[racket] Cyclic data structures in Typed Racket

Matthias Felleisen writes:

 > I guess the real problem is to create suitable default values so 
 > that the pass the type checker or to figure out a cast that forces
 > TR to accept some other value as the default value (and making sure
 > you never use the object). 
 > 
 > Undefined not completely solved. Argh. -- Matthias

A related issue is the requirement to make data structures mutable
in order to allow cyclic data. That's a lot of risk to take for
a one-time use.

Could single-assignment cells be part of a solution to both issues?
Instead of mutable cells in a struct (or list ...) , one would have
single-assignment cells. They are initialized to undefined, and
assignment is only allowed from undefined to some other value, which
means it can happen only once.

A special code block (Racket's share) would allow the creation and
initialization and guarantee that everything is fully initialized
in the end, meaning that the type checker doesn't have to worry
about possible undefined values in the rest of the code.

A variant on this idea is Clojure's transients: data structures that
are created mutable, but are locked explicitly after initialization to
become immutable. That's much more general, but probably also much
harder to trace by a type checker.

Konrad.