Freezing all dependencies when building executables with `cabal v2-install` - haskell

Question

I am building a Docker image in which I want to bundle multiple executables. Each executable is defined in a different package, in my case pandoc, pandoc-citeproc, and pandoc-crossref. The build should be as reproducible as reasonably possible on a Debian/Ubuntu based system.
What I'd like to do is use (something like) a cabal.project.freeze file to ensure that all subsequent builds will use the same packages.
I'm aware that I can fix the version of the executables:
cabal v2-install pandoc-2.7.3 pandoc-citeproc-0.16.2 pandoc-crossref-0.3.4.1
But this will not fix the versions of transitive dependencies, so rebuilding at different times may lead to subtly different build results. Can I somehow create and use a freeze file in this setup? Using v2-freeze seems to be of no use here:
$ cabal new-freeze pandoc-2.7.3 pandoc-citeproc-0.16.2 pandoc-crossref-0.3.4.1
cabal: 'freeze' doesn't take any extra arguments: pandoc-2.7.3
pandoc-citeproc-0.16.2 pandoc-crossref-0.3.4.1

Answer 1

Okay, there might be a better built-in way to do this kind of thing, but here's a hacky workaround that might be suitable for you until a real cabal expert comes along.
The basic plan will be this: temporarily create a project with the three packages you care about -- just long enough to get a freeze file -- then use some simple text-editor macros to turn the freeze file into a v2-install command. So:
% cabal unpack pandoc-2.7.3 pandoc-citeproc-0.16.2 pandoc-crossref-0.3.4.1
% echo >cabal.project packages: pandoc-2.7.3 pandoc-citeproc-0.16.2 pandoc-crossref-0.3.4.1
% cabal v2-freeze
% sed "s/^constraints: /cabal v2-install pandoc-2.7.3 pandoc-citeproc-0.16.2 pandoc-crossref-0.3.4.1 --constraint '/;s/^ \+/--constraint '/;s/,\$/' \\\\/;\$s/\$/'/" cabal.project.freeze >cabal-v2-install.sh
Woof, that last one is a mouthful. It says:
# Replace the starting "constraints" stanza with the v2-install command we want to
# run. The first line of the stanza includes a constraint, so prefix it with
# --constraint and start a quote.
s/^constraints: /cabal v2-install pandoc-2.7.3 pandoc-citeproc-0.16.2 pandoc-crossref-0.3.4.1 --constraint '/
# The line we just produced doesn't start with spaces, so this only fires on the
# remaining lines. On those lines, it prefixes --constraint and starts a quote.
s/^ \+/--constraint '/
# Close the quote begun on each line, and replace cabal's line-continuation
# character (,) with a shell's line-continuation character (\). The $ and \ are
# escaped because we are inside the current shell's ""-quoted string.
s/,\$/' \\\\/
# The last line doesn't have a line-continuation character, but still needs its
# quote closed. The two occurrences of $ are escaped because we are inside the
# current shell's ""-quoted string.
\$s/\$/'/
You could also do these manually in an editor if you wanted. At the end of this process, which you can run in a temporary directory to ease cleanup afterwards, you should have a file named cabal-v2-install.sh with a command that will select the exact same versions and flags for all packages involved, including dependencies.