diff --git a/ast/Closure.lua b/ast/Closure.lua
index 435ac72..05acc68 100644
--- a/ast/Closure.lua
+++ b/ast/Closure.lua
@@ -44,9 +44,9 @@ Closure = Runtime(Overloadable) {
 	format_parameters = function(self, state)
 		return self.func.parameters:format(state)
 	end,
-	call_compatible = function(self, state, args)
+	call_dispatched = function(self, state, args)
 		state.scope:push(self.exported_scope)
-		local exp = self.func:call_compatible(state, args)
+		local exp = self.func:call_dispatched(state, args)
 		state.scope:pop()
 		return exp
 	end,
diff --git a/ast/Function.lua b/ast/Function.lua
index 5a95b66..f5116f4 100644
--- a/ast/Function.lua
+++ b/ast/Function.lua
@@ -41,7 +41,7 @@ Function = Overloadable {
 	format_parameters = function(self, state)
 		return self.parameters:format(state)
 	end,
-	call_compatible = function(self, state, args)
+	call_dispatched = function(self, state, args)
 		state.scope:push()
 		args:bind_parameter_tuple(state, self.parameters)
 
diff --git a/ast/LuaFunction.lua b/ast/LuaFunction.lua
index 4392289..719e26b 100644
--- a/ast/LuaFunction.lua
+++ b/ast/LuaFunction.lua
@@ -34,7 +34,7 @@ LuaFunction = ast.abstract.Runtime(Overloadable) {
 	format_parameters = function(self, state)
 		return self.parameters:format(state)
 	end,
-	call_compatible = function(self, state, args)
+	call_dispatched = function(self, state, args)
 		local lua_args = { state }
 
 		state.scope:push()
diff --git a/ast/Overload.lua b/ast/Overload.lua
index 1a9b2bf..547f6db 100644
--- a/ast/Overload.lua
+++ b/ast/Overload.lua
@@ -29,7 +29,7 @@ Overload = ast.abstract.Node {
 		end
 	end,
 
-	call = function(self, state, args)
+	dispatch = function(self, state, args)
 		local failure = {} -- list of failure messages (kept until we find the first success)
 		local success, success_specificity, success_secondary_specificity = nil, -1, -1
 		-- some might think that iterating a list for every function call is a terrible idea, but that list has a fixed number of elements, so big O notation says suck it up
@@ -42,7 +42,7 @@ Overload = ast.abstract.Node {
 					if secondary_specificity > success_secondary_specificity then
 						success, success_specificity, success_secondary_specificity = fn, specificity, secondary_specificity
 					elseif secondary_specificity == success_secondary_specificity then
-						error(("more than one function match %s, matching functions were at least (specificity %s.%s):\n\t• %s\n\t• %s"):format(args:format(state), specificity, secondary_specificity, fn:format_parameters(state), success:format_parameters(state)), 0)
+						return nil, ("more than one function match %s, matching functions were at least (specificity %s.%s):\n\t• %s\n\t• %s"):format(args:format(state), specificity, secondary_specificity, fn:format_parameters(state), success:format_parameters(state))
 					end
 				end
 				-- no need to add error message for less specific function since we already should have at least one success
@@ -51,10 +51,9 @@ Overload = ast.abstract.Node {
 			end
 		end
 		if success then
-			return success:call_compatible(state, args)
+			return success, args
 		else
-			-- error
-			error(("no function match %s, possible functions were:\n\t• %s"):format(args:format(state), table.concat(failure, "\n\t• ")), 0)
+			return nil, ("no function match %s, possible functions were:\n\t• %s"):format(args:format(state), table.concat(failure, "\n\t• "))
 		end
 	end
 }
diff --git a/ast/Quote.lua b/ast/Quote.lua
index 700bb8e..7646e39 100644
--- a/ast/Quote.lua
+++ b/ast/Quote.lua
@@ -26,8 +26,14 @@ Quote = ast.abstract.Node {
 		fn(self.expression, ...)
 	end,
 
-	call = function(self, state, args)
-		assert(args.arity == 0, "Quote! does not accept arguments")
+	dispatch = function(self, state, args)
+		if args.arity == 0 then
+			return self, args
+		else
+			return nil, "Quote! does not accept arguments"
+		end
+	end,
+	call_dispatched = function(self, state, args)
 		return self.expression:eval(state)
 	end
 }
diff --git a/ast/Typed.lua b/ast/Typed.lua
index d6af880..78db976 100644
--- a/ast/Typed.lua
+++ b/ast/Typed.lua
@@ -2,7 +2,10 @@ local ast = require("ast")
 
 local operator_priority = require("common").operator_priority
 
-return ast.abstract.Runtime {
+local format_identifier
+
+local Typed
+Typed = ast.abstract.Runtime {
 	type = "typed",
 
