Re: [Haskell-cafe] Re: Cyclic data declarations
>
> 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
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[Haskell-cafe] Cyclic data declarations
I'm in the process of writing a toy compiler but I'm having some trouble trying to make my datatypes general. For example, using parsec I parse statements as: data Stmt = SIf Test [Stmt] [Stmt] | ... However, when it's time to create a control flow graph it would be nice to represent statements as (the Int's signify the node id's for either case of the if statement): data Stmt = SIf Test Int Int | ... So, in a eureka moment I decided that this should be allowable with the following declaration: data Stmt link = SIf Test link link | ... Ofcourse, the problem is trying to declare the resulting type for parsing: "parse -> Stmt [Stmt [Stmt ]]". Any hints on whether there is a way to accomplish what I'm trying to do or do I have to bite the bullet and declare two seperate datatypes? I tried being clever and declaring a 'helper' type as "type StmtRec = Stmt [StmtRec]" but to no avail... GHC won't let it slide: "Cycle in type synonym declarations"! Cheers, Michal ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
