I am trying to use Reactive to write a build system, initially for Haskell modules, which stays running and rebuilds stuff when files are modified, created etc. However, I have run into a problem with adding files to the project to be built. Suppose you generate separate Behaviors for the contents of each file (Behavior (Maybe ByteString)). Then, whenever you add a new file, a new Behavior must be dynamically created, which records Just the contents of this file, or Nothing when it did not exist, from the application startup time onwards. ("Dynamically created" because it's not possible to statically list all the filenames the developer might choose to use, obviously!) It seems to me that this entails, by the very design of Reactive, keeping around the complete contents of every version of every file, forever. *We* know that the old contents, if any, are "dead", but the garbage collector probably won't be able to detect that. If, on the other hand, you just have one single Behavior for this purpose (Behavior (Map FilePath ByteString)), this will change any time *anything* changes on the filesystem, so it doesn't directly support triggering a recomputation/recompilation based on *particular* changes. This problem seems to me to generalise to any case where you are trying to dynamically generate "sub-behaviors" from existing Behaviors, at arbitrary points in time. Is my analysis correct? Have I missed something? -- Robin