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Ethereum: Parsing JSON with jq and vm.ffi/FFI
When working with the data returned by the cast call, JSON parsing can be an important step in extracting relevant information. However, when using the command-line tool jq or the FFI (Foreign Function Interface) module vm.ffi on Ethereum, you may encounter problems parsing JSON without any obvious errors.
In this article, we will consider why these methods do not work and offer an alternative approach to achieve the desired result.
Why isn’t this working?
The main problem is how jq and vm.ffi/FFI interpret JSON data. By default, both tools expect a well-formed JSON string or object as input. If your cast call returns invalid JSON, these tools will throw an error.
To get around this limitation, we can use the jsonschema library to validate the incoming JSON data before asking it to be parsed by jq. This approach ensures that our data conforms to the expected format before requesting the extraction of certain fields.
Solution: use jsonschema and vm.parseJson
We will use the jsonschema' library, which provides a reliable way to validate JSON data against a predefined schema. In addition, we will usevm.parseJson, which allows us to parse JSON strings directly into C++ objects.
Here is an example implementation:
#include
#include
#include // Assume jsonschema is installed and linked
// Define our JSON object
struct Data {
std::string id;
std::vector status;
};
int main() {
// Sampling data from the cast call
Json::Value jsonData = Json::arrayValue;
jsonData["id"].asString("example-id");
for (size_t i = 0; i < jsonData.arraySize(); ++i) {
Json::Value field(jsonData[i]);
if (field.isObject()) {
Json::Value status(field);
if (status.isArray()) {
status[0].asString("example-status");
}
}
}
// Validate the JSON data against the schema
JsonSchema schema;
schema.loadFromJsonValue(jsonData);
std::string expectedStatus = "example-status";
if (!schema.validate(expectedStatus)) {
std::cerr << "Invalid JSON: 'status' was expected to be an array with one element." << std::endl;
return 1; // Return a non-zero exit code to indicate an error
}
// Now we can safely parse JSON data using vm.parseJson
try {
Data parsedData;
vm.parseJson(jsonData.toStyledString(), parsedData);
std::cout << "Parsed data: id = " << parsedData.id << ", status = [" << parsedData.status[0] << "]" << std::endl;
} catch (const std::exception& e) {
std::cerr << "Error parsing JSON: " << e.what() << std::endl;
return 1; // Return a non-zero exit code to indicate an error
}
return 0;
}
In this example, we define our Datastructure withidandstatusfields. We then instantiate the JSON object from the cast call, check it against the schema usingjsonschema, and safely parse the JSON data usingvm.parseJson.
Note that in a real-world scenario, you should ensure that your JSON data conforms to the expected format by checking for validation errors or by using additional checks such as JSON syntax rules.
Conclusion
Althoughjqandvm.ffi/FFI can handle some JSON input, they may not be suitable for all cases. Using jsonschema and vm.parseJson, you can create a reliable solution for parsing JSON data from cast calls on Ethereum.
Remember that you should always validate JSON data before requesting parsing to ensure that the expected structure is maintained throughout the parsing process.
Do not hesitate to ask if you have any questions or need additional clarifications!
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