Fixed-size array that contains multiple specific types?

Question

I have an array (coming from JSON) that always contains a string and an int, like so: ["foo",42]

Right now, I have to use []interface{} with assertions arr[0].(string) arr[1].(int)

I'm wondering if there's any way to specify the types expected in the array? I'm picturing something like.. [...]{string,int}

Thanks.

You can add runtime type assertions. golang.org/ref/spec#Type_assertions. You're using the first form of type assertions, but you can also check whether the assertion succeeds or fails. Also see: stackoverflow.com/questions/28015753/… — dyoo
– dyoo, Commented Jan 23, 2015 at 0:52
Wouldn't this better be done with a struct type and an attached UnmarshalJSON method (so that it can handle the [] vs {} difference)? — Dave C
– Dave C, Commented Apr 4, 2015 at 20:40

mattn · Accepted Answer · 2015-01-23 03:02:07Z

1

At the first, answer is No. But you can get values from interface{} with type you expected. How about this?

package main

import (
    "encoding/json"
    "fmt"
    "github.com/mattn/go-scan"
    "log"
)

func main() {
    text := `["foo", 42]`

    var v interface{}
    err := json.Unmarshal([]byte(text), &v)
    if err != nil {
        log.Fatal(err)
    }
    var key string
    var val int
    e1, e2 := scan.ScanTree(v, "[0]", &key), scan.ScanTree(v, "[1]", &val)
    if e1 != nil || e2 != nil {
        log.Fatal(e1, e2)
    }
    fmt.Println(key, val)
}

edited Jan 23, 2015 at 3:02

answered Jan 23, 2015 at 2:56

mattn

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