I E

Playing with GHC package databases

1 July, 2025

I wanted to do some dependency analysis of a Haskell project, and I wanted to see what it was like without installing a Cabal-specific tool such as cabal-plan (which works fine). Here’s what I played around with.

Finding the GHC package database used by Cabal

When building Haskell code with GHC and Cabal, Cabal manages GHC package databases for your project. Cabal uses the GHC_ENVIRONMENT environment variable to communicate package information to GHC.

Inspect the variable via cabal exec:

$ cabal exec -- printenv GHC_ENVIRONMENT
(output) 
/home/isaac/scotty/dist-newstyle/tmp/environment.-42133/.ghc.environment.x86_64-linux-9.6.6

It’s a temporary file that’s cleaned up after cabal exits:

$ cat $(cabal exec -- printenv GHC_ENVIRONMENT)
(output) 
cat: /home/isaac/scotty/dist-newstyle/tmp/environment.-42668/.ghc.environment.x86_64-linux-9.6.6: No such file or directory

So to read the file’s contents:

$ cabal exec -- bash -c 'cat $(printenv GHC_ENVIRONMENT) | wc -l'
(output) 
167
$ cabal exec -- bash -c 'cat $(printenv GHC_ENVIRONMENT) | head -n 20'
(output) 
-- This is a GHC environment file written by cabal. This means you can
-- run ghc or ghci and get the environment of the project as a whole.
-- But you still need to use cabal repl $target to get the environment
-- of specific components (libs, exes, tests etc) because each one can
-- have its own source dirs, cpp flags etc.
--
clear-package-db
global-package-db
package-db /home/isaac/.local/state/cabal/store/ghc-9.6.6/package.db
package-db /home/isaac/scotty/dist-newstyle/packagedb/ghc-9.6.6
package-id scotty-0.22-inplace
package-id aeson-2.2.3.0-22b3a92d15c9e101394040ae41246b217864b9947aefa8650ea84ae86f886a71
package-id OneTuple-0.4.2-7fce57870ab38b77b68efd7d8a8c569b5646e38fd6083c48482d84b2cbb4b9f8
package-id base-4.18.2.1
package-id ghc-bignum-1.3
package-id ghc-prim-0.10.0
package-id rts-1.0.2
package-id template-haskell-2.20.0.0
package-id ghc-boot-th-9.6.6
package-id pretty-1.1.3.6

This means that cabal build et al. will use package databases /home/isaac/.local/state/cabal/store/ghc-9.6.6/package.db and /home/isaac/scotty/dist-newstyle/packagedb/ghc-9.6.6.

Now that we know the package database paths, we can use ghc-pkg to query them.

$ ghc-pkg --package-db=/home/isaac/.local/state/cabal/store/ghc-9.6.6/package.db list | wc -l
(output) 
675

I think this is just the package database for everything I’ve downloaded from Hackage to date.

What about the other one?

$ ghc-pkg --package-db=/home/isaac/scotty/dist-newstyle/packagedb/ghc-9.6.6 list
(output) 
WARNING: there are broken packages.  Run 'ghc-pkg-9.6.6 check' for more details.
/home/isaac/scotty/dist-newstyle/packagedb/ghc-9.6.6
    scotty-0.22
$ ghc-pkg --package-db=/home/isaac/scotty/dist-newstyle/packagedb/ghc-9.6.6 check
(output) 
There are problems in package scotty-0.22:
  Warning: haddock-interfaces: /home/isaac/scotty/dist-newstyle/build/x86_64-linux/ghc-9.6.6/scotty-0.22/doc/html/scotty/scotty.haddock doesn't exist or isn't a file
  Warning: haddock-html: /home/isaac/scotty/dist-newstyle/build/x86_64-linux/ghc-9.6.6/scotty-0.22/doc/html/scotty doesn't exist or isn't a directory
  dependency "aeson-2.2.3.0-22b3a92d15c9e101394040ae41246b217864b9947aefa8650ea84ae86f886a71" doesn't exist
  dependency "blaze-builder-0.4.2.3-12eefb9a1a78be6e7022945ab259dd32e85b6ca455fcd003fe4e06f9179e7992" doesn't exist
  ...
  dependency "wai-extra-3.1.17-819af7b672ea64d23961a97b5fe6cf5ca65cb2c5510c7d77bbddee954acb10c5" doesn't exist
  dependency "warp-3.4.7-7055965423ff64562e3affc843bcceaf11f389a4a74c6cd55ed3940f63322e42" doesn't exist
Warning: include-dirs: /nix/store/97cajcan6faqj0kr9zhl0jin38vzcnhi-ghc-9.6.6/lib/ghc-9.6.6/lib/../lib/x86_64-linux-ghc-9.6.6/directory-1.3.8.5/include doesn't exist or isn't a directory

