You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: expression.go
    path of file: ./repos/corteza/server/system/service
    
    the code of the file until where you have to start completion: package service

import (
	"context"
	"fmt"

	"errors"

	"github.com/PaesslerAG/gval"
	"github.com/cortezaproject/corteza/server/pkg/expr"
)

type (
	expression struct {
		parser gval.Language
	}
)

func Expression() *expression {
	return 
    
func (svc *expression) Evaluate(ctx context.Context, expressions map[string]string, vars map[string]any) (out map[string]any, err error) {
	out = make(map[string]any, len(expressions))
	evalErrs := make(map[string]error, len(expressions))
	hasErr := false

	for k, e := range expressions {
		out[k], err = svc.parser.Evaluate(e, vars)
		evalErrs[k] = err
		hasErr = hasErr || err != nil
	}

	if !hasErr {
		return
	}

	err = nil
	errMsg := ""
	for k, e := range evalErrs {
		if e != nil {
			errMsg += fmt.Sprintf("%s: %s\n", k, e.Error())
		}
	}

	return nil, errors.New(errMsg)
}