Define user facing executable targets

Defines a separate external workspace `@stackage-exe` for a
`stack_snapshot` named `@stackage` that will contain aliases pointing to
the Cabal executable targets to packages in `@stackage`.

haskell/cabal.bzl

@@ -1202,17 +1202,18 @@ haskell_cabal_library(
                 build_file_builder.append(
                     """
 haskell_cabal_binary(
-    name = "{name}_exe_{exe}",
+    name = "_{name}_exe_{exe}",
     exe_name = "{exe}",
     flags = {flags},
     srcs = glob(["{dir}/**"]),
     deps = {deps},
     tools = {tools},
-    visibility = {visibility},
+    visibility = ["@{workspace}-exe//{name}:__pkg__"],
     compiler_flags = ["-w", "-optF=-w"],
     verbose = {verbose},
 )
 """.format(
+                        workspace = repository_ctx.name,
                         name = package.name,
                         exe = exe,
                         flags = package.flags,
@@ -1246,6 +1247,41 @@ _stack_snapshot = repository_rule(
     },
 )
 
+def _stack_executables_impl(repository_ctx):
+    workspace = repository_ctx.name[:-len("-exe")]
+    all_components = dicts.add(
+        _default_components,
+        repository_ctx.attr.components,
+    )
+    for (package, components) in all_components.items():
+        executables = _parse_components(package, components).exe
+        build_file_builder = []
+        build_file_builder.append(
+            """load("@{workspace}//:packages.bzl", "packages")""".format(
+                workspace = workspace,
+            ),
+        )
+        for exe in executables:
+            build_file_builder.append("""\
+alias(
+    name = "{exe}",
+    actual = "@{workspace}//:_{package}_exe_{exe}",
+    visibility = ["//visibility:public"],
+) if "{package}" in packages.keys() else None
+""".format(
+                workspace = workspace,
+                package = package,
+                exe = exe,
+            ))
+        repository_ctx.file(package + "/BUILD.bazel", executable = False, content = "\n".join(build_file_builder))
+
+_stack_executables = repository_rule(
+    _stack_executables_impl,
+    attrs = {
+        "components": attr.string_list_dict(),
+    },
+)
+
 def _stack_update_impl(repository_ctx):
     stack_cmd = repository_ctx.path(repository_ctx.attr.stack)
     _execute_or_fail_loudly(repository_ctx, [stack_cmd, "update"])
@@ -1341,6 +1377,7 @@ _fetch_stack = repository_rule(
 """Find a suitably recent local Stack or download it."""
 
 def stack_snapshot(
+        name,
         stack = None,
         extra_deps = {},
         vendored_packages = {},
@@ -1430,6 +1467,7 @@ def stack_snapshot(
       ```
 
     Args:
+      name: The name of the Bazel workspace.
       snapshot: The name of a Stackage snapshot. Incompatible with local_snapshot.
       local_snapshot: A custom Stack snapshot file, as per the Stack documentation.
         Incompatible with snapshot.
@@ -1481,6 +1519,7 @@ def stack_snapshot(
         stack = stack,
     )
     _stack_snapshot(
+        name = name,
         stack = stack,
         # Dependency for ordered execution, stack update before stack unpack.
         stack_update = "@rules_haskell_stack_update//:stack_update",
@@ -1501,6 +1540,10 @@ def stack_snapshot(
         verbose = verbose,
         **kwargs
     )
+    _stack_executables(
+        name = name + "-exe",
+        components = components,
+    )
 
 def _expand_make_variables(name, ctx, strings):
     extra_label_attrs = [