[Haskell-cafe] Instrumentation without requiring code-level changes?

Saurabh Nanda saurabhnanda at gmail.com
Fri Jan 27 05:19:43 UTC 2017

(another cross post from Reddit -

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

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.
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