S Peyton Jones, D Vytiniotis, S Weirich, and G Washburn, ICFP 2006, pp50-61 Generalized algebraic data types (GADTs), sometimes known as "guarded recursive data types" or "first-class phantom types", are a simple but powerful generalization of the data types of Haskell and ML. Recent works have given compelling examples of the utility of GADTs, although type inference is known to be difficult. Our contribution is to show how to exploit programmer-supplied type annotations to make the type inference task almost embarrassingly easy. Our main technical innovation is wobbly types, which express in a declarative way the uncertainty caused by the incremental nature of typical type-inference algorithms. [This is a version of Wobbly types: type inference for generalised algebraic data types. Relative to ICFP 2006, the PDF above also describes a simplification to the type system, at the cost of extra annotation burden. This simpler system is the one actually implemented in GHC 6.8]