Creating instances of Abstract Data Types that recursively contain each other

Question

Given two date types defined as follows:

data Foo = Foo Bar String
data Bar = Bar Foo String

How can I make foo and bar such that foo is Foo bar "foo" and bar is Bar foo "bar"?

What about when we change the types to:

data Foo = Foo Bar (MVar String)
data Bar = Bar Foo (MVar String)

Answer 1

Just using a let is enough (let in Haskell is letrec, and supports mutually recursive definitions). The mutually recursive definitions set up cycles in the heap, that look like this:

The MVar initialization doesn't really change things in any meaningful way.

import Control.Concurrent

data Foo = Foo Bar (MVar String)

data Bar = Bar Foo (MVar String)

main = do
    a <- newMVar "foo"
    b <- newMVar "bar"

    let foo = Foo bar a
        bar = Bar foo b

    return ()