I wrote my "Result" macro header, using C23 and some of the newest features:
#pragma once
#include <stdio.h>
#include <stdlib.h>
#define __RESULT_EAT_PARENS(...) __VA_ARGS__
typedef enum ResultKind
{
OK,
ERR
} ResultKind;
#define DEFINE_RESULT(ok_type, err_type) \
typedef struct Result_##ok_type##_##err_type \
{ \
ResultKind kind; \
union \
{ \
ok_type ok; \
err_type err; \
} value; \
} Result_##ok_type##_##err_type;
#define __RESULT_OK_PASTE(ok_type, err_type, ...) \
(Result_##ok_type##_##err_type) \
{ \
.kind = OK, .value = {.ok = __VA_ARGS__ } \
}
#define __RESULT_OK_IMPL(ok_type, err_type, ...) \
__RESULT_OK_PASTE(ok_type, err_type, __VA_ARGS__)
#define __RESULT_ERR_PASTE(ok_type, err_type, ...) \
(Result_##ok_type##_##err_type) \
{ \
.kind = ERR, .value = {.err = __VA_ARGS__ } \
}
#define __RESULT_ERR_IMPL(ok_type, err_type, ...) \
__RESULT_ERR_PASTE(ok_type, err_type, __VA_ARGS__)
#define Ok(types, ...) \
__RESULT_OK_IMPL(__RESULT_EAT_PARENS types, (__VA_ARGS__))
#define Err(types, ...) \
__RESULT_ERR_IMPL(__RESULT_EAT_PARENS types, (__VA_ARGS__))
#define is_ok(res) \
((res).kind == OK)
#define is_err(res) \
((res).kind == ERR)
#define expect(res, msg) \
({ \
__auto_type __res_tmp = (res); \
(is_err(__res_tmp)) \
? (fprintf(stderr, "Panic at %s:%d: %s\n", __FILE__, __LINE__, (msg)), exit(1), __res_tmp.value.ok) \
: __res_tmp.value.ok; \
})
#define expect_err(res, msg) \
({ \
__auto_type __res_tmp = (res); \
(is_ok(__res_tmp)) \
? (fprintf(stderr, "Panic at %s:%d: %s\n", __FILE__, __LINE__, (msg)), exit(1), __res_tmp.value.err) \
: __res_tmp.value.err; \
})
#define unwrap_or(res, default_val) \
({ \
__auto_type __res_tmp = (res); \
is_ok(__res_tmp) ? __res_tmp.value.ok : (default_val); \
})
#define unwrap_or_else(res, func) \
({ \
__auto_type __res_tmp = (res); \
is_ok(__res_tmp) ? __res_tmp.value.ok : (func)(); \
})
#define __RESULT_MAP_PASTE(out_ok_type, err_type, res_in, var, ...) \
({ \
__auto_type __res_tmp = (res_in); \
(is_err(__res_tmp)) \
? __RESULT_ERR_IMPL(out_ok_type, err_type, __res_tmp.value.err) \
: ({ \
typeof(__res_tmp.value.ok) var = __res_tmp.value.ok; \
__RESULT_OK_IMPL(out_ok_type, err_type, __VA_ARGS__); \
}); \
})
#define __RESULT_MAP_IMPL(out_ok_type, err_type, res_in, var, ...) \
__RESULT_MAP_PASTE(out_ok_type, err_type, res_in, var, __VA_ARGS__)
#define map(types, res_in, var, ...) \
__RESULT_MAP_IMPL(__RESULT_EAT_PARENS types, res_in, var, (__VA_ARGS__))
#define __RESULT_MAP_ERR_PASTE(ok_type, out_err_type, res_in, var, ...) \
({ \
__auto_type __res_tmp = (res_in); \
(is_ok(__res_tmp)) \
? __RESULT_OK_IMPL(ok_type, out_err_type, __res_tmp.value.ok) \
: ({ \
typeof(__res_tmp.value.err) var = __res_tmp.value.err; \
__RESULT_ERR_IMPL(ok_type, out_err_type, __VA_ARGS__); \
}); \
})
#define __RESULT_MAP_ERR_IMPL(ok_type, out_err_type, res_in, var, ...) \
__RESULT_MAP_ERR_PASTE(ok_type, out_err_type, res_in, var, __VA_ARGS__)
#define map_err(types, res_in, var, ...) \
__RESULT_MAP_ERR_IMPL(__RESULT_EAT_PARENS types, res_in, var, (__VA_ARGS__))
#define __RESULT_AND_THEN_PASTE(out_ok_type, err_type, res_in, var, ...) \
({ \
__auto_type __res_tmp = (res_in); \
(is_err(__res_tmp)) \
