Hi guys, I¹ve been using quickcheck for a while and I¹m having no problem making generators for my data types, however recently I stumbled upon a situation where I need to maintain State inside the quickcheck generator functions. Here's a simple example, imagine I've got a basic language which only has two features, variable definitions and assignments: type Script = [Statement] data Name = Name String deriving (Show,Eq) data Statement = Def Name Int | Assign Name Name deriving Show Generating random scripts is easy enough, just make each type an instance of Arbitrary with a suitable generator function, however I want to make sure that the generated scripts are syntactically valid. In this case that means no "Name" can be used in an "Assign" statement unless it has previously been defined by a "Def" statement. My knowledge of using monads is somewhat basic so I'm struggling to figure out how to use the State monad with QuickCheck. I've pasted my current effort on the web: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=9412 If anyone could spare a moment to glance at my code and give me a push in the right direction that would be much appreciated! Thanks, Alex Mac