The darcs 2.5 package uses the flat -fglasgow-exts, so it picks up MonoLocalBinds. This causes quite a few errors. The error messages are particularly bad in src/Darcs/Commands/Convert.lhs there is code of the form explicitlyPolyMorphicFun arg $- \local -> ... very long body ... let localdef = _ in ... more long body ... where failure to generalize the inner let is reported as insufficient polymorphism at the outer annotation. Glancing over the paper, it seems like the specific problem is just deferring equality constraints between ground types. Can't that be handled with a notion of constrait normalization?