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Snapshot location

:octicons-tag-24: 2.1.1

This document describes the specification of a snapshot location (in the snapshot or resolver key).

!!! info

Stack uses the [Pantry](https://hackage.haskell.org/package/pantry) to
specify the location of snapshots. Pantry is geared towards reproducible
build plans with cryptographically secure specification of snapshots.

There are essentially four different ways of specifying a snapshot location:

  1. Via a compiler version, which is a "compiler only" snapshot. This could be, for example:

    snapshot: ghc-8.6.5

  2. Via a URL pointing to a snapshot configuration file, for example:

    snapshot: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/nightly/2018/8/21.yaml`

  3. Via a local file path pointing to a snapshot configuration file, for example:

    snapshot: my-local-snapshot.yaml

  4. Via a convenience synonym, which provides a short form for some common URLs. These are:

    • GitHub: github:user/repo:path is treated as:

      https://raw.githubusercontent.com/user/repo/master/path

    • LTS Haskell: lts-X.Y is treated (by default) as:

      github:commercialhaskell/stackage-snapshots:lts/X/Y.yaml

      and, consequently, expands to:

      https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/X/Y.yaml

    • Stackage Nightly: nightly-YYYY-MM-DD is treated (by default) as:

      github:commercialhaskell/stackage-snapshots:nightly/YYYY/M/D.yaml

      and, consequently, expands to:

      https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/nightly/YYYY/M/D.yaml

!!! info "Overriding the default snapshot location base"

By default, LTS Haskell and Stackage Nightly snapshot configurations are
retrieved from the `stackage-snapshots` GitHub repository of user
`commercialhaskell`. The
[snapshot-location-base](../configure/yaml/non-project.md#snapshot-location-base)
option allows a custom location to be set.

For safer, more reproducible builds, you can optionally specify a URL together with a cryptographic hash of its content. For example:

snapshot:
  url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/12/0.yaml
  size: 499143
  sha256: 781ea577595dff08b9c8794761ba1321020e3e1ec3297fb833fe951cce1bee11

size is the number of bytes in the file and sha256 is the file's SHA256 hash. If not provided, the information will automatically be generated and stored in a lock file.