go-fetch

A small library that affords the use of simple jq/javascript/python-style accessors on nested interface{}s. go-fetch is not a replacement for properly unmarshalling JSON into appropriate structs and is intended to be used in situations where embedded data-accessing/querying is needed.

Documentation available at http://godoc.org/github.com/nikhan/go-fetch.

For example, given a map with the following structure:

{
    "foo":{
        "bar":[1,2,3]
    }
}

the third element of bar can be accessed by:

result, err := Fetch.Fetch(".foo.bar[2]", obj)

All queries must start with ., as this refers to the root of the value that is passed to go-fetch. Making a query . will return the entire value itself.

go-fetch supports bracket accessors for maps, so if you need to access a key that has characters that need to be avoided (such as a .,#,$,*,%,!), you can do so:

result, err := Fetch.Fetch(`.["foo"].bar[2]`, obj)

Fetch.Fetch() is a convenience function that runs both Fetch.Parse() and Fetch.Run(). If you have a situation where you will be running the same query over lots of values it is highly recommended that you Fetch.Parse() your query once and Fetch.Run() each value that needs to be queried.

query, _ := Fetch.Parse(`.["stop.trying"].to[0].make.fetch.happen`)
for{
    select {
        case m := <-data:
            Fetch.Run(query, m)
...

BenchmarkFetch	  				200000	     17778 ns/op
BenchmarkFetchParseOnce	  	  10000000	       168 ns/op
BenchmarkNoFetch			  20000000	       117 ns/op
BenchmarkNoFetchNoCheck		2000000000	      1.45 ns/op

The above benchmarks were run on a 2010 Macbook Pro. BenchmarkFetch is running Fetch.Fetch(). You can see that parsing the query every time can be costly. The second benchmark, BenchmarkFetchParseOnce compiles the query once with Fetch.Parse(). BenchmarkNoFetch is testing the time it takes to do all of the assertions on a map of interfaces{}. Finally, BenchmarkNoFetchNoCheck is what happens when dealing with properly typed structs.