some question about koa-swig in using koa-views

[Froguard]

how to set filters of koa-swig when I use it in koa-views
like this:

var views = require('koa-views');
app.use(views(__dirname + 'views', {
    map:{
        html: 'swig' 
    },
    filters:{
            getAppEnv:function(){
                return env.toLowerCase();
            }
    }
}));

or like this:

var views = require('koa-views');
app.use(views(__dirname + 'views', {
    map:{
        html: 'swig' 
    },
    options:{
        filters:{
            getAppEnv:function(){
                return env.toLowerCase();
            }
        }
    }
}));

two mode both are useless...Is there any problem?
how could I set filters via koa-views...
thank u!