初始化周期错误

Question

I have autogenerated code. Simplified version:

package main

// begin of A
func main(){
    ParseReader("x")
}
func parseInclude(fileName string) (interface{}, error) {
    got, _ := ParseReader(fileName)
    return got, nil
}
// end of A
type grammar struct {
    pos   int
    run  func(*parser) (interface{}, error)
}
var g = &grammar{
    pos:  1,
    run: (*parser).callonIncludeOp1,
}

type parser struct {
    filename string
    cur      current
}
func (p *parser) callonIncludeOp1() (interface{}, error) {
    return p.cur.onIncludeOp1("x")
}
func (p *parser) parse(g *grammar) (val interface{}, err error) {
    return g.pos, nil
}

type current struct {
    pos  int 
}
// B
func (c *current) onIncludeOp1(qfilename interface{}) (interface{}, error) {
    got, _ := parseInclude("x")
    return got, nil
}

func ParseReader(filename string) (interface{}, error) {
    p := &parser{ filename: filename }
    return p.parse(g)
}

I have error after compiling

./prog.go:19: initialization loop:
    /home/gCDfp4/prog.go:19 g refers to
    /home/gCDfp4/prog.go:25 (*parser).callonIncludeOp1 refers to
    /home/gCDfp4/prog.go:36 (*current).onIncludeOp1 refers to
    /home/gCDfp4/prog.go:7 parseInclude refers to
    /home/gCDfp4/prog.go:41 ParseReader refers to
    /home/gCDfp4/prog.go:19 g

I need do recursive call at grammar because i have preprocessor operator "#include" for parsing other file.

Because it is autogenerated code i can only modify code in block A or in function B.

How can i break the initialization cycle ?

Answer 1

This is the result of the package initialization where:

Dependency analysis does not rely on the actual values of the variables, only on lexical references to them in the source, analyzed transitively.

For instance, if a variable x's initialization expression refers to a function whose body refers to variable y then x depends on y.

As in: "A reference to a variable or function is an identifier denoting that variable or function."

Your example in a playground returns something more direct:

tmp/sandbox395359317/main.go:21: initialization loop:
    prog.go:21 g refers to
    prog.go:28 (*parser).callonIncludeOp1 refers to
    prog.go:21 g

There are techniques in Go for loose coupling, for instance interface.

As an example (not optimal, but at least breaks the initialization cycle), you can in //A add:

type parseIncluder interface {
    parseInclude(fileName string) (interface{}, error)
}

func (c *current) parseInclude(fileName string) (interface{}, error) {
    return parseInclude(fileName)
}

And in //B, the call to parseInclude() becomes:

got, _ := c.cParseIncluder().parseInclude("x")

See Go plaground and click on Run: no more initialization loop.

---

The OP Red Skotina used a different approach with an package init() function:

var gProxy grammar

func init() { gProxy = g }
func parseInclude(fileName string) (interface{}, error) {
    got, _ := ParseReaderProxy(fileName)
    return got, nil
}
func ParseReaderProxy(filename string) (interface{}, error) {
    p := &parser{filename: filename}
    return p.parse(gProxy)
}