate for example tree mutation code (RB tree or similar) for each node type. Embedding the header structure and keeping the links just between these embedded headers, to be able to reuse a common (compiled) piece of code is a very good idea here IMO.

pcwalton 5y ago

How do you write a generic quicksort function without generics? There are two ways I know of: (1) throw type and memory safety out the window and use void pointers plus size/alignment like qsort(3) does; (2) require that users manually write an interface with a swap(int i, int j) function like Go before generics does. Both solutions are really bad.

joe_guy 5y ago
Void pointers also add an unfortunate layer of dereferencing. In C# for example, generics are important to keep values on the stack instead of the heap, not only avoiding garbage collection but improving data locality.
- pcwalton 5y ago
  Well, a modern optimizing compiler can get rid of the indirection if it inlines qsort() and then inlines the comparison function. Of course, it needs to be able to see the source of qsort() to do so (which might be a problem if it's dynamically linked).
  - joe_guy 5y ago
    If I'm understanding correctly, you're saying a compiler will optimize out "void*" to instead be the value put into it, so theres no pointer dereference to get to it? That's what I am talking about. I don't know much about modern C optimizations.
    - jstimpfle 5y ago
      There isn't any pointer to optimize out really, I don't think - at the machine level, a pointer is a pointer is an integer number regardless of its type at the source code level. What can be measureable though is function call overhead - if you write a sort function that takes a function pointer and your sort function calls that, there is a lot of work for saving the local state, preparing all the arguments for the function, etc. That work can potentially be avoided if the compiler can inline the function call into (a copy of) your sort function.
      - joe_guy 5y ago
        At the machine level there is absolutely a difference. one means your data is right here, the other means the data is elsewhere. That poor data locality can cause cache misses and always consumes extra cycles.
        jstimpfle 5y ago
        What is "right there"? If the computer should sort an array that is located in main memory, it needs to know their addresses (pointers) to even load them. There is no way around.
        joe_guy 5y ago
        It depends on how many times removed it is. Lets say it's 4 8bit ints. It can be sequentially and directly in memory like: stack: [0x01, 0x02, 0x03, 0x03] Each int is directly next to each other in memory. But it could instead be pointers to the ints, like: stack: [0x07002412, 0x0700241A, 0x07002424, 0x070036A0] heap (or stack, really): 0x07002412: 0x01 0x0700241A: 0x02 0x07002424: 0x03 0x070036A0: 0x04 The CPU needs to dereference that pointer, and the cache is getting all screwed up. That is what "void*" is doing. You're using a pointer instead of a value. In C# this is what generics allow you to avoid. // This is like my first memory layout above. struct Foo<T> { public T MyField; } vs: // This is like the second layout above. struct Foo { public object MyField; }
- deleted 5y ago
  [deleted]
jstimpfle 5y ago
I rarely ever do need a sorting algorithm for what I do. I might not have done even one sort call in 2021. Usually the data I have is sorted by construction, or doesn't need to be sorted in any particular order. When I do need to sort, I just use qsort from libc. It's also easy to write my own version of qsort. I once measured qsort vs std::sort on plain ints (basically the most pessimistic case for qsort vs std::sort) and the difference was like 2x. If this becomes a bottleneck then it's worth investigating how to take advantage of additional context to speed up the sorting much more than any generic sorting algorithm could do anyway. Simple example: bucket sort. (I'm 100% confident sorting performance hasn't ever been noticeable in my career so far, but I've done simple optimizations like that once in a while for fun). > Both solutions are really bad. Due to what I described above, I'm actually in favour of qsort - no new code is generated, just a simple library call that takes a pointer to the comparison function. Really really easy to use.
- dnautics 5y ago
  I don't know what you do, but for our codebase, we run transactions against the database in most of the integration tests. We can't generally be sure the database won't return rows in some order it likes that aren't the rows we expect -- so if we don't have a generic sorting function we will have flaky tests.
  - jstimpfle 5y ago
    If you can't bring the database to return the values in a well defined order, absolutely go ahead and sort it! No need for _generics_ though, qsort() will do just fine. (C probably isn't great for DB interop in any case).
- tsimionescu 5y ago
  qsort is a horrible function, and infamously so. First, since it's part of the C stdlib, it is almost universally dynamically linked, precluding any chance for it to be inlined, and even worse, it calls a function pointer. Even ignoring performance, it has no support for checking the types, which in C means it can easily mess up memory badly. For example, I can call `qsort(arrayOfInts, num, sizeof(float), funcThatComparesTwoStrings)` and I'll get a really bad time, with absolutely no way for the compiler to tell me I'm doing something obviously wrong.
mpweiher 5y ago
Use Smalltalk?
- travisgriggs 5y ago
  Don't you mean "If you use a late bound language, you don't need to chase down the type rabbit hole and encounter the nasty teeth that are generics along the way"? An example of which is Smalltalk, but could also include Dart 1, Python, Elixir, CLOS/Lisp, JavaScript, etc. Smalltalk is an interesting exhibition of a similar principle though. If you did add types to Smalltalk, you'd likely solve this containment parameterization problem the same way you do nearly everything in Smalltalk: by binding some new behavior (adding a method) to some data (a class object). You'd add a message to the MetaClass types that could create flyweight class types parameterized on the fly. And when you compiled, you'd send messages to the instance of the class itself to manufacture that synthetic on-the-fly type. Hand wave, hand wave, hand wave. The "let's see where naval gazing on a unifying principle goes?" that is employed in Zig is to use the same language itself to recurse on itself when defining itself, types included. What happens in more traditional/mainstream/Algol derived languages, is that each part of the system gets a sort of sub department of the language. You have a set of keywords/syntax for expressing run time behavior. And then someone says, but how do actually bring data to life--a sort of "Creatio ex nihilo" problem--and the answer is we need a new department in the language, so we get keywords/syntax for constructors. When behavior/construction needs to be parameterized across compilation paths, we get another set of keywords/syntax. And another for type definition. And yet another for scope management. And then again with meta things. And maybe a preprocessor. I do think there's a trick to finding a sweet spot in these "turtles all the way down" systems. In Smalltalk, I was happy with how the "don't know what's going on? just follow the message sends", but at the same time still providing enough structure/composition to have to be reflecting/introspecting all the time. Forth is event simpler, but I found I had to constant recompile things in my head to resolve. I'm relatively new on the Elixir curve, and am beginning to worry this happens with macros there. Figuring out how Plug works really messes with my brain. I haven't done enough with Zig to know where it's @comptime ends up in this balance.
deleted 5y ago
[deleted]

