On 3 Aug 2013, at 02:20, Jason Dagit wrote:
>> Hi!
>> Is there any specific reason why GHC is written in a parser GENERATOR
>> (Happy) and not in MONADIC PARSER COMBINATOR (like parsec)?
>>
>> Is Happy faster / handles better errors / hase some great features or
>> anything else?
>
> One reason is that it predates monadic parser libraries.
I'm not entirely sure this is true. I reckon the development of applicative
parser combinators (used in the implementation of the nhc12 compiler, way back
in 1995 or so), is roughly contemporaneous with the development of Happy, and
its use inside ghc. (I found a release note from Sept 1997 that said ghc had
just converted its interface-file parser to use Happy.) Certainly table-driven
parsers in non-functional languages go back a lot further, and functional
combinator-based parsing was then the relative newcomer.
As to why ghc switched to Happy, the literature of the time suggests that
generated table-driven parsers were faster than combinator-based parsers. I'm
not sure I have ever seen any performance figures to back that up however. And
with the general improvement in performance of idiomatic Haskell over the last
twenty years, I'd be interested to see a modern comparison.
Regards,
Malcolm
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