Improve error messages and generate operator<< for records

[danyowdee]

This PR improves the error messages when a record cannot conform to a typeclass, because it has fields of types that don’t. In addition, it generates the C++ operator<< for enums, and introduces the show typeclass to do the same for records deriving (show).

Assuming the following Djinni file:

enum CurrencyCode {
    USD;
    EUR;
};

Amount = record {
    units: i64;
    currency: CurrencyCode;
} deriving (show)

Product = record {
    name: string;
    identifier: string;
    price: Amount;
} deriving (show)

ShoppingCart = record {
    items: list<Product>;
} deriving (show)

This PR allows you to write the following C++ code:

Product p{"Little Red Corvette", "v0KpfrJE4zw", Amount{1999, CurrencyCode::USD}};
Product q{"Breakfast Can Wait", "CzWX1gv6u2s", Amount{2013, CurrencyCode::EUR}};
ShoppingCart c{{p, q}};
std::cout << c << std::endl;
/* prints
"""ShoppingCart{ items:[
Product{ name:Little Red Corvette, identifier:v0KpfrJE4zw, price:Amount{ units:1999, currency:CurrencyCode::USD } },
Product{ name:Breakfast Can Wait, identifier:CzWX1gv6u2s, price:Amount{ units:2013, currency:CurrencyCode::EUR } },
] }"""
 */

If Amount would be missing the declaration of conformance to show, the error message from running Djinni would clearly say so:

Record 'Amount' not deriving required operation(s): 'show'

Supported Field Types

• any record deriving (show)
• numbers and strings
• any enum
• any list<T> where T is any of the above (or a map, or a list)
• any map<K,V> where K and V are any of the above (or a map)

Limitations/Known Issues

• data fields are printed as arrays of integers by default
• flag and optional fields require handwritten overloads of operator<< to be available in the translation unit of the record; failure to do so will not report an error until trying to compile the record’s .cppfile
• no specific error checking/reporting is implemented for containers: if the contained type doesn’t support show, the error may not surface until trying to compile the record’s .cpp file
• the added code is most definitely not idiomatic Scala, but rather baby steps towards learning the language

[xianwen]

Hi, @danyowdee:
I noticed you haven't signed the CLA yet, could you please sign it here: https://opensource.dropbox.com/cla/
Thanks a lot!

[danyowdee]

Done.

[xianwen]

Please update the generated files, specifically, test-suite/generated-src/cpp/color.hpp and
test-suite/generated-src/cpp/constant_enum.hpp will be affected by this diff.

[artwyman]

You should add some new cases to the test-suite to exercise this new functionality, as well as documentation about how to use it. E.g. the new "deriving" directive should be documented and demonstrated.