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

Using Go Slices as Keys in Maps: Limitations and Reasoning

While Go arrays can serve as keys in maps, their slice counterparts, despite sharing an array-like implementation, cannot be directly used in the same manner. This discrepancy raises the question: why aren't Go slices suitable for use as map keys?

The Value Type of Arrays

One explanation lies in the nature of arrays as value types. When assigning an array value to another variable, a new copy of the array is created. This means any modifications made to the assigned array will not affect the original array.

The Referential Nature of Slices

Unlike arrays, slices refer to an underlying array. Copying a slice does not create a new copy of the array. As a result, changes made to the assigned slice directly impact the original array.

Implications for Map Keys

Map keys require a notion of equality. For arrays, this is straightforward as it involves element-wise comparison. However, for slices, defining equality becomes more complex. Element-wise equality is one option, but referring to the same underlying array is another.

Additional Considerations

Moreover, inserting a slice into a map raises questions about copying. One approach would be to make an expensive copy of the underlying array. However, this is inconsistent with the behavior of slice assignments.

Expectations and Confusion

The difference in behavior between arrays and slices as map keys can lead to confusion. Programmers may have varying expectations about the outcome of code snippets utilizing slices as keys. To avoid potential misunderstandings, Go developers have opted to prohibit the use of slices as map keys.

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