Dart conditionally imported library

This is a proposal for a new guarded import syntax which allows conditionally either importing or *not* importing a library, and telling at runtime which one it was. The allowed conditions are the same as for the current conditional imports. The import works like a deferred library which is either always loaded or never loaded. We add guards to check whether the library is loaded or not. ## Specification ### Import syntax and semantics The import syntax of conditional imports is changed so you can omit the *first* URI string. In that case, it must be followed by precisely one `if (….) URI` condition, and it must then be followed by a prefix name clause `as name` . It cannot be `deferred`. ```ebnf <importSpecification> ::= `import' <configurableUri> (`deferred' `as' <identifier>)? <combinator>* `;' <configurableUri> ::= <uri> <configurationUri>* ``` becomes ```ebnf <importSpecification> ::= `import' <configurableUri> (`deferred' `as' <identifier>)? <combinator>* `;' | `import' <configurationUri> `as' <identifier> <combinator>* `;' <configurableUri> ::= <uri> <configurationUri>* ``` Example: ```dart import if (dart.library.io) "some_io_library.dart" as some; ``` The prefix name can only be shared with other conditionally imported libraries with the *same* condition. _Alternatively we can allow `<configurationUri>+` in the conditional import, to allow at most one of multiple libraries to be loaded, depending on configuration, but still allow ending up wi

Posted by lrhn on May 12, 2022, 2:48 PM

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