S. Alexander Jacobson wrote:
I'd like be able to do something like this:
$(label Salary Int) -- template haskell to define salary label main = do person <- readFile "blah" >>= return . read print $ person # salary
In this case, haskell would assume that person has only one label, salary. The read function would ignore all the other labels. If I changed the code to this:
$(label Salary Int) -- template haskell to define salary label $(label Name String)
main = do person <- readFile "blah" >>= return . read print $ person # salary print $ show (person::Name .*. Salary)
Then the code would assume that a person has both a name and a salary.
I see. You would like the pattern of using a data structure would tell the reader which data structure it should have read. This reminds me of how `read' is supposed to be used, although it doesn't quite work when polymorphism is involved (cf. Num-erals). This is an interesting problem; I should think about it. BTW, in the second example, you supply an annotation `Name .*. Salary'. If you're willing to do that, the problem can be solved then. I mean a function asShapeOf that is operationally an identity. You would use it like let _ = person `asShapeOf` (undefined::Name .*. Salary) or let _ = person `asShapeOf` (Name .*. Salary) If you find that approach appropriate, it could be easily implemented.
Separately, I would really like hrecords not to have order dependency. It seems strange to me that (Foo .*. Bar .*. HNil) is a different type from (Bar .*. Foo .*. HNil).
That is indeed strange, and inevitable to the way record polymorphism is attained. One normally does not care about this distinction, because many (most) of polymorphic record consumers are sufficiently polymorphic and so accept either type. That is, if a function accepts any subtype of the particular record type, then the issue of the order of record fields should not arise. One may think of the type of a record with permuted fields as being a subtype of the original record type. There are however many cases where the record type should be closed. Most frequently that case arises when we want to store records in a data structure (e.g., a list). This is also the case of functions like (==) which takes two arguments of the same type (and won't take a subtype). In the OOHaskell library, we have a coercion function (Actually, we have a bunch of such functions, which compute either a meet or a join of several record types). That function will rearrange the fields if necessary (as well as remove extra fields). In HList, it could be implemented via h2projectByLabels. So, one may write [rec1, coerce rec2, coerce rec3] and not to care of the order of fields in rec2 or rec3. Granted, the current implementations of the coercion functions are quite inefficient; I've been meaning to re-write them for more than a year. Perhaps now is the time...