ncmncm 5y ago

Generics are the way that useful semantics can find their way into useful libraries. A collection of useful libraries is what is needed to make a language useful. This is why C++ usage is still growing fast: almost every new feature makes it possible to write more powerful libraries that get, thereby, easier to use.

jstimpfle 5y ago
This is a blanket assertion. Why should useful semantics require generics? Why can't they come with simple fixed data structures? If you can provide a nice counterexample to the example that I gave, this would be more convincing. Absent the absolute necessity of generics to support essential semantics, I'm probably in favour of a simpler generics-less version. I don't see why "more powerful libraries" get automatically easier to use. It could often be the opposite. There is a tradition in some C++ subcultures where they try to cram in as many invariants as possible into types, concepts, templates, etc. at all costs. I tend to think that if all this heavy machinery is needed, the functionality might be too complicated from the start.
- ncmncm 5y ago
  If you were writing a library, and a language feature made it possible to make your library easier to use safely, why would you not? You might as well say, "I don't see why a more expensive dinner has to be tastier." It is always easy to make libraries that are hard to use, in any language, but those do not become popular if there are better alternatives. We need generics because users have types that they need libraries to work with. Your own example, of an intrusively-linked list, illustrates this: without generics, you could not write a linked-list library component that would be usable for that. This is why C programs are crammed with so many custom one-off hash tables: You cannot code a useful hash table library in C. Libraries start simple, and accumulate your "heavy machinery" as they are made more useful and usable for their growing family of users. A library that lacks users does not.
  - jstimpfle 5y ago
    It's more like, "I don't see why a tastier dinner has to be more expensive". It's a tradeoff situation, and while I might be willing to buy a great $50+ meal from time to time instead of just a $20 one, I don't have any inclination to pay $1000 even if the meal is a tiny little bit better than the just-great one. It's a matter of tradeoffs. Library design is a balancing act. > We need generics because users have types that they need libraries to work with. This is right where I become sceptical. Most libraries shouldn't care for the user's types at all. They should expose their own types so you can work the library - not the other way around. Please provide an actual use case where the library has to "know" about the user's types (and please don't mention std::sort. It's as common an example as is "Dog :: Animal" examples to argue for class inheritance, and is just as irrelevant). > Your own example, of an intrusively-linked list, illustrates this: without generics, you could not write a linked-list library component that would be usable for that. As I mentioned, linked list can tangentially benefit from a very very thing generics layer on top of a fixed data structure implementation. The layer does nothing more than "instanciate" the types, but there is no code generated. But then again, the added convenience / safety is minimal, I once wrote a C++ implementation that I was quite happy with, and never used it. > This is why C programs are crammed with so many custom one-off hash tables There are probably not huge issues with a design like "HashTable ht = { .hash = &foo_key_hash, .equals = &foo_key_equals }" (if quick results are what you're after), but yes it is nice to have a few lines of type-safe wrap generated over some generic container interface. An alternative reason why you'll find a good amount of custom hashtables in C code bases is, I suppose, that there are a lot of different ways to implement hash tables. Also writing a little code here might not be so bad. I've had 1 use case for a "hash" table in 2021 (glyph hash table for a new iteration of my font rendering code) and I've gotten away without even implementing it - the for-loop I've written has never shown up in any performance profile. Considering that container data structures are probably the highest profile application for generics, I'm still not convinced that generics are needed in a systems programming language...

tsimionescu 5y ago

> the code for linking e.g. nodes in a tree should be the same regardless of the data structure where the headers are embedded. Why should that be so? It's particularly NOT true for one of the simplest and most efficient data structures in any program: the array. The code that needs to work with an array needs to know exactly the size of each element of the array, so you basically can't have an efficient language that doesn't support generic arrays. Haskell maybe sometimes gets by with a very smart compiler and heavy use of laziness, but even C has support for generic arrays.

deleted 5y ago
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jstimpfle 5y ago
You can totally make array iteration generic, just pass the element size. Not saying it is necessarily a good idea. I agree that there are cases where the "there should be a single central piece of code" argument is not a strong one, because the generic code is so simple that it should be inlined into the usage code. Arrays iteration is one example - but note that for example sorting code is, while the sorting algorithm might be generic, more than just generic array iteration. Doubly linked lists already profit from shared code in some ways, and definitely I'd find it a bad idea to replicate for example tree mutation code (RB tree or similar) for each node type. Embedding the header structure and keeping the links just between these embedded headers, to be able to reuse a common (compiled) piece of code is a very good idea here IMO.