On Sat, Aug 1, 2009 at 10:25 PM, Michal D. <michal.dobrogost@gmail.com> wrote:

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] | ...

One of your types could be:
newtype Block = Block [Stmt]

Then you could have:
data Stmt = SIf Test Block Block | ...

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 | ...

Depending on the amount of code duplication, I think I'd actually make two separate data types.

data GStmt = GIf Test Int Int | ...

And have a function:
toFlowGraph :: Stmt -> GStmt

So, in a eureka moment I decided that this should be allowable with
the following declaration:

data Stmt link = SIf Test link link | ...

Clever, but I don't think I would do it this way.

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"!

I'd have to see your parser, but it shouldn't be too hard to work around this depending on how you want to solve it. By the way, for your StmtRec you probably want a newtype instead of a type.

Jason