Is it ok to use panic/recover as a means for testing successful type assertion?

Question

I've been working on a way of trying to parse through nested JSON responses without mapping the information to a predefined struct.

With a blank interface it comes back as:

map[name:My Folder parentId:1 created:2014-10-09T16:32:07+0000 deleted:false description:Sync Dir id:3 links:[map[rel:self entity:folder href:https://web.domain.org/rest/folders/3 id:3] map[href:https://web.domain.org/rest/folders/1 id:1 rel:parent entity:folder] map[entity:user href:https://web.domain.org/rest/users/1 id:1 rel:creator]] modified:2014-12-18T18:07:01+0000 permalink:https://web.domain.org/w/SpJYGQkv syncable:true type:d userId:1]

So I'm using the following to navigate this information:

func NFind(input interface{}, refs...interface{}) (output interface{}) {
    defer func() {if r := recover(); r != nil { output = nil }}()

    for _, ref := range refs {
        switch cur := ref.(type) {
            case string:
                output = input.(map[string]interface{})[cur]
            case int:
                output = input.([]interface{})[cur]
        } 
    }
    return output
}

func NMap(input interface{}) (output map[string]interface{}) {
    defer func() {if r := recover(); r != nil {}}()
    if input == nil { return nil }
    return input.(map[string]interface{})
}

func NArray(input interface{}) (output []interface{}) {
    defer func() {if r := recover(); r != nil {}}()
    if input == nil { return nil }
    return input.([]interface{})
}

func NString(input interface{}) (output string) {
    defer func() {if r := recover(); r != nil {}}()
    if input == nil { return "" }
    return input.(string)
}

func NFloat64(input interface{}) (output float64) {
    defer func() {if r := recover(); r != nil {}}()
    if input == nil { return 0 }
    return input.(float64)
} 

Is this an acceptable way of parsing information from JSON strings, or is there a more preferable method?

Here is the example of using the above to parse out the correct information I'm currently using:

func mapCache(input map[string]interface{}, valType string) {
    fmt.Println(input)
    var (
        name string
        href string
        rel string
        links []interface{}
        myMap map[string]interface{}
    )

    if name = NString(NFind(input, "name")); name == "" { return }
    if links = NArray(NFind(input, "links")); links == nil { return }

    for i := 0; i < len(links); i++ {
        if myMap = NMap(links[i]); myMap == nil { return }
        if rel = NString(myMap["rel"]); rel == "" { return }
        if rel == "self" {
            if href = NString(myMap["href"]); href == "" { return }
        }
    }
    CacheDB.Set(valType, name, href, false)
}

Any insight would be appreciated! Thanks!

Answer 1

Check the specification for type assertions:

A type assertion used in an assignment or initialization of the special form

v, ok = x.(T)
v, ok := x.(T)
var v, ok = x.(T)

yields an additional untyped boolean value. The value of ok is true if the assertion holds. Otherwise it is false and the value of v is the zero value for type T. No run-time panic occurs in this case.

This is faster and less hacky than using error handling to check the type.

So you could rewrite

func NMap(input interface{}) map[string]interface{} {
    defer func() {if r := recover(); r != nil {}}()
    if input == nil { return nil }
    return input.(map[string]interface{})
}

as

func NMap(input interface{}) map[string]interface{} {
    if m, ok := input.(map[string]interface{}); ok {
        return m
    }
    return nil
}

You might also consider a library like github.com/zazab/zhash for making map[string]interface{} easier to work with. Or, of course, try to work out how one of encoding/json's existing modes can do it.