270 lines
12 KiB
C
270 lines
12 KiB
C
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#include <stdbool.h>
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#include <stdint.h>
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/**
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* @file result.h
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* Provides an alternative to exceptions, whose semantics are slightly better
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* than exceptions
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*
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* C does not support exceptions. Many C functions instead return a status code
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* reporting what happened. Alternatively, many C libraries allow a callback to
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* be logged in order to report a detailed description of any errors that
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* occurred. However, the programming language known as Rust deliberately left
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* out exceptions; instead they opted for return values to report errors, with
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* a `RESULT` type. This type contains the actual value if there is one, or
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* some error otherwise.
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*
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* This is the approach that this file defines.
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*/
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/**
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* @brief A macro which gives the value of a `RESULT` if present, else returns
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* the result.
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* @details Do not use this twice on the same line.
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*
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* Example Usage:
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* @code
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* RESULT(int) someResultFromFn = ...;
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*
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* int value;
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*
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* RESULT_M_TRY(int, value, someResultFromFn);
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* @endcode
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*
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* @param type The type of the `RESULT(type)`
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* @param dest The location to write the new result into
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* @param result The old result
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* @return nothing. (wrapped in `do-while`)
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*
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* @see RESULT_M_TRY_DECL
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*/
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#define RESULT_M_TRY(type, dest, result) \
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do \
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{ \
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RESULT (type) _result_##type##_try_at_##__LINE__ = result; \
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if (_result_##type##_try_at_##__LINE__.err) \
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{ \
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return _result_##type##_try_at_##__LINE__; \
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} \
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dest = _result_##type##_try_at_##__LINE__.value; \
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} \
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while (0)
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/**
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* @brief A macro which gives the value of a `RESULT` if present, else returns
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* the result.
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* @details Do not use this twice on the same line.
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*
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* Example Usage:
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* @code
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* RESULT(int) myFn(int someParam) {
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* ....
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* }
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*
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* RESULT(int) function() {
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* RESULT_M_TRY_DECL(int, val, myFn(1));
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* // equivalent to:
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* RESULT(int) r = myFn(1);
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* if (!RESULT_IS_OK(int)(r)) return r;
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* int val = RESULT_UNWRAP(r);
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* }
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* @endcode
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*
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* @param type The type of the `RESULT(type)`
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* @param dest The location to write the new result into
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* @param result The old result
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* @return nothing. (wrapped in `do-while`)
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*
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* @see RESULT_M_TRY
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*/
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#define RESULT_M_TRY_DECL(type, dest, result) \
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type dest; \
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RESULT_M_TRY (type, dest, result)
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/**
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* @brief Template for a RESULT class (similar to the one that the language
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* Rust has). This is the typename.
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* @details This macro expands to a unique name per type, so `RESULT(int)` is
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* roughly equivalent to the C++ `RESULT<int>`. Note that
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* `RESULT(int*)` will not compile; if you want to use something along
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* those lines, you must typedef it first.
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*
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* For the error strings, you must use string literals or something
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* with storage duration, so the pointer doesn't become invalidated.
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*
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* @param The template parameter
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* @return `Result_ type`, so `RESULT(int)` is `Result_int` Each macro of this
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* sort follows that convention, and you can rely on it
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*/
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#define RESULT(type) Result_##type
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// Class functions
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/**
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* @brief Creates a result which holds a value of `type`
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* @details Indicates that no error occurred, and that the computed value from
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* the function can be retrieved via `RESULT_UNWRAP`.
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*
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* @param type value
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* @return Returns a `RESULT(type)` containing a `type`
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*/
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#define RESULT_OK(type) Result_##type##_Ok
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/**
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* @brief Creates a result which indicates that an error occurred.
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* @details Indicates that an error occurred in the function. Similar to an
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* exception in other languages.
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*
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* @param const char * error message
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* @return Returns a `RESULT(type)` that does not contain a `type`
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*/
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#define RESULT_ERR(type) Result_##type##_Err
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// methods
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/**
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* @brief Tests if the `RESULT` actually contains a `type` (else it was an
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* error)
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*
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* @return `true` if calling `RESULT_UNWRAP` is safe, else `false`
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*/
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#define RESULT_IS_OK(type) Result_##type##_is_ok
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/**
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* @brief Tests if the `RESULT` doesn't contain a `type` (else it was ok)
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*
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* @return `true` if calling `RESULT_UNWRAP_ERR` is safe, else `false`
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*/
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#define RESULT_IS_ERR(type) Result_##type##_is_err
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/**
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* @brief Unwraps the `RESULT` into a `type`, panicking if impossible
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* @details If `RESULT_IS_OK`, this returns the `type` value of the `RESULT`.
