[mrkeen]

Thinking in terms of monoids can be quite helpful even if you're not in pure functions land.

For instance, if you're putting together an on-disk full-text search index, immutability techniques become very relevant (since you can't insert into the middle of files like you can do with in-memory hashmaps). Just make small indexes and a (binary, associative) index-merge operation. Now you have an online&updatable search engine which doesn't need to wait for full reindexes over the data to include new documents.