Can GHC or some lint tool tell me when I've provided a type signature for a function that could be more polymorphic?
GHC doesn't do this, and a quick search of Hackage turns up nothing.
A simple, but possibly quite effective way to implement such a thing would be to load the module in GHCi, use :browse to get all the type signatures, then load a copy without any type signatures, use :browse again, and compare the two outputs; then just print all the lines that differ beyond parentheses, whitespace and alpha-renaming. However, this wouldn't work perfectly, especially if you have definitions whose types can't be inferred.
I have a feeling such a tool would turn up a lot of false positives in practice.
来源：https://stackoverflow.com/questions/9986508/can-i-get-warnings-about-overly-restrictive-type-signatures