[Haskell-cafe] Re: Partial parsers in Happy

Simon Marlow simonmarhaskell at gmail.com
Tue Apr 17 08:06:40 EDT 2007

Juan Carlos Arevalo Baeza wrote:
>    More info: I managed to do a hack that works around it, but it is 
> clearly not acceptable. Part of the Haskell code generated by Happy 
> contains this:
> ---------------------------------------------------------------------------
> -- Accepting the parse
> -- If the current token is 0#, it means we've just accepted a partial
> -- parse (a %partial parser).  We must ignore the saved token on the top of
> -- the stack in this case.
> happyAccept 0# tk st sts (_ `HappyStk` ans `HappyStk` _) =
>     happyReturn1 ans
> happyAccept j tk st sts (HappyStk ans _) =
>     (happyTcHack j (happyTcHack st)) (happyReturn1 ans)
>    That looked suspect. There's a "tk" parameter that is summarily 
> ignored! So I started mucking with it. ...

You're quite right, Happy does consume an extra token because it always has one 
token of lookahead.  The %partial feature was added mainly so that I could parse 
the header of a module in GHC without parsing the whole module, so I didn't need 
to recover and parse the rest of the module in this case.

By all means fix Happy in whatever way you think is useful, and send me the patch.


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