The following packages are broken, either because they have a problem
listed above, or because they depend on a broken package.
scotty-0.22

It’s missing a bunch of dependencies. Let’s use both package databases together:

$ ghc-pkg --package-db=/home/isaac/.local/state/cabal/store/ghc-9.6.6/package.db --package-db=/home/isaac/scotty/dist-newstyle/packagedb/ghc-9.6.6 list
(output) 
/home/isaac/.local/state/cabal/store/ghc-9.6.6/package.db
    Cabal-3.12.1.0
    Cabal-syntax-3.12.1.0
    ChasingBottoms-1.3.1.15
    Glob-0.10.2
    HUnit-1.6.2.0
    JuicyPixels-3.3.9
    JuicyPixels-3.3.9
    MemoTrie-0.6.11
    OneTuple-0.4.2
    ...
    xml-types-0.3.8
    yaml-0.11.11.2
    yaml-0.11.11.2
    zip-archive-0.4.3.2
    zlib-0.7.1.0
    zlib-0.7.1.0

/home/isaac/scotty/dist-newstyle/packagedb/ghc-9.6.6
    scotty-0.22

The second package database is just for this project’s packages.

Package database analysis

Now that we’ve got a working package database stack, we can play around. First, save the dependency graph in a flexible format:

$ ghc-pkg --package-db=/home/isaac/scotty/dist-newstyle/packagedb/ghc-9.6.6 --simple-output field scotty-0.22 id
(output) 
scotty-0.22-inplace
$ python3 - > scotty-0.22-inplace.json <<EOF 
$ python3 - > scotty-0.22-inplace.json <<EOF
import json
import subprocess

package_name = "scotty-0.22-inplace"

def graph(package_id):
  result = {}

  def go(package_id):
    depends = subprocess.run(
      [
        "ghc-pkg",
        "--no-user-package-db",
        "--package-db=/home/isaac/.local/state/cabal/store/ghc-9.6.6/package.db",
        "--package-db=/home/isaac/scotty/dist-newstyle/packagedb/ghc-9.6.6",
        "--ipid",
        "--simple-output",
        "field",
        package_id,
        "depends"
      ],
      capture_output=True,
      text=True
    ).stdout.strip()

    split_depends = [depend_id for depend_id in map(lambda x: x.strip(), depends.split(" ")) if depend_id]
  
    result[package_id] = split_depends
    for depend_id in split_depends:
      if depend_id not in result:
        go(depend_id)

  go(package_id)

  return result

print(json.dumps(graph(package_name), sort_keys=True))
EOF
(no output)

Then visualise the graph:

$ { python3 - | dot -Tsvg > scotty-0.22-inplace.svg; } <<EOF 
$ { python3 - | dot -Tsvg > scotty-0.22-inplace.svg; } <<EOF
import json

file_name = "scotty-0.22-inplace.json"

def quote(x):
  return f'"{x}"'

def strip_id(x):
  [*prefix, last] = x.split("-")
  return "-".join(prefix)

def dot(graph):
  for k, v in graph.items():
    joined_depends = "{" + " ".join([quote(strip_id(depend_id)) for depend_id in v]) + "}"    
    print(f"{quote(strip_id(k))} -> {joined_depends}")

print("digraph {")
with open(file_name) as file:
  dot(json.load(file))
print("}")
EOF
(no output)
$ dot -Tsvg scotty-0.22-inplace.dot > deps-scotty-0.22-inplace.svg
(output)
Dependency graph for my checkout of scotty-0.22

Let’s order them by transitive closure size:

