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.yamlfile 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 local packages and local snapshots are the same on those machines
stack buildin 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
- the parent snapshot (
Some of this information can be incomplete. Consider this
This information is incomplete. For example, the extra-deps may change in the
future. Instead, you could specify enough information in the
to fully resolve that package. That looks like:
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:
Users don't particularly feel like writing all of that. Therefore, it's common
to see incomplete information in a
Recursive snapshot layers¶
Snapshot files can be recursive, where
stack.yaml refers to
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
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 snapshots: # 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 completed: file: foo.yaml sha256: XXXX size: XXXX - original: lts-13.9 completed: size: 496662 url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/13/9.yaml sha256: 83de9017d911cf7795f19353dba4d04bd24cd40622b7567ff61fc3f7223aa3ea packages: - original: https://hackage.haskell.org/package/acme-missiles-0.3.tar.gz completed: size: 1442 url: https://hackage.haskell.org/package/acme-missiles-0.3.tar.gz name: acme-missiles version: '0.3' sha256: e563d8b524017a06b32768c4db8eff1f822f3fb22a90320b7e414402647b735b pantry-tree: size: 226 sha256: 614bc0cca76937507ea0a5ccc17a504c997ce458d7f2f9e43b15a10c8eaeb033
Whenever a project-level configuration file (
stack.yaml) is loaded, Stack
checks for a lock file in the same file path, with a
.lock extension added.
For example, if you command:
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
If the lock file does not exist, it will be created by:
- Loading the
- Loading all snapshot files
- Completing all missing information
- Writing out the new
When loading a Stack project all completed package or snapshot locations (even when they were completed using information from a lock file) get collected to form a new lock file in memory and compare against the one on disk, writing if there are any differences.