Why Does a Go `for` Loop Fail with a Struct Expression Initializer, and How Can It Be Fixed?

Struct Expression in For Loop Initializer

In Go, using a struct expression as an initializer in a for loop can encounter syntax errors during compilation. While pointers to structs are acceptable, local variables declared through struct expressions may raise issues.

Consider the following code snippet:

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

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

This code will trigger a compile-time error: "expected boolean or range expression, found simple statement (missing parentheses around composite literal?)". The ambiguity arises because the opening brace { can be interpreted as either the beginning of a composite literal or the opening brace of the for block.

To resolve this ambiguity, enclose the struct expression in parentheses, making it clear that it is a composite literal:

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

With this change, the code will compile successfully and execute as intended, initializing the local variable r with a new Request struct and incrementing its id value each iteration.

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