Why Use Parentheses When Initializing Go Structs Inside Statements?

Initialising Go Structs within Parentheses: Purpose and Usage

In Go, structs can be initialised using either the traditional assignment syntax, such as:

item1 := Item{1, "Foo"}

Or, alternatively, with parentheses:

item2 := (Item{2, "Bar"})

Both approaches produce identical results when inspected with reflect. However, initialising with parentheses serves a specific purpose.

One key difference arises when syntactically embedding the structure initialisation into another statement, such as an if conditional. Without parentheses, the intent becomes ambiguous:

if i := Item{3, "a"}; i.Id == 3 {
}

This results in compile-time errors, leaving it unclear whether the opening brace belongs to the composite literal or the if statement's body.

Employing parentheses resolves this ambiguity, making it clear that the composite literal enclosed within the parentheses represents the entirety of the expression to be assigned to the variable i. This prevents the compiler from misinterpreting the syntax:

if i := (Item{3, "a"}); i.Id == 3 {
}

In summary, using parentheses when initialising Go structs within another statement ensures that the code remains unambiguous and compiles successfully, particularly in scenarios involving conditional statements or other expressions that require a clear separation between the composite literal and the surrounding syntax.

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