After playing in generics playground with a dozen of cases that came to my mind, IMHO brackets seem to be more clear than parentheses for declaring type parameters. Also using the type keyword before the type parameter reduces the cognitive load drastically.
type StateFn[type T] func(T) StateFn[T] // Good vs: type StateFn(type T) func(T) StateFn(T) // Not as clear as the previous one It's hard to say if there are any practical alternatives to brackets - according to the proposal. But angle brackets are not that ugly by themselves. (I'm not sure if this approach is actually possible) The hash/number signs appear in the language specifications just once: type StateFn<#T> func(T) StateFn<#T> And the %T syntax is used to print types: type StateFn<%T> func(T) StateFn<%T> Then, that problematic example would become: a, b = w<#x,y>(z) or: a, b = w<%x,y>(z) On Tuesday, July 14, 2020 at 11:56:01 PM UTC+2 gri wrote: > We have received a variety of feedback on the generics draft design > <https://go.googlesource.com/proposal/+/refs/heads/master/design/go2draft-contracts.md> > > (blog <https://blog.golang.org/generics-next-step>). Thanks to everyone > who took the time to read it, play with generics code in the playground > <https://go2goplay.golang.org/>, file issues, and send us their thoughts. > > Not unexpectedly, many people raised concerns about the syntax, > specifically the choice of parentheses for type parameter declarations and > generic type and function instantiations. > > A typical computer keyboard provides four easily accessible pairs of > single-character symmetrical "brackets": parentheses ( and ), square > brackets [ and ], curly braces { and }, and angle brackets < and >. Go uses > curly braces to delineate code blocks, composite literals, and some > composite types, making it virtually impossible to use them for generics > without severe syntactic problems. Angle brackets require unbounded parser > look-ahead or type information in certain situations (see the end of this > e-mail for an example). This leaves us with parentheses and square > brackets. Unadorned square brackets cause ambiguities in type declarations > of arrays and slices, and to a lesser extent when parsing index > expressions. Thus, early on in the design, we settled on parentheses as > they seemed to provide a Go-like feel and appeared to have the fewest > problems. > > As it turned out, to make parentheses work well and for > backward-compatibility, we had to introduce the type keyword in type > parameter lists. Eventually, we found additional parsing ambiguities in > parameter lists, composite literals, and embedded types which required more > parentheses to resolve them. Still, we decided to proceed with parentheses > in order to focus on the bigger design issues. > > The time has come to revisit this early decision. If square brackets alone > are used to declare type parameters, the array declaration > > type A [N]E > > cannot be distinguished from the generic type declaration > > type A[N] E > > But if we are comfortable with the extra type keyword, the ambiguity > disappears: > > type A[type N] E > > (When we originally dismissed square brackets, the type keyword was not > yet on the table.) > > Furthermore, the ambiguities that arise with parentheses appear not to > arise with square brackets. Here are some examples where extra parentheses > are not needed with square brackets: > > using () using [] > > func f((T(int)) func f(T[int]) > > struct{ (T(int)) } struct{ T[int] } > > interface{ (T(int)) } interface{ T[int] } > > [](T(int)){} []T[int]{} > > To test this better understanding, and to get a feel for this alternative > notation, we will begin to make changes to our prototype implementation > such that it accepts either parentheses or square brackets (only one or the > other) in a generic Go package. Those changes will first appear as commits > to the dev.go2go branch > <https://go.googlesource.com/go/+/refs/heads/dev.go2go>, and eventually > in the playground <https://go2goplay.golang.org/>. > > If square brackets don't lead to unforeseen issues, we have another fully > explored notation to choose from, which will allow us to make a more > informed decision. > > - gri, iant > > PS: For ambiguities with angle brackets consider the assignment > > a, b = w < x, y > (z) > > Without type information, it is impossible to decide whether the > right-hand side of the assignment is a pair of expressions > > (w < x), (y > (z)) > > or whether it is a generic function invocation that returns two result > values > > (w<x, y>)(z) > > In Go, type information is not available at compile time. For instance, in > this case, any of the identifiers may be declared in another file that has > not even been parsed yet. > > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/a5fd4511-6930-442b-b9d6-c221bec7eb83n%40googlegroups.com.