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* Otherwise, this panics.
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*
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* @return `type` that is contained in the `RESULT`, or could `exit()`
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*
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* @see RESULT_EXPECT
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*/
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#define RESULT_UNWRAP(type) Result_##type##_unwrap
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/**
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* @brief Unwraps the `RESULT` into a `type`, panicking with the error message
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* if impossible
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* @details Similar to `RESULT_UNWRAP`, except this has a specialized error
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* message
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*
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* @param const char * the error message on failure
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* @return `type` contained in `RESULT`, else terminates the program
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*
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* @see RESULT_UNWRAP
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*/
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#define RESULT_EXPECT(type) Result_##type##_expect
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/**
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* @brief Unwraps the `RESULT` into a `type`, returning the supplied
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* alternative otherwise
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* @details If `RESULT_IS_OK`, this returns the `type` value of the `RESULT`.
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* Otherwise, this returns the alternative supplied in the function
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* arguments.
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*
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* @param type or_else The value returned if the unwrap failed
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* @return `type` contained in `RESULT`, or the alternative
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*
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* @see RESULT_UNWRAP
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* @see RESULT_EXPECT
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*/
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#define RESULT_UNWRAP_OR(type) Result_##type##_unwrap_or
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/**
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* @brief Unwraps the `RESULT` into an error string, panicking if impossible
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* @details If `RESULT_IS_ERR`, this returns error message of the `RESULT`.
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* Otherwise, this panics.
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*
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* @param const char * the error string
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* @return error string in the RESULT, or could terminate
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*
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* @see RESULT_EXPECT
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*/
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#define RESULT_UNWRAP_ERR(type) Result_##type##_unwrap_err
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/**
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* @brief Declares a RESULT type for use.
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* @details This declares the struct and functions for a given type. Note that
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* the functions still need to be defined. The values inside the
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* struct are implementation details, and may change at any time; do not use
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* them.
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*
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* @param template parameter
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*
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* @see RESULT_DEFINE
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*/
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#define RESULT_DECLARE(type) \
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typedef struct RESULT (type) \
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{ \
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type value; \
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/* string literals only */ \
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const char *err; \
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} RESULT (type); \
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RESULT (type) RESULT_OK (type) (type value); \
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RESULT (type) RESULT_ERR (type) (const char *err); \
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bool RESULT_IS_OK (type) (const RESULT (type) *); \
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bool RESULT_IS_ERR (type) (const RESULT (type) *); \
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type RESULT_UNWRAP (type) (const RESULT (type) *); \
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type RESULT_EXPECT (type) (const RESULT (type) *, const char *); \
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type RESULT_UNWRAP_OR (type) (const RESULT (type) *, type); \
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const char *RESULT_UNWRAP_ERR (type) (const RESULT (type) *);
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/**
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* @brief Defines all the functions for the given `RESULT(type)`
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* @details Defines each and every function necessary to use the `RESULT`.
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*
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* @param template parameter
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*
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* @see RESULT_DECLARE
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*/
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#define RESULT_DEFINE(type) \
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RESULT (type) RESULT_OK (type) (type value) \
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{ \
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RESULT (type) res = { .value = value, .err = NULL }; \
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return res; \
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} \
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RESULT (type) RESULT_ERR (type) (const char *err) \
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{ \
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RESULT (type) res = { .err = err }; \
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return res; \
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} \
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bool RESULT_IS_OK (type) (const RESULT (type) * res) \
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{ \
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return res->err == NULL; \
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} \
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bool RESULT_IS_ERR (type) (const RESULT (type) * res) \
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{ \
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return res->err != NULL; \
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} \
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type RESULT_UNWRAP (type) (const RESULT (type) * res) \
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{ \
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if (RESULT_IS_ERR (type) (res)) \
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panicf ("Attempted to unwrap empty Result of type " #type \
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". Instead had error: %s", \
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res->err); \
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return res->value; \
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} \
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type RESULT_EXPECT (type) (const RESULT (type) * res, \
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const char *message_on_err) \
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{ \
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if (RESULT_IS_ERR (type) (res)) \
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panic (message_on_err); \
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return res->value; \
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} \
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type RESULT_UNWRAP_OR (type) (const RESULT (type) * res, type else_val) \
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{ \
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if (RESULT_IS_ERR (type) (res)) \
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return else_val; \
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return res->value; \
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} \
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const char *RESULT_UNWRAP_ERR (type) (const RESULT (type) * res) \
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{ \
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if (RESULT_IS_OK (type) (res)) \
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panic ("Result was not an error; type: " #type); \
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return res->err; \
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}
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