$ { python3 - | sort -h | tail; } <<EOF 
$ { python3 - | sort -h | tail; } <<EOF
import json

file_name = "scotty-0.22-inplace.json"

def transitive_closure(graph, root):
  seen = set()

  def go(node):
    if node in seen:
      return set()
    else:
      seen.add(node)
      return set.union({node}, *[go(adjacent_node) for adjacent_node in graph[node]])

  return go(root)

def transitive_closure_sizes(graph):
  for node in graph.keys():
    print(len(transitive_closure(graph, node)), node)

with open(file_name) as file:
  transitive_closure_sizes(json.load(file))
EOF
(output) 
25 streaming-commons-0.2.2.6-3f522fa5f939db1b55145657d311c943006edb86009c26092909366b2ee75aee
28 semigroupoids-6.0.1-9dd89e384571d44fb22e2fb7072578c66e77815638622ba98f23f5f4b84282b2
33 http-api-data-0.6.1-44cab653536eecb6e1a4492b7db5ddb55bad7134ef4df6f7019d9b0d933fbf23
33 wai-logger-2.5.0-afde5370b0becd5a59db9b87d40363205352d3d195d9f7f6918e6141c5790f9b
34 http2-5.3.9-e85f1b676925943caace1517467efd2cb267ded07cdb15141f4d23011f166d32
36 semialign-1.3.1-54b91e33817ae4935235d4a598fa09630b1344e5fc8ef634e7b379a9abcbae4b
58 aeson-2.2.3.0-22b3a92d15c9e101394040ae41246b217864b9947aefa8650ea84ae86f886a71
67 warp-3.4.7-7055965423ff64562e3affc843bcceaf11f389a4a74c6cd55ed3940f63322e42
117 wai-extra-3.1.17-819af7b672ea64d23961a97b5fe6cf5ca65cb2c5510c7d77bbddee954acb10c5
127 scotty-0.22-inplace

wai-extra sure has a lot of dependencies. Which dependencies are unique to wai-extra?

$ { python3 - | sort; } <<EOF 
$ { python3 - | sort; } <<EOF
import json

file_name = "scotty-0.22-inplace.json"

def transitive_closure(graph, root, excluding={}):
  def go(node, excluding):
      result = {node}
      for adjacent_node in graph[node]:
        if adjacent_node not in excluding:
          result = set.union(result, go(adjacent_node, {}))
      return result

  return go(root, excluding)

with open(file_name) as file:
  graph = json.load(file)

wai_extra = "wai-extra-3.1.17-819af7b672ea64d23961a97b5fe6cf5ca65cb2c5510c7d77bbddee954acb10c5"
a = transitive_closure(graph, "scotty-0.22-inplace", {wai_extra})
b = transitive_closure(graph, wai_extra)
for element in b - a:
  print(element)
EOF
(output) 
ansi-terminal-1.1.2-d813dc9a9ddcc8fe046981ae4cf16e97851f81b47627fe62d97deeea8b3a895c
ansi-terminal-types-1.1-f5259ee8039cb2e4f0745aaf2a221ac1804976d88721a0bbf03fa589fc1da7b9
base64-bytestring-1.2.1.0-1d3b0d9233998c8db4e03792dfcb571d3630c51818122c1df384f008255859d8
call-stack-0.4.0-8d31f8ae063fb95797e807de0dcca55432203fbdb07105abf4ba4f3a0860ef74
colour-2.3.6-24e30114cf117673f32f0197751def539bd8f5410de72a299de6885591f0fdfd
easy-file-0.2.5-30a028cc4c6fd7862bc8a47d31985a45218a10d4422c035a8fff3448ec4801dc
fast-logger-3.2.5-ba361b4be97b9b35499712d8f507906cdebc3cc5a1036133240c2e08b10a6a56
HUnit-1.6.2.0-df563d5b792be11ca4e58e4d870dddc83eb4e3f9a52334bc71194d682e05b20e
unix-compat-0.7.3-32201a092fc238f3eccb0914b80c185b8d1d4f773a1dd8f2fb5563b7b6d76bda
wai-extra-3.1.17-819af7b672ea64d23961a97b5fe6cf5ca65cb2c5510c7d77bbddee954acb10c5
wai-logger-2.5.0-afde5370b0becd5a59db9b87d40363205352d3d195d9f7f6918e6141c5790f9b

And if we exclude wai-extra, then which dependencies are unique to aeson?

