Support value access for dictionaries/maps

[niknetniko]

We should add a special function for accessing values in a map, dictionary or object, since this is a common operation in languages that support the data structure.

In the DSL, we must be careful:

• Ideally we want to use the Python syntax, i.e. a_dictionary["some_key"].
• However, this gets tricky when using numeric keys, as there is no way to know the difference between dictionary access and array access syntactically.

Some options could include:

• Not using square brackets in the DSL for dictionary access, instead reserving some other function name, e.g. a_dictionary.dict_get. This is less ideal, since we have to introduce magic functions with reserved names.
• Distinguishing both cases based on the type of the variable, and requiring casts in cases where we don't know. First see if we know the type of the variable, then if we know the type of the key (e.g. non-numeric keys indicate dictionary), and otherwise require casts, e.g. dict(a_dictionary)[some_key].

[pdawyndt]

Maybe this is an idea. Using "magic methods" is Python-compliant and also differentiates between types on which the indexing operation is called.

>>> a_list = [1, 2, 3]
>>> list.__getitem__(a_list, 2)
3

>>> a_dict = {'A': 1, 'B': 2, 'C': 3}
>>> dict.__getitem__(a_dict, 'B')
2

Should then be converted into some nicer syntax when rendering for a specific programming language.