如何通过Erlang的预处理器有条件地排除整个子句？

dhxwm5r4 于 2022-12-08 发布在 Erlang

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I am writing some module in Erlang that is supposed to be compiled on various OTP versions. In some cases I would like to use the unsupported in older versions ++ operator pattern, but only if it is available. Therefore my attempt to this task goes as follows:

f([]) -> empty;
f([_|_]) -> cons;
-if(?OTP_RELEASE >= 23).
f([] ++ _) -> empty_append;
-endif.
f(X) -> error(nah).

The workarounds to this issue I found are:

Move the iffed clause to the beginning. This however does not work well when:
I need to add more -if s
The order of clauses forbids me from doing so for any reason.
Create a beautiful garden of ifelses with whole function copied several times. Obviously, this does not pass.

Is there some convenient way to do it? If possible, I'd appreciate a general solution, not limited to the presented case.

erlang

来源：https://stackoverflow.com/questions/64609631/how-to-conditionally-exclude-whole-clauses-via-erlangs-preprocessor

1条答案

按热度按时间

50few1ms1#

从宏中的流控制：所以如果你想使用-ifdef，你需要多次复制同一个函数。
（我猜这与它们被定义在-和.之间有关）
话虽如此，您可以执行以下操作：

f([]) -> empty;
f([_|_]) -> cons;
f([] ++ _) when ?OTP_RELEASE >= 23 -> empty_append;
f(X) -> error(nah).

您可以使用erlc -S <module>验证是否已从生成的梁ASM代码中删除永远不匹配的子句。此外，编译器将显示警告。
在此步骤中生成的警告不能选择性地忽略。从here：

Another class of warnings is generated by the compiler during optimization and code generation. They warn about patterns that will never match (such as a=b), guards that always evaluate to false, and expressions that always fail (such as atom+42).

Those warnings cannot be disabled (except by disabling all warnings).

如果你想完全避免警告，你需要类似这样的代码（注意，这种风格不会删除分支，因为OtpRelease是一个变量）：

f(Term) ->
    f(Term, ?OTP_RELEASE).

f([], _) -> empty;
f([_|_], _) -> cons;
f([] ++ _, OtpRelease) when OtpRelease >= 23 -> empty_append;
f(_, _) -> error(nah).

EDIT：在上一个示例中，OTP23的代码优化器能够删除无法访问的代码。在此特定情况下，如果f/2未导出，则其第3或第4个子句将根据?OTP_RELEASE被删除）

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如何通过Erlang的预处理器有条件地排除整个子句？

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