Add @try Builtin (#2333)

* Add @try macro
* Add @try_catch. Update release notes.

---------

Co-authored-by: Christoffer Lerno <christoffer@aegik.com>

lib/std/core/builtin.c3

@@ -410,6 +410,7 @@ macro swizzle2(v, v2, ...) @builtin
 {
 	return $$swizzle2(v, v2, $vasplat);
 }
+
 <*
  Return the excuse in the Optional if it is Empty, otherwise
  return a null fault.
@@ -434,6 +435,67 @@ macro bool @ok(#expr) @builtin
 	return true;
 }
 
+<*
+ Check if an Optional expression evaluates to a fault. If so, return it;
+ else, assign the result to an expression.
+
+ @require $defined(#v = #v) : "#v must be a variable"
+ @require $defined(#expr!) : "Expected an optional expression"
+ @require @assignable_to(#expr!!, $typeof(#v))  : `Type of #expr must be an optional of #v's type`
+*>
+macro void? @try(#v, #expr) @builtin
+{
+	var res = #expr;
+	if (catch err = res) return err?;
+	#v = res;
+}
+
+<*
+ Check if an Optional expression evaluates to a fault. If so, return true if it is the
+ expected fault, the optional if it is unexpected, or false if there was no fault and
+ the assign happened.
+
+ This can be used in like this:
+
+  while (true)
+  {
+    char[] data;
+    // Read until end of file
+    if (@try_catch(data, load_line(), io::EOF)) break;
+    .. use data ..
+  }
+
+ In this example we read until we reach an EOF, which is expected. However, if we encounter some other
+ fault, we rethrow is. Without this macro, the code is instead written like:
+
+  while (true)
+  {
+    char[]? data;
+    data = load_line();
+    if (catch err = data)
+    {
+      if (err = io::EOF) break;
+      return err?
+    }
+    .. use data ..
+  }
+
+ @require $defined(#v = #v) : "#v must be a variable"
+ @require $defined(#expr!) : "Expected an optional expression"
+ @require @assignable_to(#expr!!, $typeof(#v))  : `Type of #expr must be an optional of #v's type`
+ @return "True if it was the expected fault, false if the variable was assigned, otherwise returns an optional."
+*>
+macro bool? @try_catch(#v, #expr, fault expected_fault) @builtin
+{
+	var res = #expr;
+	if (catch err = res)
+	{
+		return err == expected_fault ? true : err?;
+	}
+	#v = res;
+	return false;
+}
+
 <*
  @require $defined(&#value, (char*)&#value) : "This must be a value that can be viewed as a char array"
  *>

releasenotes.md

@@ -32,6 +32,7 @@
 - Add `thread::fence` providing a thread fence.
 - Place output in `out` by default for projects. Use temp folder for building at the command line.
 - Allow absolute paths for `$embed`.
+- Add `@try` and `@try_catch`.
 
 ### Fixes
 - mkdir/rmdir would not work properly with substring paths on non-windows platforms.

test/unit/stdlib/core/builtintests.c3

@@ -28,6 +28,41 @@ fn void test_enum_by_name()
 	assert(@catch(enum_by_name(Tester, "GHI")) == NOT_FOUND);
 }
 
+
+faultdef SOME_FAULT, ABC_FAULT;
+
+fn void test_try_catch()
+{
+	int val;
+	int? x = ABC_FAULT?;
+	assert(@try_catch(val, x, ABC_FAULT)!!);
+	assert(val == 0);
+	assert(!@catch(@try_catch(val, x, ABC_FAULT)));
+	x = SOME_FAULT?;
+	assert(@catch(@try_catch(val, x, ABC_FAULT)) == SOME_FAULT);
+	x = 3;
+	assert(!@try_catch(val, x, ABC_FAULT)!!);
+	assert(val == 3);
+}
+
+fn void test_try_set()
+{
+	assert(enum_by_name(Tester, "ABC")!! == Tester.ABC);
+
+	Tester val;
+	if (catch @try(val, enum_by_name(Tester, "ABC"))) abort("Test failure");
+	assert(val == Tester.ABC);
+
+	Tester another;
+	if (catch err = @try(another, enum_by_name(Tester, "GHI")))
+	{
+		assert(err == NOT_FOUND);
+		return;
+	}
+
+	abort("Test failure");
+}
+
 fn void test_likely()
 {
 	int a = 2;