create repository rule for loading and initializing (#62)

install_ws_dependency.bzl

@@ -0,0 +1,39 @@
+def _generate_install_rule_impl(rctx):
+    rctx.download_and_extract(
+        url = rctx.attr.url,
+        stripPrefix = rctx.attr.stripPrefix,
+        output = ".",
+    )
+    text = """workspace(name='{ws_name}')
+load("{init_file}", "{init_function}")
+{init_expression}
+""".format(ws_name = rctx.attr.name,
+ init_file = rctx.attr.init_file,
+  init_function = rctx.attr.init_function,
+  init_expression = rctx.attr.init_expression)
+
+    rctx.file("WORKSPACE", text)
+    rctx.file("BUILD", "")
+
+_generate_install_rule = repository_rule(
+    local = True,
+    implementation = _generate_install_rule_impl,
+    attrs = {
+        "url": attr.string(mandatory = True),
+        "stripPrefix": attr.string(mandatory = False),
+        "init_file": attr.string(mandatory = True),
+        "init_function": attr.string(mandatory = True),
+        "init_expression": attr.string(mandatory = True),
+    },
+)
+
+def install_ws_dependency(repo_name, url, strip_prefix, init_file, init_function, init_expression = None):
+    init_expression = init_expression if init_expression else (init_function + "()")
+    _generate_install_rule(
+        name = repo_name,
+        url = url,
+        stripPrefix = strip_prefix,
+        init_file = init_file,
+        init_function = init_function,
+        init_expression = init_expression
+    )