Custom Snapshots¶

Custom snapshots were totally reworked with the extensible snapshots overhaul in Stack 1.6.0, see the writeup and PR #3249). This documentation covers the new syntax only.

Custom snapshots allow you to create your own snapshots, which provide a list of packages to use, along with flags, ghc-options, and a few other settings. Custom snapshots may extend any other snapshot that can be specified in a resolver field. The packages specified follow the syntax of extra-deps in the stack.yaml file, with one exception: to ensure reproducibility of snapshots, local directories are not allowed for custom snapshots (as they are expected to change regularly).

resolver: lts-8.21 # Inherits GHC version and package set
compiler: ghc-8.0.1 # Overwrites GHC version in the resolver, optional

name: my-snapshot # User-friendly name

# Additional packages, follows extra-deps syntax
packages:
- unordered-containers-0.2.7.1
- hashable-1.2.4.0
- text-1.2.2.1

# Override flags, can also override flags in the parent snapshot
flags:
  unordered-containers:
    debug: true

# Packages from the parent snapshot to ignore
drop-packages:
- wai-extra

# Packages which should be hidden (affects script command's import
# parser
hidden:
  wai: true
  warp: false

# Set GHC options for specific packages
ghc-options:
  warp:
  - -O2

If you put this in a snapshot.yaml file in the same directory as your project, you can now use the custom snapshot like this:

resolver: snapshot.yaml

This is an example of a custom snapshot stored in the filesystem. They are assumed to be mutable, so you are free to modify it. We detect that the snapshot has changed by hashing the contents of the involved files, and using it to identify the snapshot internally. It is often reasonably efficient to modify a custom snapshot, due to stack sharing snapshot packages whenever possible.

Using a URL instead of a filepath¶

For efficiency, URLs are treated differently. If I uploaded the snapshot to https://domain.org/snapshot-1.yaml, it is expected to be immutable. If you change that file, then you lose any reproducibility guarantees.

Overriding the compiler¶

The following snapshot specification will be identical to lts-7.1, but instead use ghc-7.10.3 instead of ghc-8.0.1:

resolver: lts-7.1
compiler: ghc-7.10.3

Dropping packages¶

The following snapshot specification will be identical to lts-7.1, but without the text package in our snapshot. Removing this package will cause all the packages that depend on text to be unbuildable, but they will still be present in the snapshot.

resolver: lts-7.1
drop-packages:
  - text

Specifying ghc-options¶

In order to specify ghc-options for a package, you use the same syntax as the ghc-options field for build configuration. The following snapshot specification will be identical to lts-7.1, but provides -O1 as a ghc-option for text:

resolver: lts-7.1
packages:
  - text-1.2.2.1
ghc-options:
  text: -O1

This works somewhat differently than the stack.yaml ghc-options field, in that options can only be specified for packages that are mentioned in the custom snapshot's packages list. It sets the ghc-options, rather than extending those specified in the snapshot being extended.

Another difference is that the * entry for ghc-options applies to all packages in the packages list, rather than all packages in the snapshot.

Specifying flags¶

In order to specify flags for a package, you use the same syntax as the flags field for build configuration. The following snapshot specification will be identical to lts-7.1, but it enables the developer cabal flag:

resolver: lts-7.1
packages:
  - text-1.2.2.1
flags:
  text:
    developer: true