There was a long discussion of these issues a while back under the title 'updating labelled fields'. The most recent message was from SPJ:
http://www.haskell.org//pipermail/glasgow-haskell-users/2002-May/003374.html You should probably take a look at the whole thread... On Thu, 17 Feb 2005, S. Alexander Jacobson wrote: > I don't know what it takes to get this sort of change into > circulation, but I assume it starts with a clear and specific > description of the problem, exploration of alternatives, and a > specific proposed solution. So here goes: > > ---------- > Proposal: Allow "\=" for field update in record update syntax > > Specifically, intepret > > rec {field\=fn} > > as > > rec {field = fn (field rec)} > > Jusitification: > > It is extremely common when updating records to be updating a field > value based on its prior value rather than simply assigning a new > value from nothing. Unforunately the syntax supports the later > clearly but makes the former awkward to read, understand, and debug: > > myFun db myType1 = > db { tMyType1 = insert myType1 $ tMyType1 db > nextId = 1 + (nextId db) > } > > One solution to this problem that does not involve changing syntax is > to generate field update functions for every field of a type e.g.: > > update_tMyType1 f s = s {tMyType1= f $ myType1 s} > update_nextId f s = s {nextId = f $ nextId s} > > Once you have those functions, myFun looks much better: > > myFun db myType = update_nextId (+1) $ update_tMyType1 f db > where f= insert myType1 > > But, generating the update_ functions is not programming; its > just manual syntax generation. And, even if you have template haskell > to do it, > > * it is annoying to have to find/write the TH code to implement > * update_ is incredibly verbose > * shortening it risks proliferation of psuedo-syntax: > u_tMyType, u__tMyType, or utMyType? > * it adds clutter to have to call the TH for each record type, and > * it pollutes the name space/increases risk of name collision > > Rather than force the user to jump through these hoops, why not make > the syntax flow naturally from the existing record update syntax? It > makes sense to use \= for this task as it is the functional > generalization of C or Java's += or *= . Then myFun would have the > elegant and clear syntax: > > myFun db myType1 =db {tMyType1\=insert myType1, nextId\=(+1)} > > -Alex- > > > ______________________________________________________________ > S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com > _______________________________________________ > Haskell mailing list > Haskell@haskell.org > http://www.haskell.org/mailman/listinfo/haskell > -- Hal Daume III | [EMAIL PROTECTED] "Arrest this man, he talks in maths." | www.isi.edu/~hdaume _______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