 	expression = nil,
@@ -14,6 +17,15 @@ return ast.abstract.Runtime {
 	end,
 
 	_format = function(self, state, prio, ...)
+		-- try custom format
+		if state and state.scope:defined(format_identifier) then
+			local custom_format = format_identifier:eval(state)
+			local args = ast.ArgumentTuple:new(self)
+			local fn, d_args = custom_format:dispatch(state, args)
+			if fn then
+				return custom_format:call(state, d_args):format(state, prio, ...)
+			end
+		end
 		return ("type(%s, %s)"):format(self.type_expression:format(state, operator_priority["_,_"], ...), self.expression:format_right(state, operator_priority["_,_"], ...))
 	end,
 
@@ -22,3 +34,8 @@ return ast.abstract.Runtime {
 		fn(self.expression, ...)
 	end
 }
+
+package.loaded[...] = Typed
+format_identifier = ast.Identifier:new("format")
+
+return Typed
diff --git a/ast/abstract/Node.lua b/ast/abstract/Node.lua
index ff65e0a..17cb758 100644
--- a/ast/abstract/Node.lua
+++ b/ast/abstract/Node.lua
@@ -64,7 +64,7 @@ Node = class {
 	-- to be preferably used during construction only
 	set_source = function(self, source)
 		local str_source = tostring(source)
-		if self.source == "?" then
+		if self.source == "?" and str_source ~= "?" then
 			self.source = str_source
 			self:traverse(traverse.set_source, str_source)
 		end
@@ -156,16 +156,39 @@ Node = class {
 		return t
 	end,
 
+	-- call the node with the given arguments
 	-- return result AST
 	-- arg is a ArgumentTuple node (already evaluated)
-	-- redefine if relevant
+	-- do not redefine; instead redefine :dispatch and :call_dispatched
 	call = function(self, state, arg)
+		local dispatched, dispatched_arg = self:dispatch(state, arg)
+		if dispatched then
+			return dispatched:call_dispatched(state, dispatched_arg)
+		else
+			error(("can't call %s %s: %s"):format(self.type, self:format(state), dispatched_arg), 0)
+		end
+	end,
+	-- find a function that can be called with the given arguments
+	-- return function, arg if a function is found that can be called with arg. The returned arg may be different than the input arg.
+	-- return nil, message if no matching function found
+	dispatch = function(self, state, arg)
+		-- by default, look for custom call operator
 		if state.scope:defined(custom_call_identifier) then
 			local custom_call = custom_call_identifier:eval(state)
-			return custom_call:call(state, arg:with_first_argument(self))
-		else
-			error("trying to call a "..self.type..": "..self:format(state))
+			local dispatched, dispatched_arg = custom_call:dispatch(state, arg:with_first_argument(self))
+			if dispatched then
+				return dispatched, dispatched_arg
+			else
+				return nil, dispatched_arg
+			end
 		end
+		return nil, "not callable"
+	end,
+	-- call the node with the given arguments
+	-- this assumes that this node was correctly dispatched to (was returned by a previous call to :dispatch)
+	-- you can therefore assume that the arguments are valid and compatible with this node
+	call_dispatched = function(self, state, arg)
+		error(("%s is not callable"):format(self.type))
 	end,
 
 	-- merge any changes back into the main branch
diff --git a/ast/abstract/Overloadable.lua b/ast/abstract/Overloadable.lua
index 7b19d08..d8891c7 100644
--- a/ast/abstract/Overloadable.lua
+++ b/ast/abstract/Overloadable.lua
@@ -1,3 +1,5 @@
+-- for nodes that can be put in an Overload
+
 local ast = require("ast")
 
 return ast.abstract.Node {
@@ -9,19 +11,21 @@ return ast.abstract.Node {
 	compatible_with_arguments = function(self, state, args)
 		error("not implemented for "..self.type)
 	end,
-	-- same as :call, but assumes :compatible_with_arguments was checked before the call
-	call_compatible = function(self, state, args)
-		error("not implemented for "..self.type)
-	end,
 
 	-- return string
 	format_parameters = function(self, state)
 		return self:format(state)
 	end,
 
-	-- default for :call
-	call = function(self, state, args)
-		assert(self:compatible_with_arguments(state, args))
-		return self:call_compatible(state, args)
-	end
+	-- can be called either after a successful :dispatch or :compatible_with_arguments
+	call_dispatched = function(self, state, args)
+		error("not implemented for "..self.type)
+	end,
+
+	-- default for :dispatch
+	dispatch = function(self, state, args)
+		local s, err = self:compatible_with_arguments(state, args)
+		if s then return self, args
+		else return nil, err end
+	end,
 }