Like this? Either use a type annotation to specify what 't0' should be based on these potential instance(s): instance Foo Bar -- Defined in 'Foo.Bar' ... and possibly more from other modules that the compiler has not yet encountered or define the required instance 'Foo t0' Not sure how best to present this. To explain this properly it's going to take several lines :\ --- Some other more general suggestions: it'd be nice to have - a unique tag for each GHC error, like 'ambiguous-type-variable' to improve searchability of error messages from GHC. The tag would also remain constant while the message may change over time. - a wiki that documents all the GHC errors. Not merely beginner-level advice, but also explanations of what causes them in all its gory details (so discussions like this could be pasted into that, for example). There is a stub already on https://wiki.haskell.org/GHC/Error_messages but it looks largely abandoned :(