/*
* Copyright (c) Meta Platforms, Inc. and affiliates.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#pragma once
#include <folly/CPortability.h>
#include <folly/ScopeGuard.h>
#include <folly/dynamic.h>
namespace folly {
/**
* DynamicParser provides a tiny DSL for easily, correctly, and losslessly
* parsing a folly::dynamic into any other representation.
*
* To make this concrete, this lets you take a JSON config that potentially
* contains user errors, and parse __all__ of its valid parts, while
* automatically and __reversibly__ recording any parts that cause errors:
*
* {"my values": {
* "an int": "THIS WILL BE RECORDED AS AN ERROR, BUT WE'LL PARSE THE REST",
* "a double": 3.1415,
* "keys & values": {
* "the sky is blue": true,
* "THIS WILL ALSO BE RECORDED AS AN ERROR": "cheese",
* "2+2=5": false,
* }
* }}
*
* To parse this JSON, you need no exception handling, it is as easy as:
*
* folly::dynamic d = ...; // Input
* int64_t integer; // Three outputs
* double real;
* std::map<std::string, bool> enabled_widgets;
* DynamicParser p(DynamicParser::OnError::RECORD, &d);
* p.required("my values", [&]() {
* p.optional("an int", [&](int64_t v) { integer = v; });
* p.required("a double", [&](double v) { real = v; });
* p.optional("keys & values", [&]() {
* p.objectItems([&](std::string widget, bool enabled) {
* enabled_widgets.emplace(widget, enabled);
* });
* });
* });
*
* Your code in the lambdas can throw, and this will be reported just like
* missing key and type conversion errors, with precise context on what part
* of the folly::dynamic caused the error. No need to throw:
* std::runtime_error("Value X at key Y caused a flux capacitor overload")
* This will do:
* std::runtime_error("Flux capacitor overload")
*
* == Keys and values are auto-converted to match your callback ==
*
* DynamicParser's optional(), required(), objectItems(), and
* arrayItems() automatically convert the current key and value to match the
* signature of the provided callback. parser.key() and parser.value() can
* be used to access the same data without conversion.
*
* The following types are supported -- you should generally take arguments
* by-value, or by-const-reference for dynamics & strings you do not copy.
*
* Key: folly::dynamic (no conversion), std::string, int64_t
* Value: folly::dynamic (no conversion), int64_t, bool, double, std::string
*
* There are 21 supported callback signatures, of three kinds:
*
* 1: No arguments -- useful if you will just call more parser methods.
*
* 5: The value alone -- the common case for optional() and required().
* [&](whatever_t value) {}
*
* 15: Both the key and the value converted according to the rules above:
* [&](whatever_t key, whatever_t) {}
*
* NB: The key alone should be rarely needed, but these callback styles
* provide it with no conversion overhead, and only minimal verbosity:
* [&](const std::string& k, const folly::dynamic&) {}
* [&]() { auto k = p.key().asString(); }
*
* == How `releaseErrors()` can make your parse lossless ==
*
* If you write parsing code by hand, you usually end up with error-handling
* resembling that of OnError::THROW -- the first error you hit aborts the
* whole parse, and you report it.
*
* OnError::RECORD offers a more user-friendly alternative for "parse,
* serialize, re-parse" pipelines, akin to what web-forms do. All
* exception-causing parts are losslessly recorded in a parallel
* folly::dynamic, available via releaseErrors() at the end of the parse.
*
* Suppose we fail to look up "key1" at the root, and hit a value error in
* "key2": {"subkey2": ...}. The error report will have the form:
*
* {"nested": {
* "key_errors": {"key1": "explanatory message"},
* "value": <whole input>,
* "nested": { "key2": { "nested": {
* "subkey2": {"value": <original value>, "error": "message"}
* } } }
* }}
*
* Errors in array items are handled just the same, but using integer keys.
*
* The advantage of this approach is that your parsing can throw wherever,
* and DynamicParser isolates it, allowing the good parts to parse.
*
* Put another way, this makes it easy to implement a transformation that
* splits a `folly::dynamic` into a "parsed" part (which might be your
* struct meant for runtime use), and a matching "errors" part. As long as
* your successful parses are lossless, you can always reconstruct the
* original input from the parse output and the recorded "errors".
*
* == Limitations ==
*
* - The input dynamic should be an object or array. wrapError() could be
* exposed to allow parsing single scalars, but this would not be a
* significant usability improvement over try-catch.
*
* - Do NOT try to parse the same part of the input dynamic twice. You
* might report multiple value errors, which is currently unsupported.
