Type checking a multithreaded functional language with session types

We define a language whose type system, incorporating session types, allows com-plex protocols to be specified by types and verified by static typechecking. A ses-sion type, associated with a communication channel, specifies the state transitionsof a protocol and also the data types of messages associated with transitions; thustypechecking can verify both correctness of individual messages and correctness ofsequences of transitions. Previously, session types have mainly been studied in thecontext of the π-calculus; instead, our formulation is based on a multi-threaded func-tional language with side-effecting input/output operations. Our typing judgementsstatically describe dynamic changes in the types of channels, and our function typesnot only specify argument and result types but also describe changes in channels.We formalize the syntax, semantics and type checking system of our language, andprove subject reduction and runtime type safety theorems.