gh-115: Add operator REFUTE.

Author: python-processing-unit

docs/SPECIFICATION.html

@@ -841,6 +841,8 @@
 
 - `BOOL: ASSERT(ANY: value)` = MUST raise a runtime error if `value` is falsey according to the language's truthiness rules. On success, it MUST return `TRUE`.
 
+- `BOOL: REFUTE(~BOOL: cond)` = MUST raise a runtime error if `cond` is truthy according to the language's truthiness rules. On success, it MUST return `TRUE`.
+
 - `THROW()` or `THROW(INT|STR: a1, ..., INT|STR: aN)` = MUST raise a runtime error. When one or more arguments are supplied, the error message MUST be formed by concatenating those arguments using the same rendering rules as `PRINT`, but without a trailing newline. When no arguments are supplied, the error message MUST be `"Exception thrown"`.
 
 - `BOOL: MAIN()` = MUST return `TRUE` when the call site is in the primary program source and `FALSE` when the call site is executing from imported module code. The result MUST depend on the source location of the call expression rather than on the dynamic caller stack.

src/builtins.c

@@ -6448,6 +6448,16 @@ static Value builtin_assert(Interpreter* interp, Value* args, int argc, Expr** a
     return value_bool(true);
 }
 
+// REFUTE(~BOOL: cond):BOOL
+static Value builtin_refute(Interpreter* interp, Value* args, int argc, Expr** arg_nodes, Env* env, int line, int col) {
+    (void)arg_nodes; (void)env;
+
+    if (value_truthiness(args[0])) {
+        RUNTIME_ERROR(interp, "Refutation failed", line, col);
+    }
+    return value_bool(true);
+}
+
 static Value builtin_throw(Interpreter* interp, Value* args, int argc, Expr** arg_nodes, Env* env, int line, int col) {
     (void)arg_nodes; (void)env;
 
@@ -9102,6 +9112,7 @@ static BuiltinFunction builtins_table[] = {
 
     // Control
     {"ASSERT", 1, 1, builtin_assert},
+    {"REFUTE", 1, 1, builtin_refute},
     {"THROW", 0, -1, builtin_throw},
 
     // Variables

tests/cases/failing/refute-true.pre

@@ -0,0 +1 @@
+REFUTE(TRUE)

tests/cases/passing/refute.pre

@@ -0,0 +1,4 @@
+REFUTE(FALSE)
+REFUTE(0d0)
+REFUTE(0d0.0)
+REFUTE("")