? __RESULT_ERR_IMPL(out_ok_type, err_type, __res_tmp.value.err) \
: ({ \
typeof(__res_tmp.value.ok) var = __res_tmp.value.ok; \
__VA_ARGS__ \
}); \
})
#define __RESULT_AND_THEN_IMPL(out_ok_type, err_type, res_in, var, ...) \
__RESULT_AND_THEN_PASTE(out_ok_type, err_type, res_in, var, __VA_ARGS__)
#define and_then(types, res_in, var, ...) \
__RESULT_AND_THEN_IMPL(__RESULT_EAT_PARENS types, res_in, var, (__VA_ARGS__))
Everything goes go well so for. But then I add:
#define __RESULT_AND_THEN_PASTE(out_ok_type, err_type, res_in, var, ...) \
({ \
__auto_type __res_tmp = (res_in); \
(is_err(__res_tmp)) \
? __RESULT_ERR_IMPL(out_ok_type, err_type, __res_tmp.value.err) \
: ({ \
typeof(__res_tmp.value.ok) var = __res_tmp.value.ok; \
__VA_ARGS__ \
}); \
})
#define __RESULT_AND_THEN_IMPL(out_ok_type, err_type, res_in, var, ...) \
__RESULT_AND_THEN_PASTE(out_ok_type, err_type, res_in, var, __VA_ARGS__)
#define and_then(types, res_in, var, ...) \
__RESULT_AND_THEN_IMPL(__RESULT_EAT_PARENS types, res_in, var, (__VA_ARGS__))
I wrote a test to show the problem:
#include "result.h" // The header file with the macro definitions
#include <stdio.h>
// --- Type Definitions ---
typedef const char *CString;
// --- 1. Define the required Result type ---
// Result<int, CString>
DEFINE_RESULT(int, CString);
// --- Helper Functions for and_then ---
/**
* @brief A function for the and_then test (Ok -> Ok or Ok -> Err)
* Simulates an operation that might fail.
*/
Result_int_CString sophisticated_op(int i)
{
printf(" -> (Called sophisticated_op(%d))\n", i);
if (i > 10)
{
// Return a new Ok
return Ok((int, CString), i * 2);
}
else
{
// Return an Err
return Err((int, CString), "value too small");
}
}
/**
* @brief A function for the and_then test (Ok -> Err)
*/
Result_int_CString sophisticated_err_op(int i)
{
printf(" -> (Called sophisticated_err_op(%d))\n", i);
// Always returns Err
return Err((int, CString), "sophisticated op failed");
}
// --- Main Test Function ---
int main()
{
// --- Setup: Basic values ---
// These macros (Ok, Err) work correctly.
Result_int_CString res_ok = Ok((int, CString), 42);
Result_int_CString res_err = Err((int, CString), "file not found");
printf("\n--- 4. Chaining (and_then) ---\n");
printf("Compilation is expected to fail on the following 3 lines:\n");
// 4a. Ok(42) |> and_then(sophisticated_op) -> Ok(84)
// COMPILATION FAILS HERE
Result_int_CString chain1 = and_then(
(int, CString), // Target type (U, E)
res_ok, // Input Ok(42)
val, // Bind variable
sophisticated_op(val) // Expression -> Ok(84)
);
// 4b. Ok(42) |> and_then(sophisticated_err_op) -> Err("...")
// COMPILATION FAILS HERE
Result_int_CString chain2 = and_then(
(int, CString), // Target type (U, E)
res_ok, // Input Ok(42)
val,
sophisticated_err_op(val) // Expression -> Err(...)
);
// 4c. Err(...) |> and_then(sophisticated_op) -> Err(...) (short-circuit)
// COMPILATION FAILS HERE
Result_int_CString chain3 = and_then(
(int, CString), // Target type (U, E)
res_err, // Input Err("file not found")
val, // Bind variable
sophisticated_op(val) // Expression - should not execute
);
printf("...If compilation somehow succeeded, printing results:\n");
// These lines just print the results.
