Template Haskell cannot represent type wildcards
There does not appear to be a way for Template Haskell code to generate partial type signature wildcards. The type quasiquote
[t|_|] results in
Wildcard not allowed, and use in an expression quasiquote like
[|() :: _|] results in
Exotic form of type not (yet) handled by Template Haskell _.
It would be useful if TH code could partially constrain the type of the expressions it was generating, without needing to determine the type completely.