| I'm sure there's an easy answer to this, but I'm wondering: why is the | CallStack feature implemented with implicit parameters instead of just | a magical constraint? Whenever I use this feature, I don't want to | have to enable -XImplicitParams and then make sure I get the name | right. What would be wrong with, e.g., | | > undefined :: AppendsCallStack => a | | Seems simpler. Is it problems with a nullary class? Hmm. Actually I think that's quite a good idea. The call-stack idea started here https://ghc.haskell.org/trac/ghc/wiki/ExplicitCallStack/ImplicitLocations. We want to be able to get hold of the current call stack. It obviously doesn't make sense to say currentCallStack :: CallStack But as the call stack is an implicit parameter (operationally), it obviously DOES make sense to say ?currentStack :: (?currentStack :: CallStack) => CallStack if ?currentStack is an implicit parameter. And it went from there. There are disadvantages: * Need to use -XImplicitParams (Richard's point) * Need to align two names: foo :: (?loc :: CallStack) => Int -> Int foo x = if x<0 then error "urk" else -x won't work, because 'error' needs (?callStack :: CallStack) * The special cases in the type checker need a 2-level pattern match: for the magic "IP" class, and then the magic "CallStack" type * In principle you might have multiple call stacks kicking around at the same time boo :: (?a::CallStack, ?b::CallStack) => Int -> Int Now I'm not really sure what is supposed to happen about solving these constraints. Perhaps it could be a feature, but it's not one anyone has asked for, and even having to think about it makes my head hurt. Your alternative suggestion is to have a magic nullary class, the ICallStack class ("I" for implicit) so that class ICallStack where callStack :: CallStack At least that's is the implementation, but all the user can see is the overloaded function callStack :: ICallStack => CallStack The solving rules, the CallStack type, and functions for printing it, would be precisely as now. I like this. What about others? (We'd have to think about what to do for 8.0 but first lets see what we want.) Simon