// They require is_ok, unwrap_or, and expect_err from result.h.
if (is_ok(chain1))
{
printf("and_then(Ok(42), op_ok): value = %d\n", unwrap_or(chain1, -1));
}
if (is_err(chain2))
{
printf("and_then(Ok(42), op_err): error = \"%s\"\n", expect_err(chain2, "should be err"));
}
if (is_err(chain3))
{
printf("and_then(Err, op_ok): error = \"%s\"\n", expect_err(chain3, "should be err"));
}
printf("All tests finished.\n");
return 0;
}
I compiled it using clang and got this error:
karesis@Celestina:~/Projects/cprint$ clang -Wall -Wextra -std=c23 src/test_result.c -o test
src/test_result.c:141:31: error: expected ';' after expression
141 | Result_int_CString chain1 = and_then(
| ^
src/result.h:134:5: note: expanded from macro 'and_then'
134 | __RESULT_AND_THEN_IMPL(__RESULT_EAT_PARENS types, res_in, var, (__VA_ARGS__))
| ^
src/result.h:131:63: note: expanded from macro '__RESULT_AND_THEN_IMPL'
131 | __RESULT_AND_THEN_PASTE(out_ok_type, err_type, res_in, var, __VA_ARGS__)
| ^
src/test_result.c:150:31: error: expected ';' after expression
150 | Result_int_CString chain2 = and_then(
| ^
src/result.h:134:5: note: expanded from macro 'and_then'
134 | __RESULT_AND_THEN_IMPL(__RESULT_EAT_PARENS types, res_in, var, (__VA_ARGS__))
| ^
src/result.h:131:63: note: expanded from macro '__RESULT_AND_THEN_IMPL'
131 | __RESULT_AND_THEN_PASTE(out_ok_type, err_type, res_in, var, __VA_ARGS__)
| ^
src/test_result.c:159:31: error: expected ';' after expression
159 | Result_int_CString chain3 = and_then(
| ^
src/result.h:134:5: note: expanded from macro 'and_then'
134 | __RESULT_AND_THEN_IMPL(__RESULT_EAT_PARENS types, res_in, var, (__VA_ARGS__))
| ^
src/result.h:131:63: note: expanded from macro '__RESULT_AND_THEN_IMPL'
131 | __RESULT_AND_THEN_PASTE(out_ok_type, err_type, res_in, var, __VA_ARGS__)
| ^
3 errors generated.
karesis@Celestina:~/Projects/cprint$
I tried running just the preprocessor (clang -Wall -Wextra -std=c23 -E test_result.c > expanded.txt) and saw the "ghost ,":
...
Result_int_CString chain1 = ({ __auto_type __res_tmp = (res_ok); (((__res_tmp).kind == ERR)) ? (Result_int_CString) { .kind = ERR, .value = {.err = __res_tmp.value.err } } : ({ typeof(__res_tmp.value.ok) val = __res_tmp.value.ok; (sophisticated_op(val)), }); });
# 64 "test_mre.c"
Result_int_CString chain2 = ({ __auto_type __res_tmp = (res_ok); (((__res_tmp).kind == ERR)) ? (Result_int_CString) { .kind = ERR, .value = {.err = __res_tmp.value.err } } : ({ typeof(__res_tmp.value.ok) val = __res_tmp.value.ok; (sophisticated_err_op(val)), }); });
# 73 "test_mre.c"
Result_int_CString chain3 = ({ __auto_type __res_tmp = (res_err); (((__res_tmp).kind == ERR)) ? (Result_int_CString) { .kind = ERR, .value = {.err = __res_tmp.value.err } } : ({ typeof(__res_tmp.value.ok) val = __res_tmp.value.ok; (sophisticated_op(val)), }); });
...
