test.coffee

class Model  
  constructor: ->
    @data = {}
  
  set: (key, val) ->  
    @data[key] = val

  get: (key) ->
    @data[key]

  @property: (property, options={setter:true}) ->
    (value) ->
      if options.setter? and value?
        @set property, value
      @get(property) ? options.default

class Foo extends Model
  foo: @property 'foo'

  bar: @property('bar', setter:false)

  baz: @property('baz', default:'Baz')



f = new Foo()
f.foo('fa')
console.log f.foo()
console.log f.data

f.data.foo = 'fb'
console.log f.foo()
console.log f.data

f.bar('ba')
console.log f.bar()
console.log f.data

f.data.bar = 'bb'
console.log f.bar()
console.log f.data


console.log f.baz()
console.log f.data

f.baz('ca')
console.log f.baz()
console.log f.data

f.data.baz = 'cb'
console.log f.baz()
console.log f.data

you're probably right that {setter: undefined} should not be considered a request for getter-only. simplest of all is just options.setter != false, it's fine if we ignore 0 and ''

sounds good