Add result tuple assert

src/ludus/prelude.ld

@@ -903,6 +903,13 @@ fn assert! {
 		else panic! string ("Assert failed: ", msg, " with ", value)
 }
 
+
+fn assert {
+	"Asserts a condition. Returns a result tuple: `(:ok, value)` if the value is truthy, and `(:err, value)` if it is falsy. Or, give it an error message, and that's what'll be in the error tuple instead."
+	(value) -> if value then (:ok, value) else (:err, value)
+	(msg, value) -> if value then (:ok, value) else (:err, msg)
+}
+
 &&& Turtle & other graphics
 
 & some basic colors
@@ -1282,6 +1289,7 @@ ns prelude {
 	unwrap!
 	unwrap_or
 	assert!
+	assert
 	colors
 	forward!, fd!
 	back!, bk!