Go golang, syntax error: unexpected ++, expecting :

  func test(args ...string) {
    var msg map[string] interface{}

    i := 0
    msg["product"] = args[i++]
    msg["key"] = args[i++]
    msg["signature"] = args[i++]
    msg["string_to_sign"] = args[i++]
  }

  go build utils.go

after compile, I get the error message

    ./utils.go:28: syntax error: unexpected ++, expecting :
    ./utils.go:28: missing statement after label
    ./utils.go:29: syntax error: unexpected ++, expecting :
    ./utils.go:30: syntax error: unexpected ++, expecting :
    ./utils.go:31: syntax error: unexpected ++, expecting :
    ./utils.go:36: syntax error: unexpected ++, expecting :
    ./utils.go:37: syntax error: unexpected ++, expecting :

why can't I put i++ in index of slice? is there any limitation in index of slice?

> Go Frequently Asked Questions (FAQ)
>
> Why are ++ and -- statements and not expressions? And why postfix,
> not prefix?
>
> Without pointer arithmetic, the convenience value of pre- and postfix
> increment operators drops. By removing them from the expression
> hierarchy altogether, expression syntax is simplified and the messy
> issues around order of evaluation of ++ and -- (consider f(i++) and
> p[i] = q[++i]) are eliminated as well. The simplification is
> significant. As for postfix vs. prefix, either would work fine but the
> postfix version is more traditional; insistence on prefix arose with
> the STL, a library for a language whose name contains, ironically, a
> postfix increment.
>
> The Go Programming Language Specification
>
> IncDec statements
>
> The "++" and "--" statements increment or decrement their operands by
> the untyped constant 1. As with an assignment, the operand must be
> addressable or a map index expression.
>
> IncDecStmt = Expression ( "++" | "--" ) .
>
> The following assignment statements are semantically equivalent:
>
> IncDec statement Assignment
> x++ x += 1
> x-- x -= 1

Write,

func test(args ...string) {
	var msg map[string]interface{}
	i := 0
	msg["product"] = args[i]
	i++
	msg["key"] = args[i]
	i++
	msg["signature"] = args[i]
	i++
	msg["string_to_sign"] = args[i]
}

Which, in your particular case, simplifies to,

func test(args ...string) {
	var msg map[string]interface{}
	msg["product"] = args[0]
	msg["key"] = args[1]
	msg["signature"] = args[2]
	msg["string_to_sign"] = args[3]
}

According to Language Specification, http://golang.org/ref/spec#IncDec_statements, i++ is a IncDec statements, which is a statement, but not a expression.As for args[index], index must be a expression. You want more details , just read it Go Language Specification, it's just what the language demand.

As other people have said i++ is a statement in go, not an expression as it is in C. Go has a different way of expressing the same intent using multiple assignment:

func test(args ...string) {
    msg := make(map[string]string)
    i := 0

    msg["product"], i = args[i], i+1
    msg["key"], i = args[i], i+1
    msg["signature"], i = args[i], i+1
    msg["string_to_sign"], i = args[i], i+1

    fmt.Printf("%v\n", msg)
}

Your definition of map would have failed at runtime too.

++ operator is disappointing. This is my hack:

func test(args ...string) {
    i := 0
    inc := func(i *int) int { *i++; return i }
    var msg map[string] interface{}

    msg["product"] = args[inc(&i)]
    msg["key"] = args[inc(&i)]
    msg["signature"] = args[inc(&i)]
    msg["string_to_sign"] = args[inc(&i)]
  }