nice. "gen" here is akin to the existing forward declaration of recursive
inner functions. it says, "you are about to see something special and you
need to know *this* about it."
On Tue, Jul 14, 2020 at 11:06 PM Bakul Shah <ba...@iitbombay.org> wrote:
> I don't much like square brackets or angle brackets or guillemets. But
> here is a different way
> of *reducing* the need for parentheses at least for the common case:
>
> A proposal for fewer irritating parentheses!
>
> One thing to note is that generic functions & types are *different* from
> existing things like const, func, type, var. As such they should have their
> own declaration marker. For example
>
> gen T type pair struct { a, b T } // contrast with type pair(type T) ...
> gen T,U type W struct { a T; b U } // contrast with type W(type T, U) ...
> gen T func Print(s []T) {...} // print a slice of T
>
> These function/type/method generics are used by *prepending* the type:
>
> var x int pair // a pair of ints
> var y (int, int pair) W // here we have to use parentheses
> int Print([]int{1,2,3}) // print a slice of ints
> qq := int pair pair{{1,2},{3,4}} // a pair of a pair of ints
> ww := (int, int) W pair{{1,2},{3,4}}
>
> This use may seem weird if you are used to C/C++. I find it more readable
> than having to deal with extra parentheses. "int pair" clearly says a
> pair of ints and so on. What is more, if in future types are allowed to be
> *inferred* for generic function calls, you can simply drop the type prefix.
>
> If there is still a parsing ambiguity, I'd suggest adding a - as in
> int-pair.
>
> Additional type syntax rule:
>
> type: ... | type generic-type| (type-list) generic-type
>
> or
>
> type: ... | type "-" generic-type | (type-list) "-" generic-type
>
> FWIW I thought of this four weeks ago (June 16).
>
> On Jul 14, 2020, at 3:29 PM, 'gri' via golang-nuts <
> golang-nuts@googlegroups.com> wrote:
>
> Correction: The last paragraph in the post below should have said: "In Go,
> type information is not available at *parse* time". (Of course, type
> information is available at compile time.)
>
> On Tuesday, July 14, 2020 at 2:56:01 PM UTC-7 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/97a2914f-3e54-4994-974f-135e11f11117n%40googlegroups.com
> <https://groups.google.com/d/msgid/golang-nuts/97a2914f-3e54-4994-974f-135e11f11117n%40googlegroups.com?utm_medium=email&utm_source=footer>
> .
>
>
> --
> 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/3E782FE9-D854-45D8-B454-EC869409EA5D%40iitbombay.org
> <https://groups.google.com/d/msgid/golang-nuts/3E782FE9-D854-45D8-B454-EC869409EA5D%40iitbombay.org?utm_medium=email&utm_source=footer>
> .
>
--
*Michael T. jonesmichael.jo...@gmail.com <michael.jo...@gmail.com>*
--
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/CALoEmQy%2BhprcNRC4C1miSY69qkfOyme4jUmhEvaaCK0P5qC5jg%40mail.gmail.com.
