SQL from functions inside schemas

[georgolden]

My schema Task.js:

Entity: {},

creation: 'datetime',  
startDate: {
    type: 'datetime',
    required: false,
    default: (task) => task.creation,
  },

SQL:

"startDate" timestamp with time zone NULL DEFAULT '(task) => task.creation',

Have I made a mistake using functions inside a default property of a field, that type is not a number? I saw in sources metarhia/metadomain schema File.js that use fucntion like this:

Registry: {},

filename: { type: 'string', index: true },    
size: 'number',
compression: {  
  format: 'string', 
  size: 'number',  
  ratio: (file) => file.compression.size / file.size,  
},

This is a very simple example. I am testing right now all possible cases, that are coming in to my mind.
When can I use calculated fields in schemes with metasql?