code: func interpolationFuncFile() ast.Function {
  	return ast.Function{
  		ArgTypes:   []ast.Type{ast.TypeString},
  		ReturnType: ast.TypeString,
  		Callback: func(args []interface{}) (interface{}, error) {
  			path, err := homedir.Expand(args[0].(string))
  			if err != nil {
  				return "", err
  			}
  			data, err := ioutil.ReadFile(path)
  			if err != nil {
  				return "", err
  			}
  
  			return string(data), nil
  		},
  	}
  }
docs: // interpolationFuncFile implements the "file" function that allows
  // loading contents from a file.