Avoiding "No explicit method ..." warnings

George Russell ger@tzi.de
Tue, 21 Jan 2003 13:30:37 +0100

This isn't a bug, just a suggestion.  It's not even a very important
suggestion, but one that might be worth implementing if it's easy and you can
find the the time.  Or perhaps I am just doing things the wrong way?

The point is that I sometimes have something like the following situation

class ComplicatedClass x where
   simpleTitleFn :: x -> String
   muchMoreComplicatedTitleFn :: extra arguments -> x -> IO (WithError (Source blah blah blah String)
   muchMoreComplicatedTitleFn _ x = [ ... some expression involving simpleTitleFn ...]

The idea is that only muchMoreComplicatedTitleFn must always work; however instances may
choose to implement it directly, or implement the much simpler function simpleTitleFn
(if that does all they want).  

At the moment the situation is that someone who defines just "muchMoreComplicatedTitleFn"
will get an unnecessary warning message from the compiler about "No explicit method or
default method for simpleTitleFn".  I suggest instead introducing a new class of
optional method (for example, via a pragma {-# OPTIONAL_METHOD simpleTitleFn #-}) which
compiles exactly as normal except that (1) no warning is given for instances which don't
define it; (2) a warning is given whenever anyone outside the class declaration *uses*