$ { python3 - | sort; } <<EOF 
$ { python3 - | sort; } <<EOF
import json

file_name = "scotty-0.22-inplace.json"

def transitive_closure(graph, root, excluding={}):
  def go(node, excluding):
      result = {node}
      for adjacent_node in graph[node]:
        if adjacent_node not in excluding:
          result = set.union(result, go(adjacent_node, {}))
      return result

  return go(root, excluding)

with open(file_name) as file:
  graph = json.load(file)

wai_extra = "wai-extra-3.1.17-819af7b672ea64d23961a97b5fe6cf5ca65cb2c5510c7d77bbddee954acb10c5"
aeson = "aeson-2.2.3.0-22b3a92d15c9e101394040ae41246b217864b9947aefa8650ea84ae86f886a71"
a = transitive_closure(graph, "scotty-0.22-inplace", {aeson,wai_extra})
b = transitive_closure(graph, aeson)
for element in b - a:
  print(element)
EOF
(output) 
aeson-2.2.3.0-22b3a92d15c9e101394040ae41246b217864b9947aefa8650ea84ae86f886a71
assoc-1.1.1-91e0bc5b0eb5cdad4ce61452d6155d452cac5d786af875847301af32b934f253
bifunctors-5.6.2-cdf5e9d975abe82ed95add7544f5e6e48940d9e7f5951180b3dcacba9ad97937
character-ps-0.1-87eb2c6d1a544cf19d3be82f4399a28864ed8d7542e2de535b42fc8056a9d5c9
comonad-5.0.9-ec7e1b2474b620c56c05b73796c5dce04e68441d59edb217a30e1bb77e48c942
contravariant-1.5.5-6a59bcf4e69a8f892227aa56684b44d1735596355500cba48bb9556495af7a7a
data-fix-0.3.4-7d597ffefc30f7a138edc24dc1e03bebbf99520c26ee1af6318a323b3bc526a8
distributive-0.6.2.1-c13d5bb01b367cf9f192f77e0e5d90e4a6cda304cc5697ea52f5925c88d80b68
dlist-1.0-3f3e12dbb9340ca35fec4abee0f3e37d2efd77c79010ac403b24fe666a366fda
generically-0.1.1-f217083ba3fc5ebe022288f26b2a769e0b39440c221ecae8e8a84336b627c7fa
indexed-traversable-0.1.4-86fd612af77d0540ca42d5223fc9600bb51bf5e9f6d7ffe74f58e4eb6346acc2
indexed-traversable-instances-0.1.2-9831eee38ead8950ef755ff4425a82f4e822cb30329849d29c400d6fdfa9f48f
network-uri-2.6.4.2-f34e13d200dc845553cbada86fe6571c3b03af72b2be5233ec82dae21b560e8e
OneTuple-0.4.2-7fce57870ab38b77b68efd7d8a8c569b5646e38fd6083c48482d84b2cbb4b9f8
parsec-3.1.16.1
QuickCheck-2.15.0.1-c94e2ced9f5f0dd3f594a04519b82805819b2b0379b9af74da0bd21726807fc4
semialign-1.3.1-54b91e33817ae4935235d4a598fa09630b1344e5fc8ef634e7b379a9abcbae4b
semigroupoids-6.0.1-9dd89e384571d44fb22e2fb7072578c66e77815638622ba98f23f5f4b84282b2
StateVar-1.2.2-0d5154d6e1d634f4e8767a7881908c34dedbc6cace8d2fe22118078ab9b12887
strict-0.5.1-0102bddb9fe467e451549ac50f5ddc9163563e9539eeed3d6ee08b5d9346b275
text-short-0.1.6-662ac4f8bb2f349ae6d506b1834638219dce4453004fb077f179cb16b3c6e2b3
th-abstraction-0.7.1.0-961929bb0a2cfcc168b2a8158c99139722497385235d4359a7dcd4d353e5957a
th-compat-0.1.6-0605ad4fe4730665b7b05d178af0de7dd3cb888b4f51e363b3ce11c5e6c1b6ed
these-1.2.1-2c66a71e09a4f0d388e0495b32ada8a64764c07eb4b90121bb1482fc8aefd0a1
vector-0.13.2.0-36660ea3d0c11b81bc8d2fd49bdacdb1fe74eb64b339694a83e4d99c8cad7b5a
vector-stream-0.1.0.1-545e7e06db03dece0246a6bae97757021fce10507a67c3d3b06190d1367c5623
witherable-0.5-bd7f0d1e02c3180a6977cb316a3de72faf9fa6ec35511c54587f2ae6843f9d9a

Thoughts and ideas