It's like (using chain1 as example):
Result_int_CString chain1 =
({
__auto_type __res_tmp = (res_ok);
(
((__res_tmp).kind == ERR)
)
? (Result_int_CString){
.kind = ERR,
.value = {.err = __res_tmp.value.err}
}
: ({
typeof(__res_tmp.value.ok) val = __res_tmp.value.ok;
(sophisticated_op(val)), /* <--- Problematic Comma Here */
});
});
I have no idea why there's a comma there. It is supposed to be a ; so I try to add a ;:
#define __RESULT_AND_THEN_PASTE(out_ok_type, err_type, res_in, var, ...) \
({ \
__auto_type __res_tmp = (res_in); \
(is_err(__res_tmp)) \
? __RESULT_ERR_IMPL(out_ok_type, err_type, __res_tmp.value.err) \
: ({ \
typeof(__res_tmp.value.ok) var = __res_tmp.value.ok; \
__VA_ARGS__; /* <--- I add a `;` */ \
}); \
})
But another bug occurred:
karesis@Celestina:~/Projects/cprint$ clang -Wall -Wextra -std=c23 test_mre.c -o test
test_mre.c:55:31: error: expected expression
55 | Result_int_CString chain1 = and_then(
| ^
./result.h:132:5: note: expanded from macro 'and_then'
132 | __RESULT_AND_THEN_IMPL(__RESULT_EAT_PARENS types, res_in, var, (__VA_ARGS__))
| ^
./result.h:129:5: note: expanded from macro '__RESULT_AND_THEN_IMPL'
129 | __RESULT_AND_THEN_PASTE(out_ok_type, err_type, res_in, var, __VA_ARGS__)
| ^
./result.h:124:30: note: expanded from macro '__RESULT_AND_THEN_PASTE'
124 | __VA_ARGS__; /* <--- I add a `;` */ \
| ^
test_mre.c:64:31: error: expected expression
64 | Result_int_CString chain2 = and_then(
| ^
./result.h:132:5: note: expanded from macro 'and_then'
132 | __RESULT_AND_THEN_IMPL(__RESULT_EAT_PARENS types, res_in, var, (__VA_ARGS__))
| ^
./result.h:129:5: note: expanded from macro '__RESULT_AND_THEN_IMPL'
129 | __RESULT_AND_THEN_PASTE(out_ok_type, err_type, res_in, var, __VA_ARGS__)
| ^
./result.h:124:30: note: expanded from macro '__RESULT_AND_THEN_PASTE'
124 | __VA_ARGS__; /* <--- I add a `;` */ \
| ^
test_mre.c:73:31: error: expected expression
73 | Result_int_CString chain3 = and_then(
| ^
./result.h:132:5: note: expanded from macro 'and_then'
132 | __RESULT_AND_THEN_IMPL(__RESULT_EAT_PARENS types, res_in, var, (__VA_ARGS__))
| ^
./result.h:129:5: note: expanded from macro '__RESULT_AND_THEN_IMPL'
129 | __RESULT_AND_THEN_PASTE(out_ok_type, err_type, res_in, var, __VA_ARGS__)
| ^
./result.h:124:30: note: expanded from macro '__RESULT_AND_THEN_PASTE'
124 | __VA_ARGS__; /* <--- I add a `;` */ \
| ^
3 errors generated.
karesis@Celestina:~/Projects/cprint$
It seems that it actually needs a expression there. clang -E shows:
...
Result_int_CString chain1 = ({ __auto_type __res_tmp = (res_ok); (((__res_tmp).kind == ERR)) ? (Result_int_CString) { .kind = ERR, .value = {.err = __res_tmp.value.err } } : ({ typeof(__res_tmp.value.ok) val = __res_tmp.value.ok; (sophisticated_op(val)),; }); });
# 64 "test_mre.c"
Result_int_CString chain2 = ({ __auto_type __res_tmp = (res_ok); (((__res_tmp).kind == ERR)) ? (Result_int_CString) { .kind = ERR, .value = {.err = __res_tmp.value.err } } : ({ typeof(__res_tmp.value.ok) val = __res_tmp.value.ok; (sophisticated_err_op(val)),; }); });
# 73 "test_mre.c"
Result_int_CString chain3 = ({ __auto_type __res_tmp = (res_err); (((__res_tmp).kind == ERR)) ? (Result_int_CString) { .kind = ERR, .value = {.err = __res_tmp.value.err } } : ({ typeof(__res_tmp.value.ok) val = __res_tmp.value.ok; (sophisticated_op(val)),; }); });
...