*
* - optional() does not support defaulting. This is unavoidable, since
* DynamicParser does not dictate how you record parsed data. If your
* parse writes into an output struct, then it ought to be initialized at
* construction time. If your output is initialized to default values,
* then you need no "default" feature. If it is not initialized, you are
* in trouble anyway. Suppose your optional() parse hits an error. What
* does your output contain?
* - Uninitialized data :(
* - You rely on an optional() feature to fall back to parsing some
* default dynamic. Sadly, the default hits a parse error. Now what?
* Since there is no good way to default, DynamicParser leaves it out.
*
* == Future: un-parsed items ==
*
* DynamicParser could support erroring on un-parsed items -- the parts of
* the folly::dynamic, which were never asked for. Here is an ok design:
*
* (i) At the start of parsing any value, the user may call:
* parser.recursivelyForbidUnparsed();
* parser.recursivelyAllowUnparsed();
* parser.locallyForbidUnparsed();
* parser.locallyAllowUnparsed();
*
* (ii) At the end of the parse, any unparsed items are dumped to "errors".
* For example, failing to parse index 1 out of ["v1", "v2", "v3"] yields:
* "nested": {1: {"unparsed": "v2"}}
* or perhaps more verbosely:
* "nested": {1: {"error": "unparsed value", "value": "v2"}}
*
* By default, unparsed items are allowed. Calling a "forbid" function after
* some keys have already been parsed is allowed to fail (this permits a
* lazy implementation, which has minimal overhead when "forbid" is not
* requested).
*
* == Future: multiple value errors ==
*
* The present contract is that exactly one value error is reported per
* location in the input (multiple key lookup errors are, of course,
* supported). If the need arises, multiple value errors could easily be
* supported by replacing the "error" string with an "errors" array.
*/
namespace detail {
// Why do DynamicParser error messages use folly::dynamic pseudo-JSON?
// Firstly, the input dynamic need not correspond to valid JSON. Secondly,
// wrapError() uses integer-keyed objects to report arrary-indexing errors.
std::string toPseudoJson(const folly::dynamic& d);
} // namespace detail
/**
* With DynamicParser::OnError::THROW, reports the first error.
* It is forbidden to call releaseErrors() if you catch this.
*/
struct FOLLY_EXPORT DynamicParserParseError : public std::runtime_error {
explicit DynamicParserParseError(folly::dynamic error)
: std::runtime_error(folly::to<std::string>(
"DynamicParserParseError: ", detail::toPseudoJson(error))),
error_(std::move(error)) {}
/**
* Structured just like releaseErrors(), but with only 1 error inside:
* {"nested": {"key1": {"nested": {"key2": {"error": "err", "value": 5}}}}}
* or:
* {"nested": {"key1": {"key_errors": {"key3": "err"}, "value": 7}}}
*/
const folly::dynamic& error() const { return error_; }
private:
folly::dynamic error_;
};
/**
* When DynamicParser is used incorrectly, it will throw this exception
* instead of reporting an error via releaseErrors(). It is unsafe to call
* any parser methods after catching a LogicError.
*/
struct FOLLY_EXPORT DynamicParserLogicError : public std::logic_error {
template <typename... Args>
explicit DynamicParserLogicError(Args&&... args)
: std::logic_error(folly::to<std::string>(std::forward<Args>(args)...)) {}
};
class DynamicParser {
public:
enum class OnError {
// After parsing, releaseErrors() reports all parse errors.
// Throws DynamicParserLogicError on programmer errors.
RECORD,
// Throws DynamicParserParseError on the first parse error, or
// DynamicParserLogicError on programmer errors.
THROW,
};
// You MUST NOT destroy `d` before the parser.
DynamicParser(OnError on_error, const folly::dynamic* d)
: onError_(on_error), stack_(d) {} // Always access input through stack_
/**
* Once you finished the entire parse, returns a structured description of
* all parse errors (see top-of-file docblock). May ONLY be called once.
* May NOT be called if the parse threw any kind of exception. Returns an
* empty object for successful OnError::THROW parsers.
*/
folly::dynamic releaseErrors() { return stack_.releaseErrors(); }
/**
* Error-wraps fn(auto-converted key & value) if d[key] is set. The
* top-of-file docblock explains the auto-conversion.
*/
template <typename Fn>
void optional(const folly::dynamic& key, Fn);
// Like optional(), but reports an error if d[key] does not exist.
template <typename Fn>
void required(const folly::dynamic& key, Fn);
/**
* Iterate over the current object's keys and values. Report each item's
* errors under its own key in a matching sub-object of "errors".
*/
template <typename Fn>
void objectItems(Fn);
/**
* Like objectItems() -- arrays are treated identically to objects with
* integer keys from 0 to size() - 1.
