This weekend a user came into #ghc asking how to profile their Template Haskell splices using GHC’s cost-center profiler. While this is not so hard in the simple case where you have no dependencies, doing so with a real package took a surprisingly long time and was non-obvious to even GHC developers. Consequently, I felt this might be helpful to write down the prescription.

In short, the method we arrived at takes advantage of GHC’s external interpreter coupled with cabal-install’s support for producing package environment files. To demonstrate the approach, we will profile the Template Haskell-based lens derivation logic of the optics-th package, using the T799 module as our source of splices.

We first start by setting up a working directory of the project:

$ cabal unpack optics-th
$ cd optics-th-0.3
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