[Haskell-cafe] Cyclic data declarations

Michal D. michal.dobrogost at gmail.com
Sun Aug 2 01:25:40 EDT 2009

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"!



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