How to work around "select is not iterable" in a macro?

I have a macro called ispc_cc_library. I call it in a BUILD.bazel file this way:

COMMON_DEFINES = select({
    "@platforms//os:osx": [
        "OIDN_BNNS",
        "OIDN_STATIC_LIB",
        "OIDN_FILTER_RT",
        "OIDN_FILTER_RTLIGHTMAP",
    ],
    "//conditions:default": [
        "OIDN_DNNL",
        "OIDN_STATIC_LIB",
        "OIDN_FILTER_RT",
        "OIDN_FILTER_RTLIGHTMAP",
    ],
})

ispc_cc_library(
    name = "input_reorder_ispc",
    srcs = [
        "core/color.isph",
        "core/image.isph",
        "core/input_reorder.ispc",
        "core/math.isph",
        "core/reorder.isph",
        "core/tensor.isph",
        "core/vec.isph",
    ],
    out = "input_reorder_ispc.h",
    defines = COMMON_DEFINES,
    ispc_main_source_file = "core/input_reorder.ispc",
)

When I call the Bazel build command (bazel build //...), I get this error:

ERROR: Traceback (most recent call last):
        File "/private/var/tmp/_bazel_vertexwahn/998288db447b21c21cab4093cf36fa19/external/oidn/BUILD.bazel", line 48, column 16, in <toplevel>
                ispc_cc_library(
        File "/private/var/tmp/_bazel_vertexwahn/998288db447b21c21cab4093cf36fa19/external/rules_ispc/ispc.bzl", line 5, column 11, in ispc_cc_library
                if len(defines) > 0:
Error in len: select is not iterable

The implementation of ispc_cc_library looks like this:

def ispc_cc_library(name, out, ispc_main_source_file, srcs, defines = [], target_compatible_with = [], **kwargs):
    generted_header_filename = out

    ispc_defines_list = ""
    if len(defines) > 0:
        ispc_defines_list = "-D" + " -D".join(defines)

    native.genrule(
        name = "%s_ispc_gen" % name,
        srcs = srcs,
        outs = [name + ".o", generted_header_filename],
        cmd = select({
            "@platforms//os:linux": "$(location @ispc_linux_x86_64//:ispc) %s --target=avx2 --target-os=linux --arch=x86-64 --addressing=64 --pic $(locations %s) --header-outfile=$(location %s) -o $(location %s.o)" % (ispc_defines_list, ispc_main_source_file, generted_header_filename, name),
            "@rules_ispc//:osx_arm64": "$(location @ispc_osx_arm64//:ispc) %s --target=neon --target-os=macos --arch=aarch64 --addressing=64 --pic $(locations %s) --header-outfile=$(location %s) -o $(location %s.o)" % (ispc_defines_list, ispc_main_source_file, generted_header_filename, name),
            "@rules_ispc//:osx_x86_64": "$(location @ispc_osx_x86_64//:ispc) %s --target=sse2 --target-os=macos --arch=x86-64 --addressing=64 --pic $(locations %s) --header-outfile=$(location %s) -o $(location %s.o)" % (ispc_defines_list, ispc_main_source_file, generted_header_filename, name),
            "@platforms//os:windows": "$(location @ispc_windows_x86_64//:ispc) %s --target=avx2 --target-os=windows --arch=x86-64 --addressing=64 $(locations %s) --header-outfile=$(location %s) -o $(location %s.o)" % (ispc_defines_list, ispc_main_source_file, generted_header_filename, name),
        }),
        tools = select({
            "@platforms//os:linux": ["@ispc_linux_x86_64//:ispc"],
            "@rules_ispc//:osx_arm64": ["@ispc_osx_arm64//:ispc"],
            "@rules_ispc//:osx_x86_64": ["@ispc_osx_x86_64//:ispc"],
            "@platforms//os:windows": ["@ispc_windows_x86_64//:ispc"],
        }),
        target_compatible_with = target_compatible_with,
    )
    native.cc_library(
        name = name,
        srcs = [name + ".o"],
        hdrs = [name + ".h"],
        defines = defines,
        target_compatible_with = target_compatible_with,
        **kwargs

It works when I remove the select from COMMON_DEFINES, e.g. on macOS this way:

'''
COMMON_DEFINES = select({
    "@platforms//os:osx": [
        "OIDN_BNNS",
        "OIDN_STATIC_LIB",
        "OIDN_FILTER_RT",
        "OIDN_FILTER_RTLIGHTMAP",
    ],
    "//conditions:default": [
        "OIDN_DNNL",
        "OIDN_STATIC_LIB",
        "OIDN_FILTER_RT",
        "OIDN_FILTER_RTLIGHTMAP",
    ],
})
'''

COMMON_DEFINES = [
    "OIDN_BNNS",
    "OIDN_STATIC_LIB",
    "OIDN_FILTER_RT",
    "OIDN_FILTER_RTLIGHTMAP",
]

As Bazel reports select is not iterable what is the right way to solve this issue? Do I need to convert ispc_cc_library macro to a rule? Or is there a easy workaround?

Is it breaking because macros are evaluated during loading phase and the select is resolved (afterwards) during the analysis phase?

Steps to reproduce:

git clone https://github.com/Vertexwahn/rules_ispc.git
cd rules_ispc/tests/defines
bazel build //defines:main # should work
# now remove comment from line 16 in rules_ispc/tests/defines/BUILD.bazel -> #defines = COMMON_DEFINES,  # this is currenlty not working
# build again
bazel build //defines:main # should fail

>Solution :

You are correct that the problem is the select isn’t resolved until the loading phase, which is after the macro is done. Another way to look at it: if you build the target for the host and the target, your macro is only evaluated once, but the select may resolve differently.

Your situation looks like you should write a rule. The simple approach is wrap invoking your rule and native.cc_library in a macro together. I think all of your logic should be easy to split like that.

You could also integrate with the C/C++ rules and effectively re-implement cc_library using the built-in helpers. For a simple usage of cc_library like you have, that’s pretty straightforwards. I recommend copying the boilerplate from the rules_cc my_c_compile example.

Another approach to this kind of problem is passing around the dicts that go inside a select. You can manipulate those from macros (prepend to every element of the values, add more elements to the end of every value, wrap a string formatting operation around every element of the values, etc), and then wrap them in native.select at the end. That’s kind of a pain to debug though, I would not recommend it here.