(another cross post from Reddit - https://www.reddit.com/r/haskell/comments/5qfd36/instrumentation_without_req... ) Is it possible to collect instrumentation data from a Hakell application without requiring the core application to change code? Here are some examples of instrumentation data: * Time taken to serve an HTTP request (not average time, but individual times, for individual requests) along with the incoming request params+headers and outgoing response length. * Time taken to make an SQL query along with the actual query (again, not average times, but individual times, for individual queries) * Time taken to fetch data from Redis along with the Redis command * Time taken to render an HTML page along with path to the HTML file Most dynamic languages (like Ruby) allow monkey-patching OR "decorating" existing functions during runtime. Most instrumentation agents (like Skylight, Newrelic), simply monkey-patch the API surfaces of known libraries to collect this data. Therefore, using these instrumentation agents is a one line code change (just include the gem, or drop the JAR, or whatever). What is the equivalent of this in Haskell? Here's what I've tried so far [1] , but it has the following disadvantages: * Requires changes to the application's cabal file to replace the non-instrumented versions of the libraries with the instrumented version (replace scotty with instrumentedscotty, in my example) * Requires the application to explicitly import the instrumented versions of the libraries (replace import Scotty with import InstrumentedScotty, in my example) * Forces all the code interacting with instrumented APIs to be run in a MonadIO environment - because of the underlying calls to getCurrentTime I'm sure there's a better way, but I couldn't get my hands on it! [1] https://gist.github.com/saurabhnanda/27592da0269bc35569ec6239e1a91b75 -- Saurabh.