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authorBjörn Gustavsson <[email protected]>2015-09-29 09:24:32 +0200
committerTomas Abrahamsson <[email protected]>2018-05-09 08:38:32 +0200
commit109b1dc2e346efc37f8c5cfecbf0fa2a476b14fa (patch)
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parent55e852080d459b9b2be18cf41de60c3338f266af (diff)
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epp: Implement the -if and -elif preprocessor directives
Libraries or applications that support more than one major release of OTP may need to use conditional compilation of Erlang source code. Here are few examples where it would be necessary or desirable: * To support a new data type or language feature only available in the latest major release (real-world examples: maps and the stacktrace syntax). * To avoid warnings for deprecated functions. * To avoid dialyzer warnings. Previously, to do conditional compilation, one would have to use a parse transform or some external tool such as 'autoconf'. To simplify conditional compilation, introduce the -if and -elif preprocessor directives, to allow code like this to be written: -if(?OTP_RELEASE =:= 21). %% Code that will only work in OTP 21. -else. %% Fallback code. -endif. What kind of expressions should be allowed after an -if? We certainly don't want to allow anything with a side effect, such as a '!' or a 'receive'. We also don't want it to be possible to call erlang:system_info/1, as that could make the code depedent on features of the run-time system that could change very easily (such as the number of schedulers). Requiring the expression to be a guard expression makes most sense. It is to explain in the documentation and easy for users to understand. For simplicity of implementation, only a single guard expression will be supported; that is, the ',' and ';' syntax for guards is not supported. To allow some useful conditions to be written, there is a special built-in function: defined(Symbol) tests whether the preprocessor symbol is defined, just like -ifdef. The reason for having this defined/1 is that the defined test can be combined with other tests, for example: 'defined(SOME_NAME) andalso ?OTP_RELEASE > 21'.
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