Stitch: The Sound Type-Indexed Type Checker (Author's Cut)

RICHARD A. EISENBERG, Bryn Mawr College, USA A classic example of the power of generalized algebraic datatypes (GADTs) to verify a delicate implementation is the type-indexed expression AST. This functional pearl refreshes this example, casting it in modern Haskell using many of GHC’s bells and whistles. The Stitch interpreter is a full executable interpreter, with a parser, type checker, common-subexpression elimination, and a REPL. Making heavy use of GADTs and type indices, the Stitch implementation is clean, idiomatic Haskell and serves as an existence proof that Haskell’s is advanced enough for the use of fancy types in a practical setting.