On Tuesday 15 March 2011 20:28:38, aditya siram wrote:
Untested, but you might try:
instance (Num t) => YesNo t where ....
Careful with that. That doesn't work as one expects at first, if you have that and try to also have an instance YesNo Bool where yesno = id you'll get a compiler error. For instance selection, only the part after the "=>" in instance (Num t) => YesNo t where ... is taken into account, so that basically says "every type [because every type is unifiable with 't'] is an instance of YesNo - but if you try to use yesno on a type which is not an instance of Num, you'll get a compile error". You can allow both instance declarations by turning on OverlappingInstances, but that's not something to do light-heartedly because that opens the way for a number of other problems. Regarding the original question, that sort of boilerplate is often unnecessary because in real programmes, the type of yesno's argument can usually be determined from the context, which isn't available at the ghci prompt. At the prompt, you could specify the type of the argument directly, ghci> yesno (10 :: Int) True which may be less inconvenient than adding instances to satisfy ghci's defaulting algorithm. Otherwise, - if you know you don't need any other instances, deech's suggestion is great - if you know that using OverlappingInstances is safe, deech's suggestion is great - if you need other instances and can't/don't want to use OverlappingInstances, you can reduce the burden of the boilerplate by using preprocessor macros or Template Haskell (the latter is GHC only, so if you want portability, it's macros or code generation).