Algebraic effects based concurrency library in SML. Needs MLton. Uses one-shot continuations (MLton.Thread) to implement the concurrency library. The are monomorphic and polymorphic extensions of deep and shallow handlers. Each variant is illustrated with a simple cooperative concurrency library.
In the monomorphic API, the argument and return type of continuations and handlers are fixed. The polymorphic versions use universal types. Universal types are used to enumlate polymorphic thread-local storage, absent in MLton.Thread library, using a global reference. Particularly, the universal type implementation is "unsafe" (core-dumps on incorrect cast) as opposed to "safe" universal types; unsafe variant is necessary to maintain the polymorphism of global reference (value restriction?) (But is this really necessary?). However, the use of universal types is completely hidden within the library implementation, and is safe. The unsafe universal types are implemented using serialization and requires MLton with serialization support.
The polymorphic functions carry extra type variables when compared to the OCaml signatures since Standard ML does not have existential types to abstract the type variables.