从zipkintracer获取traceId

I am using zipkin-go-opentracing, which is an implementation of the opentracing API for zipkin in go.

For (reasons) I need to get the traceId from a span. So the question: given a opentracing.Span, how do I get the TraceId? Everything I've tried has given me some kind of type assertion error.

Thanks,

import (
    "github.com/opentracing/opentracing-go"
    "github.com/openzipkin/zipkin-go-opentracing"
)
func IdFromSpan(aspan interface{}) uint64 {
    zspan := aspan.(zipkintracer.Span)
    return zspan.Context().TraceID
}

I'm not sure this is the right way to do it but this should normally works

stdopentracing "github.com/opentracing/opentracing-go"
zipkin "github.com/openzipkin/zipkin-go-opentracing"

[...]

var traceID string

sp := stdopentracing.SpanFromContext(ctx)

if sp != nil {
    traceID = sp.Context().(zipkin.SpanContext).TraceID.ToHex()
}