timewarp

In-memory implementations are very useful, especially for larger tests, but, again, I don't see anything in Haskell that makes it easier. For example, on the JVM tests often use Google's Jimfs[1] in-memory file system (the JVM has pluggable file systems implemented as Java interfaces), and for relational databases they use H2[2]. None of this requires any code changes, perhaps thanks to the JVM's extreme pluggability. The JVM uses SPIs for almost everything, from encryption to filesystems to sockets -- you can fake the entire OS (almost) in Java, and what you can't fake easily, you can still fake with an agent (unrelated, but starting in Java 9, even the JIT is pluggable, so you can control machine-code generation at any granularity you want). Just since my conversation here with chadaustin, I've written a small library that uses an agent