get expression - unclear syntax

mapbox-gl-js version: 0.44.2

Steps to Trigger Behavior

Get expression syntax is not defined well in the documentation: it is not clear when it is necessary to use type expressions (for ex. issue 6479 ).
For now, my code looks like this (and it doesn't work without type expressions):
['get', 'line-opacity', ['object', ['get', ['string', ['get', FeatureProperties.LineType]], ['literal', this.lineStyles]]]]
And it is not clear when I have to use ['object',{...}], ['literal', {...}] or ['string',{...}]. And at the same time, in case expression I can write something like this:
'line-color': [ 'case', ['has', 'line-color'], ['get', 'line-color']]
without this type-casting magic. I would expect expressions to work in the same manner and a better description in the official documentation.

Expected Behavior

The same behavior for all expressions, proper description in the docs

Actual Behavior