proposal: Go 2: remove map from keywords

[minkofski]

Motivation

Go 1 is tightly designed with 25 keywords:

// dependency management
package import

// values
const var

// flow controls
if else break continue 
switch case default 
for range 
goto fallthrough

// function calls
func return defer

// data structures
type struct map interface

// concurrencies
go chan select

All keywords have their value and orthogonal except the map which dedicated to a hash table data structure but also share similarities array and slice, according to the language specification:

ArrayType   =       "[" ArrayLength "]" ElementType .
SliceType   =       "["             "]" ElementType .
MapType     = "map" "["   KeyType   "]" ElementType .

Comparing to these three definitions, map prefix is redundant because "[" XXX "]" ElementType . is rich enough to represent various meanings depends on the difference of XXX.

Proposal

I propose to remove map from keywords, where a Go 2 map type is:

Go2MapType = "["   KeyType   "]" ElementType .

Compatability

go fix can walk the whole program and drop all parsed map keyword from the program.

[mvdan]

I don't agree with this proposal. When I read [foo]string, right now it's unambiguous; foo must be a constant like const foo = 10; var _ [foo]string, thus meaning an array.

But with your proposal. it would be ambiguous to the reader, as it could also be type foo int; var _ [foo]string, thus meaning a map.

[maj-o]

I am the one who gave thumb up, so I have do argue why. Because I love what Mr. Griesemer showed 2006. If we can declare our Indexes on our own, surely we can remove map from the core, like complex numbers. And yes, go fix can and should fix this.
This would reduce the size of applications not using map. This would also open a door for SSE optimizations, 'cause data (ElementType) and index (KeyType) and all of its methods are known.
For me this is a thumb up, though it is strange at first look.

[ianlancetaylor]

For language change proposals, please fill out the template at https://go.googlesource.com/proposal/+/bd3ac287ccbebb2d12a386f1f1447876dd74b54d/go2-language-changes.md .

When you are done, please reply to the issue with @gopherbot please remove label WaitingForInfo.

Thanks!

[henvic]

@maj-o, can you explain why we can't achieve the same level of optimization at compile time, please?

[nirui]

Hey, just wondering, maybe when generic became a thing, map can then be supported through a package rather than a builtin keyword? (Move first-class map to a package?)

[ianlancetaylor]

@niruix Yes, a good generics implementation would support writing map as a package. That could be done with the current design draft, for example.

But the keyword version is still going to be more convenient to use.