Define default GHC version

This makes the default GHC version publicly accessible and introduces a new
repository which uses the default GHC version if not already defined.

MODULE.bazel

@@ -67,7 +67,6 @@ rules_haskell_dependencies = use_extension(
 use_repo(
     rules_haskell_dependencies,
     "os_info",
-    "rules_haskell_ghc_version",
     "rules_haskell_worker_dependencies",
     "rules_haskell_stack_update",
     "rules_haskell_stack",
@@ -286,6 +285,7 @@ register_toolchains(
 stack_snapshots = use_extension(
     "@rules_haskell//extensions:stack_snapshot.bzl",
     "stack_snapshot",
+    dev_dependency = True,
 )
 
 [
@@ -344,6 +344,13 @@ stack_snapshots.package(
     ]
 ]
 
+ghc_version = use_extension("@rules_haskell//extensions:ghc_version.bzl", "ghc_version", dev_dependency=True)
+
+use_repo(
+    ghc_version,
+    "rules_haskell_ghc_version",
+)
+
 stack_snapshots.stack_snapshot_json(label = "@rules_haskell//:stackage_snapshot.json")
 
 stack_snapshots.snapshot(

haskell/ghc.bzl

@@ -0,0 +1,6 @@
+
+# If you change this, change stackage's version in the start script
+# (see stackage.org).
+
+# Currently, we are using GHC 9.2.x as default.
+DEFAULT_GHC_VERSION = "9.2.5"

haskell/ghc_bindist.bzl

@@ -18,10 +18,9 @@ load(
 )
 load(":private/validate_attrs.bzl", "check_deprecated_attribute_usage")
 load(":private/ghc_bindist_generated.bzl", "GHC_BINDIST")
+load("//haskell:ghc.bzl", "DEFAULT_GHC_VERSION")
 
-# If you change this, change stackage's version in the start script
-# (see stackage.org).
-_GHC_DEFAULT_VERSION = "9.2.5"
+_GHC_DEFAULT_VERSION = DEFAULT_GHC_VERSION
 
 GHC_BINDIST_STRIP_PREFIX = \
     {

haskell/private/ghc_ci.bzl

@@ -1,14 +1,27 @@
+load("@rules_haskell//haskell:ghc.bzl", "DEFAULT_GHC_VERSION")
+
+def _init_ghc_version_repository(repository_ctx, GHC_VERSION):
+    repository_ctx.file("BUILD")
+    repository_ctx.file("ghc_version.bzl", content = "GHC_VERSION = {}".format(repr(GHC_VERSION)))
+
 def _ghc_version_impl(repository_ctx):
-    ghc_version = repository_ctx.os.environ.get("GHC_VERSION")
+    GHC_VERSION = repository_ctx.os.environ.get("GHC_VERSION")
 
-    if ghc_version:
-        print("Using GHC version {} from env variable `GHC_VERSION`".format(ghc_version))
+    if GHC_VERSION:
+        print("Using GHC version {} from env variable `GHC_VERSION`".format(GHC_VERSION))
 
-    repository_ctx.file("BUILD")
-    repository_ctx.file("ghc_version.bzl", content = "GHC_VERSION = {}".format(repr(ghc_version)))
+    _init_ghc_version_repository(repository_ctx, GHC_VERSION)
 
 ghc_version = repository_rule(
     implementation = _ghc_version_impl,
     environ = ["GHC_VERSION"],
     configure = True,
 )
+
+
+def _ghc_default_version_impl(repository_ctx):
+    _init_ghc_version_repository(repository_ctx, DEFAULT_GHC_VERSION)
+
+ghc_default_version = repository_rule(
+    implementation = _ghc_default_version_impl,
+)

haskell/repositories.bzl

@@ -7,7 +7,7 @@ load(
     ":private/versions.bzl",
     "check_bazel_version_compatible",
 )
-load(":private/ghc_ci.bzl", "ghc_version")
+load(":private/ghc_ci.bzl", "ghc_default_version")
 
 _rules_nixpkgs_version = "0c1f8f5470c7f292b7620762e224f53d837929d3"
 _rules_nixpkgs_sha256 = "9e3898a33c5f21f634aa9e2d45620e7c4b6d54d16d473571a891193bbd4725ca"
@@ -18,7 +18,10 @@ _rules_sh_sha256 = "d668bb32f112ead69c58bde2cae62f6b8acefe759a8c95a2d80ff6a85af5
 def rules_haskell_dependencies_bzlmod():
     """Provide rules_haskell dependencies which are not available as bzlmod modules."""
 
-    ghc_version(name = "rules_haskell_ghc_version")
+    maybe(
+        ghc_default_version,
+        name = "rules_haskell_ghc_version"
+    )
 
     # Dependency of com_google_protobuf.
     # TODO(judahjacobson): this is a bit of a hack.