I checked my code again and again, but I found nothing wrong:
#pragma once
#include <stdio.h>
#include <stdlib.h>
#define __RESULT_EAT_PARENS(...) __VA_ARGS__
typedef enum ResultKind
{
OK,
ERR
} ResultKind;
#define DEFINE_RESULT(ok_type, err_type) \
typedef struct Result_##ok_type##_##err_type \
{ \
ResultKind kind; \
union \
{ \
ok_type ok; \
err_type err; \
} value; \
} Result_##ok_type##_##err_type;
#define __RESULT_OK_PASTE(ok_type, err_type, ...) \
(Result_##ok_type##_##err_type) \
{ \
.kind = OK, .value = {.ok = __VA_ARGS__ } \
}
#define __RESULT_OK_IMPL(ok_type, err_type, ...) \
__RESULT_OK_PASTE(ok_type, err_type, __VA_ARGS__)
#define __RESULT_ERR_PASTE(ok_type, err_type, ...) \
(Result_##ok_type##_##err_type) \
{ \
.kind = ERR, .value = {.err = __VA_ARGS__ } \
}
#define __RESULT_ERR_IMPL(ok_type, err_type, ...) \
__RESULT_ERR_PASTE(ok_type, err_type, __VA_ARGS__)
#define Ok(types, ...) \
__RESULT_OK_IMPL(__RESULT_EAT_PARENS types, (__VA_ARGS__))
#define Err(types, ...) \
__RESULT_ERR_IMPL(__RESULT_EAT_PARENS types, (__VA_ARGS__))
#define is_ok(res) \
((res).kind == OK)
#define is_err(res) \
((res).kind == ERR)
#define expect(res, msg) \
({ \
__auto_type __res_tmp = (res); \
(is_err(__res_tmp)) \
? (fprintf(stderr, "Panic at %s:%d: %s\n", __FILE__, __LINE__, (msg)), exit(1), __res_tmp.value.ok) \
: __res_tmp.value.ok; \
})
#define expect_err(res, msg) \
({ \
__auto_type __res_tmp = (res); \
(is_ok(__res_tmp)) \
? (fprintf(stderr, "Panic at %s:%d: %s\n", __FILE__, __LINE__, (msg)), exit(1), __res_tmp.value.err) \
: __res_tmp.value.err; \
})
#define unwrap_or(res, default_val) \
({ \
__auto_type __res_tmp = (res); \
is_ok(__res_tmp) ? __res_tmp.value.ok : (default_val); \
})
#define unwrap_or_else(res, func) \
({ \
__auto_type __res_tmp = (res); \
is_ok(__res_tmp) ? __res_tmp.value.ok : (func)(); \
})
#define __RESULT_MAP_PASTE(out_ok_type, err_type, res_in, var, ...) \
({ \
__auto_type __res_tmp = (res_in); \
(is_err(__res_tmp)) \
? __RESULT_ERR_IMPL(out_ok_type, err_type, __res_tmp.value.err) \
: ({ \
typeof(__res_tmp.value.ok) var = __res_tmp.value.ok; \
__RESULT_OK_IMPL(out_ok_type, err_type, __VA_ARGS__); \
}); \
})
#define __RESULT_MAP_IMPL(out_ok_type, err_type, res_in, var, ...) \
__RESULT_MAP_PASTE(out_ok_type, err_type, res_in, var, __VA_ARGS__)
#define map(types, res_in, var, ...) \
__RESULT_MAP_IMPL(__RESULT_EAT_PARENS types, res_in, var, (__VA_ARGS__))
#define __RESULT_MAP_ERR_PASTE(ok_type, out_err_type, res_in, var, ...) \
({ \
__auto_type __res_tmp = (res_in); \
(is_ok(__res_tmp)) \
? __RESULT_OK_IMPL(ok_type, out_err_type, __res_tmp.value.ok) \
: ({ \
typeof(__res_tmp.value.err) var = __res_tmp.value.err; \
__RESULT_ERR_IMPL(ok_type, out_err_type, __VA_ARGS__); \
}); \
})
#define __RESULT_MAP_ERR_IMPL(ok_type, out_err_type, res_in, var, ...) \
__RESULT_MAP_ERR_PASTE(ok_type, out_err_type, res_in, var, __VA_ARGS__)
#define map_err(types, res_in, var, ...) \
__RESULT_MAP_ERR_IMPL(__RESULT_EAT_PARENS types, res_in, var, (__VA_ARGS__))
#define __RESULT_AND_THEN_PASTE(out_ok_type, err_type, res_in, var, ...) \
