proposal: spec: `for int, *E := range *[]E`

[myaaaaaaaaa]

Go Programming Experience

Experienced

Other Languages Experience

C, C++, Python, Javascript

Related Idea

• Has this idea, or one like it, been proposed before?
• Does this affect error handling?
• Is this about generics?
• Is this change backward compatible? Breaking the Go 1 compatibility guarantee is a large cost and requires a large benefit

Has this idea, or one like it, been proposed before?

• proposal: Go 2: add &range in for statements #21537

Unlike the above proposal, which requires new syntax and is non-orthogonal, this one aims to accomplish the same thing by just adding a new supported type to range.

Proposal

Currently, ranging over slices and arrays only provides read access since the elements are copied by value. Adding a way to modify them when using range would enable loops to omit using index variables entirely for most cases, apart from the rare cases when index arithmetic is actually necessary.

This would, among other things, prevent accidentally using the wrong index when writing nested loops.

func ExampleRange() {
	nums := make([]int, 5)


	for _, n := range nums {
		n = 2 // n is a copy so this doesn't modify the slice
	}
	fmt.Println(nums)
	//Output:
	//[0 0 0 0 0]


	for _, n := range &nums {
		*n = 2 // Proposed solution
	}
	fmt.Println(nums)
	//Output:
	//[2 2 2 2 2]
}

Language Spec Changes

 Range expression                                              1st value                2nd value
 
-array or slice             a  [n]E, *[n]E, or []E             index    i  int          a[i]       E
+array or slice             a  [n]E, or []E                    index    i  int          a[i]       E
+pointer to array or slice  p  *[n]E, or *[]E                  index    i  int          &p[i]      *E
 string                     s  string type                     index    i  int          see below  rune
 map                        m  map[K]V                         key      k  K            m[k]       V
 channel                    c  chan E, <-chan E                element  e  E
 integer value              n  integer type, or untyped int    value    i  see below
 function, 0 values         f  func(func() bool)
 function, 1 value          f  func(func(V) bool)              value    v  V
 function, 2 values         f  func(func(K, V) bool)           key      k  K            v          V

Informal Change

N/A

Is this change backward compatible?

Unfortunately Go already supports ranging over array pointers (!), so this would change the behavior of such loops. A gofix rule should be able to update affected loops:

 for i, v := range arrayPtr {
+	v := *v

 	// ...
 }

Slice pointers are new and therefore backward compatible.

Orthogonality: How does this change interact or overlap with existing features?

This change arguably increases orthogonality (compared to the current range-over-array-pointer), since it more closely follows the convention that taking a pointer of a value should allow for modifying it.

Would this change make Go easier or harder to learn, and why?

N/A

Cost Description

Tools that understand the type of the expression in range statements would need to be updated to the new semantics.

Changes to Go ToolChain

vet, gopls

Performance Costs

N/A (likely minimal)

Prototype

N/A

[gabyhelp]

Related Issues and Documentation

• proposal: Go 2: Add ability to use the slice expression [a:b] for more readable for loops and intervals. #64079 (closed)
• spec: add range over int, range over func #61405 (closed)
• proposal: Go 2: spec: for range with defined types #47707 (closed)
• proposal: spec: improve for-loop ergonomics #24282
• proposal: Go 2: function values as iterators #43557 (closed)
• proposal: Go 2: expand power of range for slices/arrays #36816 (closed)
• proposal: Go 2: add &range in for statements #21537 (closed)
• proposal: Go 2: fix range loopclosure bug #37061 (closed)
• proposal: Go 2: range-over-function + range over types implementing iterator interface #65742 (closed)
• proposal: Go 2: add a new iterator syntax, package, interfaces #54047 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[seankhliao]

I believe the closing comment for #21537 also applies here, this is just a different syntax but with the same limitations.

[ianlancetaylor]

Also, as the proposal points out, this change is not backward compatible. We are not going to make a language change that causes valid code to have one meaning in release 1.X and a different meaning in release 1.X+1. (We did such a change once, as described at https://go.dev/wiki/LoopvarExperiment, but that was a special case that we aren't going to repeat.)