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doc/../haskell.section.md: Make a bit clearer and more beginner friendly

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maralorn 2023-07-15 02:17:41 +02:00
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doc/languages-frameworks/haskell.section.md
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@ -45,16 +45,17 @@ The attribute names in `haskellPackages` always correspond with their name on
Hackage. Since Hackage allows names that are not valid Nix without escaping,
you need to take care when handling attribute names like `3dmodels`.
For packages that are part of [Stackage], we use the version prescribed by a
Stackage solver (usually the current LTS one) as the default version. For all
other packages we use the latest version from Hackage. See
[below](#haskell-available-versions) to learn which versions are provided
exactly.
For packages that are part of [Stackage] (a curated set of known to be
compatible packages), we use the version prescribed by a Stackage snapshot
(usually the current LTS one) as the default version. For all other packages we
use the latest version from [Hackage](https://hackage.org) (the repository of
basically all open source Haskell packages). See [below](#haskell-available-
versions) for a few more details on this.
Roughly half of the 16K packages contained in `haskellPackages` don't actually
build and are marked as broken semi-automatically. Most of those packages are
deprecated or unmaintained, but sometimes packages that should build, do not
build. Very often fixing them is not a lot of work.
Roughly half of the 16K packages contained in `haskellPackages` don’t actually
build and are [marked as broken semi-automatically](https://github.com/NixOS/nixpkgs/blob/haskell-updates/pkgs/development/haskell-modules/configuration-hackage2nix/broken.yaml).
Most of those packages are deprecated or unmaintained, but sometimes packages
that should build, do not build. Very often fixing them is not a lot of work.
<!--
TODO(@sternenseemann):
@ -126,19 +127,23 @@ Every package set also re-exposes the GHC used to build its packages as `haskell
### Available package versions {#haskell-available-versions}
We aim for a “blessed” package set which only contains one version of each
package, like Stackage (and based on it) but with more packages. Normally in
nixpkgs the number of building Haskell packages is roughly two to three times
the size of Stackage. For choosing the version to use for a certain package we
use the following rules:
package, like [Stackage], which is a curated set of known to be compatible
packages. We use the version information from Stackage snapshots and extend it
with more packages. Normally in Nixpkgs the number of building Haskell packages
is roughly two to three times the size of Stackage. For choosing the version to
use for a certain package we use the following rules:
1. By default, for every package `haskellPackages.foo` is the newest version
found on Hackage (at the time of the last update of our package set).
2. If the Stackage snapshot that we use (usually the newest LTS snapshot)
contains a package, we use the Stackage version as default version for that
package.
3. For some packages, which are not on Stackage, we have manual overrides to
set the default version to a version older than the newest on Hackage. We do
this to get them or their reverse dependencies to compile in our package set.
1. By default, for `haskellPackages.foo` is the newest version of the package
`foo` found on [Hackage](https://hackage.org), which is the central registry
of all open source Haskell packages. Nixpkgs contains a reference to a pinned
Hackage snapshot, thus we use the state of Hackage as of the last time we
updated this pin.
2. If the [Stackage] snapshot that we use (usually the newest LTS snapshot)
contains a package, [we use instead the version in the Stackage snapshot as
default version for that package.](https://github.com/NixOS/nixpkgs/blob/haskell-updates/pkgs/development/haskell-modules/configuration-hackage2nix/stackage.yaml)
3. For some packages, which are not on Stackage, we have if necessary [manual
overrides to set the default version to a version older than the newest on
Hackage.](https://github.com/NixOS/nixpkgs/blob/haskell-updates/pkgs/development/haskell-modules/configuration-hackage2nix/main.yaml)
4. For all packages, for which the newest Hackage version is not the default
version, there will also be a `haskellPackages.foo_x_y_z` package with the
newest version. The `x_y_z` part encodes the version with dots replaced by
@ -146,9 +151,12 @@ underscores. When the newest version changes by a new release to Hackage the
old package will disappear under that name and be replaced by a newer one under
the name with the new version. The package name including the version will
also disappear when the default version e.g. from Stackage catches up with the
newest version from Hackage.
5. For some packages, we also manually add other `haskellPackages.foo_x_y_z`
versions, if they are required for a certain build.
newest version from Hackage. E.g. if `haskellPackages.foo` gets updated from
1.0.0 to 1.1.0 the package `haskellPackages.foo_1_1_0` becomes obsolete and
gets dropped.
5. For some packages, we also [manually add other `haskellPackages.foo_x_y_z`
versions](https://github.com/NixOS/nixpkgs/blob/haskell-updates/pkgs/development/haskell-modules/configuration-hackage2nix/main.yaml),
if they are required for a certain build.
Relying on `haskellPackages.foo_x_y_z` attributes in derivations outside
nixpkgs is discouraged because they may change or disappear with every package