({ \
__auto_type __res_tmp = (res_in); \
(is_err(__res_tmp)) \
? __RESULT_ERR_IMPL(out_ok_type, err_type, __res_tmp.value.err) \
: ({ \
typeof(__res_tmp.value.ok) var = __res_tmp.value.ok; \
__VA_ARGS__; /* <--- I add a `;` */ \
}); \
})
#define __RESULT_AND_THEN_IMPL(out_ok_type, err_type, res_in, var, ...) \
__RESULT_AND_THEN_PASTE(out_ok_type, err_type, res_in, var, __VA_ARGS__)
#define and_then(types, res_in, var, ...) \
__RESULT_AND_THEN_IMPL(__RESULT_EAT_PARENS types, res_in, var, (__VA_ARGS__))karesis@Celestina:~/Projects/cprint$
Is this a weirdfeature of clang? (add , after a VAR__ARGS) or have I made a mistake? I would be extremely grateful for any help or insights!
MRE:
#include <stdio.h>
#define __RESULT_EAT_PARENS(...) __VA_ARGS__
#define PRINT_ARGS_MACRO(out_ok_type, err_type, res_in, var, ...) \
printf(" arg 1 (out_ok_type): %s\n", #out_ok_type); \
printf(" arg 2 (err_type): %s\n", #err_type); \
printf(" arg 3 (res_in): %s\n", #res_in); \
printf(" arg 4 (var): %s\n", #var); \
printf(" arg 5 (VA_ARGS): %s\n", #__VA_ARGS__)
#define and_then_buggy(types, res_in, var, ...) \
PRINT_ARGS_MACRO(__RESULT_EAT_PARENS types, res_in, var, (__VA_ARGS__))
int main()
{
printf("--- use macro ---\n");
and_then_buggy(
(int, CString), // types
"res_ok", // res_in
"val", // var
"my_func(val)" // ...
);
printf("\n--- expected ---\n");
printf(" arg 1 (out_ok_type): int\n");
printf(" arg 2 (err_type): CString\n");
printf(" arg 3 (res_in): \"res_ok\"\n");
printf(" arg 4 (var): \"val\"\n");
printf(" arg 5 (VA_ARGS): (\"my_func(val)\")\n");
return 0;
}
__VA_ARGS__ is empty, because types is expanded to the value (int, CString) and is eaten by out_ok_type in __RESULT_AND_THEN_IMPL. This happens because only a literal like int, CString is split into two values, but not a value containing a comma. __RESULT_EAT_PARENS does not help, because parenthesis are a part of a value in types and they do not invoke the macro with parameters. You would see it if you test the top macros individually.
It's unclear why and_then is defined with 3 explicit parameters, and then use pass (int, CString). and_then should be defined with 4 explicit parameters.
Simple test
#define __RESULT_EAT_PARENS(...) __VA_ARGS__
#define __RESULT_AND_THEN_IMPL(out_ok_type, err_type, res_in, var, ...) \
printf("%s\n", #out_ok_type); \
printf("%s\n", #err_type); \
printf("%s\n", #res_in); \
printf("%s\n", #var); \
printf("%s\n", #__VA_ARGS__)
#define and_then(types, res_in, var, ...) \
__RESULT_AND_THEN_IMPL(__RESULT_EAT_PARENS types, res_in, var, (__VA_ARGS__))
int main() {
and_then((int, CString), // Target type (U, E)
res_err, // Input Err("file not found")
val, // Bind variable
sophisticated_err_op(val) // Expression -> Err(...)
);
}
outputs
__RESULT_EAT_PARENS (int, CString)
res_err
val
(sophisticated_err_op(val))
that is not what you want.
You might want __RESULT_AND_THEN_IMPL with 3 explicit parameters.
#define __RESULT_AND_THEN_IMPL(types, res_in, var, ...) \
__RESULT_AND_THEN_PASTE(types, res_in, var, __VA_ARGS__)