Teal's ambiguous grammar

[parke]

Does it bother you that Teal's grammar is ambiguous? For example:

local type foo = nil print ( foo )

In the above line, local type and foo = nil can be parsed as separate statements according to Teal's grammar.

This is due to the fact that type is a Name, not a keyword. (In addition to type, it appears there are 7 other Names that are used as faux-keywords in Teal's grammar.)

At present, the below line is also grammatically and syntactically correct (at least according to the grammar), yet it too results in syntax errors:

local record foo = nil

Whereas, local record ; foo = nil generates non-syntax errors.

It might be possible to remove the ambiguity by replacing this:

+      ‘local’ attnamelist [‘:’ typelist] [‘=’ explist] |

with the below two lines (as has been done for global):

*      ‘local’ attnamelist ‘:’ typelist [‘=’ explist] |
*      ‘local’ attnamelist ‘=’ explist |

I have not been able to come up with a "real world" example where the ambiguity is likely to cause a serious problem.

It might be worth pointing out on the grammar page that the 8 faux-keywords are Names that get special treatment in certain contexts, and that Teal's reserved keywords are exactly the same as Lua's reserved keywords.

[hishamhm]

That's a fun analysis! And yeah, the ambiguity doesn't really bother me. There's no way to make extensions that are guaranteed not to break in the future anyway, since Lua itself may change, so instead of trying to guess by using free sigils and the like (which may conflict with a future Lua version!), I went with a practical approach of adding these extensions which are readable, and which, as you said, are unlikely to cause real-world conflicts. Further, we keep them as soft keywords, so that they can still be used in non-conflicting contexts. Lua itself has an ambiguous grammar with a tie-break decided by the parser implementation (in function calls of parenthesised expressions), so there's even precedent for that. If somebody hits a non-contrived real-world issue, we might revisit that, but so far I think Teal's additions to the Lua grammar have been pretty unproblematic!

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