Author: Ian Lance Taylor
Created: January 2011
Last updated: April 2016
Discussion at https://golang.org/issue/15292
Go should support some form of generic programming. Generic programming enables the representation of algorithms and data structures in a generic form, with concrete elements of the code (such as types) factored out. It means the ability to express algorithms with minimal assumptions about data structures, and vice-versa (paraphrasing Jazayeri, et al).
People can write code once, saving coding time. People can fix a bug in one instance without having to remember to fix it in others. Generics avoid boilerplate: less coding by copying and editing.
Generics save time testing code: they increase the amount of code that can be type checked at compile time rather than at run time.
Every statically typed language in current use has generics in one form or another (even C has generics, where they are called preprocessor macros; example).
Go already supports a form of generic programming via interfaces. People can write an abstract algorithm that works with any type that implements the interface. However, interfaces are limited because the methods must use specific types. There is no way to write an interface with a method that takes an argument of type T, for any T, and returns a value of the same type. There is no way to write an interface with a method that compares two values of the same type T, for any T. The assumptions that interfaces require about the types that satisfy them are not minimal.
Interfaces are not simply types; they are also values. There is no way to use interface types without using interface values, and interface values aren’t always efficient. There is no way to create a slice of the dynamic type of an interface. That is, there is no way to avoid boxing.
Generics permit type-safe polymorphic containers. Go currently has a very limited set of such containers: slices, and maps of most but not all types. Not every program can be written using a slice or map.
Look at the functions SortInts, SortFloats, SortStrings in the sort package. Or SearchInts, SearchFloats, SearchStrings. Or the Len, Less, and Swap methods of byName in package io/ioutil. Pure boilerplate copying.
The copy and append functions exist because they make slices much more useful. Generics would mean that these functions are unnecessary. Generics would make it possible to write similar functions for maps and channels, not to mention user created data types. Granted, slices are the most important composite data type, and that’s why these functions were needed, but other data types are still useful.
It would be nice to be able to make a copy of a map. Right now that function can only be written for a specific map type, but, except for types, the same code works for any map type. Similarly, it would be nice to be able to multiplex one channel onto two, without having to rewrite the function for each channel type. One can imagine a range of simple channel manipulators, but they can not be written because the type of the channel must be specified explicitly.
Generics let people express the relationship between function parameters and results. Consider the simple Transform function that calls a function on every element of a slice, returning a new slice. We want to write something like
func Transform(s []T, f func(T) U) []U
but this can not be expressed in current Go.
In many Go programs, people only have to write explicit types in function signatures. Without generics, they also have to write them in another place: in the type assertion needed to convert from an interface type back to the real type. The lack of static type checking provided by generics makes the code heavier.
Any implementation of generics in Go should support the following.
Generics affect the whole language. It is necessary to evaluate every single language construct to see how it will work with generics.
Generics affect the whole standard library. It is desirable to have the standard library make effective use of generics. Every existing package should be reconsidered to see whether it would benefit from using generics.
It becomes tempting to build generics into the standard library at a very low level, as in C++ std::basic_string<char, std::char_traits<char>, std::allocator<char> >. This has its benefits—otherwise nobody would do it—but it has wide-ranging and sometimes surprising effects, as in incomprehensible C++ error messages.
As Russ pointed out, generics are a trade off between programmer time, compilation time, and execution time.
Go is currently optimizing compilation time and execution time at the expense of programmer time. Compilation time is a significant benefit of Go. Can we retain compilation time benefits without sacrificing too much execution time?
Unless we choose to optimize execution time, operations that appear cheap may be more expensive if they use values of generic type. This may be subtly confusing for programmers. I think this is less important for Go than for some other languages, as some operations in Go already have hidden costs such as array bounds checks. Still, it would be essential to ensure that the extra cost of using values of generic type is tightly bounded.
Go has a lightweight type system. Adding generic types inevitably makes the type system more complex. It is essential that the result remain lightweight.
The upsides of the downsides are that Go is a relatively small language, and it really is possible to consider every aspect of the language when adding generics. At least the following sections of the spec would need to be extended: Types, Type Identity, Assignability, Type assertions, Calls, Type switches, For statements with range clauses.
Only a relatively small number of packages will need to be reconsidered in light of generics: container/*, sort, flag, perhaps bytes. Packages that currently work in terms of interfaces will generally be able to continue doing so.
Generics will make the language safer, more efficient to use, and more powerful. These advantages are harder to quantify than the disadvantages, but they are real.
Keys(map[K]V) []K, Values(map[K]V) []Vsync.Poolmapcar and friendsCheck,CheckEqual}ioutil.NopCloser, but preserving other methods instead of restricting to the passed-in interface (see the ReadFoo variants of bytes.Buffer)proto.Clonefunc [T] Diff(x, y []T) []rangestring and []byte variants; many points that could benefit need high performance, though, and generics should provide that benefitI won’t discuss a specific implementation proposal here: my hope is that this document helps show people that generics are worth having provided the downsides can be kept under control.
The following documents are my previous generics proposals, presented for historic reference. All are flawed in various ways.