Why Does Using a Struct in a Go For Loop Initializer Cause a Compile-Time Error?

Struct in for Loop Initializer: Composite Literal Ambiguity

In Go, using a struct expression in a for loop initializer can lead to syntax errors if not handled properly. While a pointer to a struct works fine, declaring a local struct variable requires specific syntax.

To illustrate the issue, consider the following code snippet:

type Request struct {
    id   int
    line []byte
    err  error
}

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

This code raises a compile-time error:

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

The ambiguity arises because the opening brace can be interpreted as the start of either a composite literal or the for block. To resolve this, use parentheses around the composite literal:

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

This syntax explicitly declares the composite literal, avoiding the ambiguity during parsing.

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