[Haskell-cafe] Parsing in Practice

Philippa Cowderoy flippa at flippac.org
Tue Oct 18 11:33:49 EDT 2005

On Tue, 18 Oct 2005, Tom Hawkins wrote:

> However, I have a few concerns with Parsec.  First is performance; what 
> factor of slow-down should I expect?  Second is bug prevention.  I don't have 
> much experience writing LL(n) grammars, so how easy is it to introduce bugs 
> in a Parsec grammar?  Even though I hate debugging LALR(1) parsing 
> ambiguities, it prevents problems.

I can't give you figures on performance I'm afraid - I think I saw some 
somewhere but I honestly can't remember where.

It's possible to shoot yourself in the foot with the 
no-backtracking-by-default behaviour, and I've done so a couple of times 
when not thinking although I don't think I've ever spent more than a 
couple of hours spotting it. If you've got a decent chunk of test data or 
don't mind generating it with QuickCheck odds are you can spot it 
reasonably quickly when it happens. You can always litter try everywhere 
and then remove it when you're confident you can do so, too. More 
generally I find it fairly easy to 'just write the grammar' with Parsec, 
though I've not done anything overly complex and I've still done silly 
things like use *bold* as the bold syntax in a wiki clone then realise I 

  * lists
  * like
  * this

which the library doesn't really do anything to help you spot until you 
try out the list syntax and watch it parse as bold because you listed the 
bold parser first. It might be interesting to try writing a variant of 
Parsec designed to search itself for possibilities like that - it's never 
going to be possible to fix it entirely though because being monadic it's 
also comfortably capable of parsing anything a turing machine can. IIRC 
there's a reasonably well-understood type of grammar that corresponds to 
the first-order fragment of Parsec though?

> Also, it appears the GHC developers chose Happy for Haskell.  What was their 
> rational?

Amongst other things, Parsec didn't exist at the time.

flippa at flippac.org

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