src/promise.lua: Add promise.interpose

doc/cqueues.tex

@@ -1295,6 +1295,10 @@ \subsubsection{\fn{signal[]}}
 
 Returns a new promise object. $f$ is an optional function to run asynchronously, to which any subsequent arguments are passed. $f$ is called using \fn{pcall}, and the return values of \fn{pcall} are passed directly to \fn{promise:set}.
 
+\subsubsection[\fn{promise.interpose}]{\fn{promise.interpose(name, function)}}
+
+Add or interpose a promise class method. Returns the previous method, if any.
+
 \subsubsection[\fn{promise:status}]{\fn{promise:status()}}
 
 Returns ``pending'' if the promise is yet unresolved, ``fulfilled'' if the promise has been resolved (\fn{promise:get} will return the values), or ``rejected'' if the promise failed (\fn{promise:get} will throw an error).

src/promise.lua

@@ -34,6 +34,12 @@ local loader = function(loader, ...)
 		return (mt == promise and "promise") or nil
 	end -- promise.type
 
+	function promise.interpose(method, newfunc)
+		local oldfunc = promise[method]
+		promise[method] = newfunc
+		return oldfunc
+	end
+
 	function promise:set(ok, ...)
 		assert3(self.state == "pending", "attempt to set value of resolved promise")