// To avoid data races, this should only happen inside func init()
lang.Unmarshallers[ /* your type name */ ] = /* your readIndex func */

// See documentation on lang.UnmarshalData for more details
v, err := lang.UnmarshalData(p *lang.Process, dataType string)

package example

import (
    "encoding/json"

    "github.com/lmorg/murex/lang"
)

func init() {
    // Register data-type
    lang.Unmarshallers["example"] = unmarshal
}

// Describe unmarshaller
func unmarshal(p *lang.Process) (interface{}, error) {
    // Read data from STDIN. Because JSON expects closing tokens, we should
    // read the entire stream before unmarshalling it. For formats like CSV or
    // jsonlines which are more line based, we might want to read STDIN line by
    // line. However given there is just one data return, you still effectively
    // head to read the entire file before returning the structure. There are
    // other APIs for iterative returns for streaming data - more akin to the
    // traditional way UNIX pipes would work.
    b, err := p.Stdin.ReadAll()
    if err != nil {
        return nil, err
    }

    var v interface{}
    err = json.Unmarshal(b, &v)

    // Return the Go data structure or error
    return v, err
}