[Haskell-cafe] types and braces

[Josef Svenningsson]

Conor,

I'd like to point out a few things that may help you on the way.

On Wed, Apr 15, 2009 at 8:58 PM, Conor McBride
<conor at strictlypositive.org> wrote:
>> I don't immediately see what the clash in that context would be - I
>> *think* what you propose should be doable. I'd be interested to know
>> what you come up with, or I might have a look at it myself when I find
>> a few minutes to spare.
>
> I've found that I can add a production
>
> atype :: { Type }
>  ...
>  | '{' trueexp '}'
>
> if I remove the productions for record declarations
>
> constr1 :: { ConDecl }
>      | con '{' '}'                   { RecDecl $1 [] }
>      | con '{' fielddecls '}'        { RecDecl $1 (reverse $3) }
>
> which suggests that it is indeed the syntax
>
>  data Moo = Foo {goo :: Boo Hoo}
>
> which is in apparent conflict with my proposed extension.
> The current parser uses the type parser btype to parse the
> initial segment of constructor declarations, so my change
> causes trouble.
>
> Further trouble is in store from infix constructors
>
>  data Noo = Foo {True} :*: Foo {False}
>
> should make sense, but you have to look quite far to
> distinguish that from a record.
>
> So I don't see that my proposed extension introduces a
> genuine ambiguity, but it does make the parser a bit
> knottier.
>
Remember that even though your parser in unambiguous that doesn't mean
that happy will be able to handle it. Happy deals with LALR grammars
and you have to confine yourself to that restriction in order to make
happy happy. Also, your example above suggests that your grammar might
require an infinite lookahead, something which happy doesn't deal
with.

Having said all this, there is a magic flag which you can give to
happy which will make all these headaches go away. The incantation is
--glr which makes happy produce generalized lr parsers which can deal
even with ambiguous grammars. I've never used this myself so I can't
give you any further advice than to point you in the general
direction. The happy manual is your friend.

Happy happy hacking.

Josef