maprule() is an improved version of
native.genrule(), with the following advantages:
- Maprule can process source files in parallel,
creating separate actions for each of them.
- Maprule does not require declaring all output
files. Instead you declare templates for the
output files yielded for each source. Therefore
N source files and M templates yield N*M
outputs.
- Maprule supports both Bash and cmd.exe syntax
for its commands via the specialized rules
bash_maprule and cmd_maprule.
- Maprule's cmd attribute does deliberately not
support $(location) expression nor Make
Variables, in order to avoid issues and
challenges with quoting. (In case of cmd.exe
passing empty arguments is impossible). These
paths can be passed as envvars instead.
- Maprule's add_env attribute does support
$(location) expressions (and some extra
placeholders) and is the idiomatic way to pass
execpaths of labels in "tools" or "srcs" (the
shared sources available for all actions) to the
command.
See https://github.com/bazelbuild/bazel/issues/4319
Even though it's not great to use type checks, they are frequently useful for
checking input types of macros.
Because there is no standard way of checking types, at least 2 types of checks
are used:
- `type(foo) == type([])`
- `type(foo) == "list"`
The first option is not very readable and the second option seem to be relying
on an Bazel implementation detail. Encapsulating type checks into this library
enables consistent and easy to understand type checking without explicitly
relying on implementation details.
This version is hash-based (implemented on top of a dictionary) and doesn't suffer the performance problems of the current version. It will eventually replace the old one after a deprecation period.
Thomas Van Lenten