Tracking down transitive dependencies in Haskell projects

Mostly a note to myself because I know I’m going to need this later, and I’m going to forget how to do it otherwise.

The question is: I’m depending on some package transitively in a Haskell project built using Cabal, and I want to know how that package ended up in the list of dependencies.

The tool I wanted was gvpr. From the man page:


gvpr - graph pattern scanning and processing language


gvpr (previously known as gpr) is a graph stream editor inspired by awk. It copies input graphs to its output, possibly transforming their structure and attributes, creating new graphs, or printing arbitrary information. The graph model is that provided by libc‐graph(3). In particular, gvpr reads and writes graphs using the dot language.

ghc-pkg helpfully includes a command to output a directed graph of package dependencies in the dot language: ghc-pkg dot. In this graph, an edge from A to B indicates that A depends on B. Therefore, we want to select all of the edges which point towards the package we’re interested in. Here, I wanted to know which package was depending on cpphs, so I did this:

$ cabal sandbox hc-pkg dot | \
    gvpr 'E[$"cpphs*"]' | \
    dot -Tsvg > deps.svg

That argument to gvpr is saying to select edges (E) where the head node ($.head) had a name starting with “cpphs”. The star means “match any string of 0 or more characters”; it is necessary because the version appears in the node name, eg “cpphs-1.19”.

The result:

The output of the above command