Blog / Go Language Spec

Duplicate map keys

September 25, 2023

It’s Monday again. That means I’ll be live-coding again! I hope you can join me.

Composite literals

… For map literals, all elements must have a key. It is an error to specify multiple elements with the same field name or constant key value. For non-constant map keys, see the section on evaluation order.

I expect we can all agree this makes sense. Keys in a map literal must be unique.

var animals = map[string]string{
	"cow":  "moo",
	"duck": "quack",
	"pig":  "oink",
	"cow":  "moo2", // invalid: duplicate key "cow" in map literal
}

But there’s an exception… Keys in a map literal aren’t required to be constants. And if you use a non-constant map key, as in the below example, you can end up with duplicate keys, in which case the evaluation order rules apply to determine which value will apply:

var cow = "cow"
var animals = map[string]string{
	"cow":  "moo",
	"duck": "quack",
	"pig":  "oink",
	cow:    "moo2", // Non-constant duplicate key allowed, but only one value takes effect
}
fmt.Println(animals) // Prints: map[cow:moo2 duck:quack pig:oink]

Quotes from The Go Programming Language Specification Version of August 2, 2023

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Composite keys

This week’s episode of the Cup o’ Go podcast is out! Keep up with the Go community in just 15 minutes per week! Have a listen! Composite literals … The key is interpreted as a field name for struct literals, an index for array and slice literals, and a key for map literals. For map literals, all elements must have a key. If you’ve written any Go code, you probably already know about keys in maps:

Array and slice literals

Composite literals … For array and slice literals the following rules apply: Each element has an associated integer index marking its position in the array. An element with a key uses the key as its index. The key must be a non-negative constant representable by a value of type int; and if it is typed it must be of integer type. An element without a key uses the previous element’s index plus one.

Struct literals

Composite literals … For struct literals the following rules apply: A key must be a field name declared in the struct type. Probably self-evident. You can’t use field names that aren’t defined in the struct type. An element list that does not contain any keys must list an element for each struct field in the order in which the fields are declared. If any element has a key, every element must have a key.

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