Simon Marlow
Simon - GHC provides some protection against mistyped pragma names, in the form of the -Wunrecognised-pragmas warning, but only for {-# ... #-} pragmas. If tools decide to use their own pragma syntax, they don't benefit from this. That's one downside, in addition to the others that Neil mentioned.
You might say we shouldn't care about mistyped pragma names. If the user accidentally writes {- HLNIT -} and it is silently ignored, that's not our problem. OK, but we cared about it enough for the pragmas that GHC understands to add the special warning, and it's reasonable to expect that HLint users also care about it.
If this is the case then in my opinion HLint should be the one that checks for mis-spelling. If we look beyond HLint, there is no way that GHC could know generally what tokens are misspelled pragmas and which are tool names. I'm trying to view the pragma question from the perspective of setting a precedent for other tools. If a dozen Haskell tools were to approach us tomorrow and ask for similar treatment to HLint it's clear that hardcoding pragma lists in the lexer would be unsustainable. Is this likely to happen? Of course not. However, it is an indication to me that the root cause of this current debate is our lack of a good extensible pragmas. It seems to me that introducing a tool pragma convention, from which tool users can claim namespaces at will, is the right way to fix this. Cheers, - Ben