code: func interpolationFuncPathExpand() ast.Function {
  	return ast.Function{
  		ArgTypes:   []ast.Type{ast.TypeString},
  		ReturnType: ast.TypeString,
  		Callback: func(args []interface{}) (interface{}, error) {
  			return homedir.Expand(args[0].(string))
  		},
  	}
  }
docs: // interpolationFuncPathExpand will expand any `~`'s found with the full file path