ExtVar support

[sbarzowski]

Only through internal API. Also no TLA support at the moment.

[sbarzowski]

@sparkprime @lwander

[coveralls]

Coverage increased (+0.7%) to 83.754% when pulling aa3635e on sbarzowski:external-vars into 650825c on google:master.

[lwander]

Missing word(s) after It

[sbarzowski]

Thanks, fixed

[coveralls]

Coverage increased (+0.7%) to 83.754% when pulling e184785 on sbarzowski:external-vars into 650825c on google:master.

[sparkprime]

It should be a runtime error. Current behavior of C++ is broken. We need to document and test
each case (import, importstr, ext-code, tla-code).

It should be allowed to import from a non-existent file in a branch that never gets executed. That is the semantics of the language.

[sparkprime]

So yeah I think if you want to eagerly pre-parse the AST, you have to defer any syntax error until actual use. The C++ implementation re-parses it every time it's referred to I think?

[sbarzowski]

The C++ implementation re-parses it every time it's referred to I think?

No. I think Go and C++ implementation handle caching in pretty much the same way:

/** Import another Jsonnet file.
     *
     * If the file has already been imported, then use that version.  This maintains
     * referential transparency in the case of writes to disk during execution.  The
     * cache holds a thunk in order to cache the resulting value of execution.
     *
     * \param loc Location of the import statement.
     * \param file Path to the filename.
     */
    HeapThunk *import(const LocationRange &loc, const LiteralString *file)
    {
        ImportCacheValue *input = importString(loc, file);
        if (input->thunk == nullptr) {
            // EVALUATION CODE
        }
        return input->thunk;
    }

/** Import a file as a string.
     *
     * If the file has already been imported, then use that version.  This maintains
     * referential transparency in the case of writes to disk during execution.
     *
     * \param loc Location of the import statement.
     * \param file Path to the filename.
     * \param found_here If non-null, used to store the actual path of the file
     */
    ImportCacheValue *importString(const LocationRange &loc, const LiteralString *file)
    {
        std::string dir = dir_name(loc.file);

        const UString &path = file->value;

        std::pair<std::string, UString> key(dir, path);
        ImportCacheValue *cached_value = cachedImports[key];
        if (cached_value != nullptr)
            return cached_value;
        // [code that actually fetches it]
        cachedImports[key] = input_ptr;
        return input_ptr;
    }

[sbarzowski]

Updated TODO to reflect the decision.

[coveralls]

Coverage increased (+0.7%) to 83.913% when pulling 750b2d1 on sbarzowski:external-vars into 93bcf64 on google:master.