On Sun, Aug 2, 2009 at 9:06 PM, Michal D.
<michal.dobrogost@gmail.com> wrote:
>
> newtype StmtRec = StmtRec (Stmt [StmtRec])
>
That's pretty much were I threw in the towel last night. Except I had
a bunch of places where I had to add the extra constructor statements.
I wish there was a solution that didn't require these... they really
butcher pattern matching clarity.
> will do. More generally, you can use
>
> newtype Fix f = In { out :: f (Fix f) }
>
> and define
>
> type StmtRec = Fix ([] `O` Stmt)
>
> where O denotes composition of functors
>
> newtype O f g a = O (f (g a))
>
Thanks for that! This provoked some thought on my part about what
exactly is going on. I think I could solve this if I added some way to
identify that a type parameter is actually referring to the whole
type. Say we had a reserved word "fixpoint" for this. Then we'd have
something like:
data Stmt x = SIf x x
then when we actually go to use it, it would be referred to as the type:
"Stmt [fixpoint]"
Which would get treated exactly like the data declaration:
data Stmt = SIf [Stmt] [Stmt]
I'll need to add the newtype declaration for the code but I'd be
interested if anyone had further thoughts on this topic. I have an
implementation of both approaches on a toy parser, but I doubt
anyone's interested in seeing that.
Michal