Tracing handler for extracting top expressions

gistfile1.txt

Red []

detect: function [
	event  [word!]
	code   [any-block! none!]
	offset [integer!]
	value  [any-type!]
	ref	   [any-type!]
	frame  [pair!]
][
	range: []
	stack: []
	switch event [
		set return exit	fetch [
			if find [set return] event [take/last stack]
			if empty? stack [		
				if event = 'exit [append range offset]
				either 1 >= length? range [append range offset][
					if range/1 < range/2 [
						code: head code
						print ["expr:" mold copy/part at code range/1 + 1 range/2 - range/1]
					]
					remove range
				]
			]
		]
		open  [append/only stack :value]
	]
]
mark-expr: function [code][do/trace :code :detect]

mark-expr [print a: 1 b: 3 print either a > 2 [a + 3][a - 1]]