*/
template <typename Fn>
void arrayItems(Fn);
/**
* The key currently being parsed (integer if inside an array). Throws if
* called outside of a parser callback.
*/
inline const folly::dynamic& key() const { return stack_.key(); }
/**
* The value currently being parsed (initially, the input dynamic).
* Throws if parsing nullptr, or parsing after releaseErrors().
*/
inline const folly::dynamic& value() const { return stack_.value(); }
/**
* By default, DynamicParser's "nested" object coerces all keys to
* strings, whether from arrayItems() or from p.optional(some_int, ...),
* to allow errors be serialized to JSON. If you are parsing non-JSON
* dynamic objects with non-string keys, this is problematic. When set to
* true, "nested" objects will report integer keys for errors coming from
* inside arrays, or the original key type from inside values of objects.
*/
DynamicParser& setAllowNonStringKeyErrors(bool b) {
allowNonStringKeyErrors_ = b;
return *this;
}
private:
/**
* If `fn` throws an exception, wrapError() catches it and inserts an
* enriched description into stack_.errors_. If lookup_key is non-null,
* reports a key lookup error in "key_errors", otherwise reportse a value
* error in "error".
*
* Not public because that would encourage users to report multiple errors
* per input part, which is currently unsupported. It does not currently
* seem like normal user code should need this.
*/
template <typename Fn>
void wrapError(const folly::dynamic* lookup_key, Fn);
void reportError(const folly::dynamic* lookup_k, const std::exception& ex);
template <typename Fn>
void parse(const folly::dynamic& key, const folly::dynamic& value, Fn fn);
// All of the above business logic obtains the part of the folly::dynamic
// it is examining (and the location for reporting errors) via this class,
// which lets it correctly handle nesting.
struct ParserStack {
struct Pop {
explicit Pop(ParserStack* sp)
: key_(sp->key_), value_(sp->value_), stackPtr_(sp) {}
void operator()() noexcept; // ScopeGuard requires noexcept
private:
const folly::dynamic* key_;
const folly::dynamic* value_;
ParserStack* stackPtr_;
};
struct PopGuard {
explicit PopGuard(ParserStack* sp) : pop_(in_place, sp) {}
~PopGuard() { pop_ && ((*pop_)(), true); }
private:
Optional<Pop> pop_;
};
explicit ParserStack(const folly::dynamic* input)
: value_(input),
errors_(folly::dynamic::object()),
subErrors_({&errors_}) {}
// Not copiable or movable due to numerous internal pointers
ParserStack(const ParserStack&) = delete;
ParserStack& operator=(const ParserStack&) = delete;
ParserStack(ParserStack&&) = delete;
ParserStack& operator=(ParserStack&&) = delete;
// Lets user code nest parser calls by recording current key+value and
// returning an RAII guard to restore the old one. `noexcept` since it
// is used unwrapped.
PopGuard push(const folly::dynamic& k, const folly::dynamic& v) noexcept;
// Throws DynamicParserLogicError if used outside of a parsing function.
inline const folly::dynamic& key() const;
// Throws DynamicParserLogicError if used after releaseErrors().
inline const folly::dynamic& value() const;
// Lazily creates new "nested" sub-objects in errors_.
folly::dynamic& errors(bool allow_non_string_keys) noexcept;
// The user invokes this at most once after the parse is done.
folly::dynamic releaseErrors();
// Invoked on error when using OnError::THROW.
[[noreturn]] void throwErrors();
private:
friend struct Pop;
folly::dynamic releaseErrorsImpl(); // for releaseErrors() & throwErrors()
// Null outside of a parsing function.
const folly::dynamic* key_{nullptr};
// Null on errors: when the input was nullptr, or after releaseErrors().
const folly::dynamic* value_;
// An object containing some of these keys:
// "key_errors" -- {"key": "description of error looking up said key"}
// "error" -- why did we fail to parse this value?
// "value" -- a copy of the input causing the error, and
// "nested" -- {"key" or integer for arrays: <another errors_ object>}
//
// "nested" will contain identically structured objects with keys (array
// indices) identifying the origin of the errors. Of course, "input"
// would no longer refer to the whole input, but to a part.
folly::dynamic errors_;
// We only materialize errors_ sub-objects when needed. This stores keys
// for unmaterialized errors, from outermost to innermost.
std::vector<const folly::dynamic*> unmaterializedSubErrorKeys_;
// Materialized errors, from outermost to innermost
std::vector<folly::dynamic*> subErrors_; // Point into errors_
};
OnError onError_;
ParserStack stack_;
bool allowNonStringKeyErrors_{false}; // See the setter's docblock.
};
} // namespace folly
#include <folly/experimental/DynamicParser-inl.h>