Warning about variables with leading underscore that are used anyway
I use -Wall all the time, which includes -fwarn-unused-binds and -fwarn-unused-matches that warn about variable bindings that are not used. It already spotted lots of mistakes for me. You can suppress the warning by prepending an underscore '_' to a variable name. However, I have recently seen code, where variable names with leading underscores are regularly used, where other programmers might have chosen trailing underscores or primes. I suspect that the programmer was not aware, that he disabled warnings about unused bindings this way. Thus I like to have a warning about underscored variables that are used in the sense of the definition given for -fwarn-unused-binds in http://www.haskell.org/ghc/docs/latest/html/users_guide/options-sanity.html. We still have to decide whether this warning should be part of -Wall or -fwarn-unused-binds or whether there should be a separate option like -fwarn-used-underscored-binds.