Hi all, Olaf Chitil wrote:
Hence I support Jon in that ghc should only allow those type variables a wider scope that have been explicitly declared with the non-Haskell 98 keyword "forall".
I'm inclined to support Jon and Olaf here. I'm pretty sure there's lot of code out there that use "-fglasgow-exts" for all sorts of reasons totally unrelated to scoped type variables. Moreover, I suspect that type signatures for local definitions are quite common too. I, at least, use them a fair bit. So, unless it is 100% clear that the proposed additional way of introducing scoped type variables really is the way to go, I would say go for the version that breaks the fewest programs. If nothing else, if it does turn out to really be the right way, it would seem fairly easy to later generalize the rule, by not requiring an explicit forall (and thus recovering the property that the current type signature syntax is just a syntactic shorthand) than go the other way. Personally, I always get a bit worried when there is too much "magic" going on, e.g. when the meaning of syntactic constructs are stretched in various ways in attempts to "do the right thing" tacitly. I believe it usually better, in the long run at least, to state intentions more explicitly. E.g., in this case, maybe it isn't such a bad idea after all to differentiate syntactically between binding occurrences and referring occurrences of scoped type variables? Thus, also from this perspective it would seem that requiring an explicit forall for introducing scoped type variables would be the better option as it could be argued that intentions are stated a bit more explicitly that way, although maybe not in as explicitly as could be desired. It is, however, unfortunate that a signature without outer foralls no longer would be just a syntactic shorthand for signatures with the foralls mentioned explicitly. Best regards, /Henrik -- Henrik Nilsson School of Computer Science and Information Technology The University of Nottingham nhn@cs.nott.ac.uk This message has been scanned but we cannot guarantee that it and any attachments are free from viruses or other damaging content: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.