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Lock Files

Stack attempts to provide reproducible build plans. This involves reproducibly getting the exact same contents of source packages and configuration options (like Cabal flags and GHC options) for a given set of input files. There are a few problems with making this work:

  • Entering all of the information to fully provide reproducibility is tedious. This would include things like Hackage revisions, hashes of remote tarballs, etc. Users don't want to enter this information.
  • Many operations in Stack rely upon a "snapshot hash," which transitively includes the completed information for all of these dependencies. If any of that information is missing when parsing the stack.yaml file or snapshot files, it could be expensive for Stack to calculate it.

To address this, we follow the (fairly standard) approach of having a lock file. The goal of the lock file is to cache completed locations of project, snapshot packages and snapshots themselves so that:

  • These files can be stored in source control
  • Users on other machines can reuse these lock files and get identical build plans given that the used project packages and local snapshots are the same on those machines
  • Rerunning stack build in the future is deterministic in the build plan, not depending on mutable state in the world like Hackage revisions


    If, for example, a tarball available remotely is deleted or the hash changes, it will not be possible for Stack to perform the build. However, by deterministic, we mean it either performs the same build or fails, never accidentally doing something different.

This document explains the contents of a lock file, how they are used, and how they are created and updated.

stack.yaml and snapshot files

Relevant to this discussion, Stack's project-level configuration file (stack.yaml, by default) specifies:

Some of this information can be incomplete. Consider this stack.yaml file:

snapshot: lts-19.22
- .
- acme-missiles-0.3

This information is incomplete. For example, the extra-deps may change in the future. Instead, you could specify enough information in the stack.yaml file to fully resolve that package. That looks like:

- hackage: acme-missiles-0.3@sha256:2ba66a092a32593880a87fb00f3213762d7bca65a687d45965778deb8694c5d1,613
    size: 226
    sha256: 614bc0cca76937507ea0a5ccc17a504c997ce458d7f2f9e43b15a10c8eaeb033

The lts-19.22 information is also incomplete. While we assume in general that Haskell LTS snapshots never change, there's nothing that prohibits that from happening. Instead, the complete version of that key is:

- url:
  size: 619399
  sha256: 5098594e71bdefe0c13e9e6236f12e3414ef91a2b89b029fd30e8fc8087f3a07

Users don't particularly feel like writing all of that. Therefore, it's common to see incomplete information in a stack.yaml file.

Recursive snapshot layers

Snapshot files can be recursive, where stack.yaml refers to foo.yaml, which refers to bar.yaml, which refers to baz.yaml. A local snapshot file can refer to a remote snapshot file (available via an HTTP(S) URL).

We need to encode information from all of these snapshot layers and the stack.yaml file in the lock file, to ensure that we can detect if anything changes.


In addition to acting as a pure correctness mechanism, the design of a lock file given here also works as a performance improvement. Instead of requiring that all snapshot files be fully parsed on each Stack invocation, we can store information in the lock file and bypass parsing of the additional files in the common case of no changes.

Lock file contents

The lock file contains the following information:

  • Completed package locations for extra-deps and packages in snapshot files


    This only applies to immutable packages. Mutable packages are not included in the lock file.

  • Completed information for the snapshot locations

It looks like the following:

# Lock file, some message about the file being auto-generated
  # Starts with the snapshot specified in stack.yaml,
  # then continues with the snapshot specified in each
  # subsequent snapshot file
  - original:
      foo.yaml # raw content specified in a snapshot file
      file: foo.yaml
      sha256: XXXX
      size: XXXX
  - original:
      size: 496662
      sha256: 83de9017d911cf7795f19353dba4d04bd24cd40622b7567ff61fc3f7223aa3ea

- original:
    size: 1442
    name: acme-missiles
    version: '0.3'
    sha256: e563d8b524017a06b32768c4db8eff1f822f3fb22a90320b7e414402647b735b
      size: 226
      sha256: 614bc0cca76937507ea0a5ccc17a504c997ce458d7f2f9e43b15a10c8eaeb033

Creation procedure

Whenever a project-level configuration file (stack.yaml, by default) is loaded, Stack checks for a lock file in the same file path, with a .lock extension added. For example, if you command:

stack --stack-yaml my-stack.yaml build


stack --stack-yaml my-stack.yaml build --dry-run

then Stack will use a lock file in the location my-stack.yaml.lock. For the rest of this document, we'll assume that the files are simply stack.yaml and stack.yaml.lock.

If the lock file does not exist, subject to Stack's --lock-file option, it will be created by:

  • Loading the stack.yaml
  • Loading all snapshot files
  • Completing all missing information
  • Writing out the new stack.yaml.lock file to the disk

Update procedure

Whenever a project-level configuration file (stack.yaml, by default) is loaded, all completed package or snapshot locations (even those completed using information from a lock file) get collected to form a new lock file in memory. Subject to Stack's --lock-file option, that new lock file is compared against the one on disk and, if there are any differences, written out to the disk.