Why Can\'t Go Slices Be Used as Map Keys?

Limitations of Using Go Slices as Map Keys

Go slices, which are essentially implementations of Go arrays, share similarities with arrays in terms of their value-type nature. However, their suitability as map keys differs significantly from that of arrays. This distinction stems from the fact that slices reference underlying arrays, which introduces complexities that are not present with arrays.

As Nigel Tao explains, copying a slice is significantly faster than copying an array, thanks to its O(1) time complexity compared to the O(length) complexity of arrays. This difference highlights the distinction between value types (arrays) and reference types (slices).

When defining map keys, it becomes necessary to establish a notion of equality. For arrays, equality is straightforwardly determined by comparing individual elements. However, for slices, there are multiple ways to define equality. Consider element-wise equality or equality based on the underlying array storage they reference.

Moreover, the act of inserting a key into a map creates uncertainty regarding whether an expensive copy of the entire backing array is required. While copying could align with expectations, it would deviate from the assignment behavior of slices.

To illustrate these complexities, consider the following example:

m := make(map[[]int]bool)
s0 := []int{6, 7, 8}
s1 := []int{6, 7, 8}
s2 := s0
m[s0] = true
s2[0] = 9
println(m[s0])
println(m[s1])
println(m[s2])

The different expectations programmers may have about the outcome demonstrate the potential for confusion that arises when using slices as map keys. To avoid such ambiguities, the choice has been made to disallow the use of slices as map keys in Go.

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