Why Does Go Throw an Error When Initializing a Struct in a For Loop?

Confusion with Struct in For Loop Initializer

In Go, a syntax error arises when initializing a struct in a for loop. Using a pointer to a struct works fine, but local variables are sometimes necessary. This article delves into the issue and provides a solution.

The problematic code snippet:

for r := Request{}; r.err == nil; r.id++ {
    r.line, r.err = input.ReadSlice(0x0a)
    channel <- r
}

Simplifying the code, we get:

for r := Request{}; r.err == nil; r.id++ {
    r.line, r.err = input.ReadSlice(0x0a)
    channel <- r
}

This code results in the following error:

expected boolean or range expression, found simple statement (missing parentheses around composite literal?) (and 1 more errors)

The error stems from the ambiguity in parsing the opening brace ('{'). It could either be part of a composite literal or the start of the for block.

To resolve this ambiguity, parentheses are added around the composite literal:

for r := (Request{}); r.err == nil; r.id++ {
    r.line, r.err = input.ReadSlice(0x0a)
    channel <- r
}

By enclosing the struct initialization in parentheses, we explicitly indicate that it is a composite literal and not part of the for loop syntax.

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