Use

The following illustrates what ~some.key would return given different data structures:

// A non-observable JS object:
{some: {key: "value"}}
   //-> "value"

// A non-observable JS object w/ a function at the end
{some: {key: function(){ return "value"; }}}
   //-> "value"

// A non-observable JS object with intermediate functions:
{some: function(){ return {key: "value"}}}
   //-> "value"

// A observable can-map
{some: new DefineMap({key: "value"})}
   //-> canCompute("value")

// A method on an observable can-map that reads observables
var Some = DefineMap.extend({
    value: "string",
    key: function(){ return this.value; }
})
{some: new Some({value: "value"})}
   //-> compute(function(){ return this.value; })

Notice that ~ should only be used once in a value lookup expression.