Apart from this, HSE now also parses any unrecognized pragma in option (e.g. LANGUAGE), declaration (e.g. RULES) or expression (e.g. SCC) position, allowing user-customized pragmas. Unrecognized pragmas in other positions will (unfortunately) give a parse error. If this ever means a problem for you - let me know!
This is likely to be a problem for me :-)
I certainly have OPTIONS_DERIVE and CATCH pragmas that I've inserted into various programs over time.
Are you sure that would be a problem? Seems to me that OPTIONS_DERIVE would appear in the same position as any other OPTIONS pragma, in which case HSE will handle it just fine. I don't know where CATCH would appear, but intuitively it sounds like something that would appear as an expression (or statement which is really just an expression), in which case HSE will handle it too. It's only pragmas appearing in "surprising" locations that would cause HSE to balk.
I think failing on an unrecognised pragma is probably a bad idea, when ignoring a pragma is usually perfectly safe.
Well, I have a dilemma here. To ignore a pragma, I need to ignore it at the lexer stage, since once I've passed a lexed token to the parser it needs to be handled in some way, or a parse error ensues. It's not hard to have a set of recognized pragmas that pass the lexer stage, and discard any others as comments. However, I am not fully satisfied with this approach, the reason being that I've had feature requests for HSE to allow for custom, user-defined pragmas. I can't ignore unrecognized pragmas *and* allow custom pragmas. The safe way is obviously to be conservative, but it is also the way that gives the users the least power over pragmas. There is also the option to lex all pragmas, and then pollute the parser so that every primitive could be an unrecognized pragma followed by the actual primitive. That would make it safe in all instances, and user-custom pragmas could be allowed in predefined positions, and discarded by the parser in others. But the cost would be quite high in terms of parser complexity. I could potentially also add the possibility for unknown pragmas to appear in all other positions too - types, patterns etc. But that too would be quite a lot of extra machinery for something that would most likely be used very rarely at best. I'm torn and welcome all input on this matter. Cheers, /Niklas