Hi Edward, I suppose you're right, I could have made this a little more detailed. I keep reading in and processing data in an accumulating way, ie. lets say I have a DB table (don't take this literally please, just an example), into in-memory records of type data MyRecord = MyRecord { name :: String, birthDate :: LocalDate} in a second step I read in Gender information so now I'd need an extended Record data MyRecord = MyRecord { name :: String, birthDate :: LocalDate, gender :: Gender } and so on for another 12 times. In other words I need to extend the record. I had been using tuples, ie: (String, LocalDate) -> Gender -> (String, LocalDate, Gender) but after 6 or so *extensions* things get a tad messy. Anyway this is where HList really shines, but it is a bit akward when you want to state the type of an HList record, ie. type Entg = Record (HCons (LVPair (Label HZero) DRG) (HCons (LVPair (Label (HSucc HZero)) BelegungsTyp) (HCons (LVPair (Label (HSucc (HSucc HZero))) EntgeltBetrag) (HCons (LVPair (Label (HSucc (HSucc (HSucc HZero)))) Zuschläge) (HCons (LVPair (Label (HSucc (HSucc (HSucc (HSucc HZero))))) Abschläge) HNil))))) which happens to be my actual stage 1 data type (Entg), one that has 5 "fields" initial and in several distinct steps 8 more are *added*. So I wonder if there is a more simple way to do this, ie to *extend* the *type* signatures of extend records without such a lot of typing. Günther Am 07.09.2009, 02:44 Uhr, schrieb Edward Z. Yang :

Excerpts from Günther Schmidt's message of Sun Sep 06 19:40:05 -0400 2009:

...

I keep accumulating values and right now use plain tuples for that. I end up with a 12 element tuple and things are a bit messy.

Hi, you may want to consider using the Writer monad. [1]

...

I'd like to use extensible Records from HList instead, thing is I'd like to keep putting type signatures in my code. As it turns out that seems to be where it gets messy with HList.

Perhaps more precisely explaining your problem domain would be useful.

Cheers, Edward

[1] http://www.haskell.org/all_about_monads/html/writermonad.html

2009/9/6 Günther Schmidt :

I keep reading in and processing data in an accumulating way, ie. lets say I have a DB table (don't take this literally please, just an example), into in-memory records of type

data MyRecord = MyRecord {        name :: String,        birthDate :: LocalDate}

in a second step I read in Gender information so now I'd need an extended Record

data MyRecord = MyRecord {        name :: String,        birthDate :: LocalDate,        gender :: Gender }

and so on for another 12 times.

In other words I need to extend the record.

I had been using tuples, ie:

       (String, LocalDate) -> Gender -> (String, LocalDate, Gender)

but after 6 or so *extensions* things get a tad messy.

<snip>

So I wonder if there is a more simple way to do this, ie to *extend* the *type* signatures of extend records without such a lot of typing.

Do you actually need the intermediate data? More specifically, do you need the data to be partial records? One simple way to represent a partial record is a function which takes the missing fields as arguments and returns the complete record, i.e., a constructor like MyRecord :: String -> LocalDate -> Gender -> MyRecord. So a partial result with the name filled in would have type LocalDate -> Gender -> MyRecord. More specifically, if the computations which produce the fields are described using a monad or applicative functor, e.g., getName :: Process String, you can just use <$> and <*> to glue them together. MyRecord <$> getName <*> getDate <*> getGender Or, if your process abstraction is a monad, you can do more complex operations. do name <- getName date <- getDate gender <- getGender name date return $ MyRecord name date gender Instead of constructing intermediate records, all the data is kept in variables until the record is complete. -- Dave Menendez http://www.eyrie.org/~zednenem/