On Fri, May 27, 2011 at 3:11 PM, Eric Rasmussen
<ericrasmussen@gmail.com> wrote:
Stephen, thanks for the link! The paper was an interesting read and definitely gave me some ideas.
Tillmann -- you are correct in that it's very similar to a database.
I frequently go through this process:
1) Receive a flat file (various formats) of tabular data
2) Create a model of the data and a parser for the file
3) Code utilities that allow business users to filter/query/accumulate/compare the files
The models are always changing, so one option would be to inspect a user-supplied definition, build a SQLite database to match, and use Haskell to feed in the data and run queries. However, I'm usually dealing with files that can easily be loaded into memory, and generally they aren't accessed with enough frequency to justify persisting them in a separate format.
"Worth it" in what terms? You're either going to have to encode the relationships yourself, or else automate the process.
It's actually worked fine in the past to code a custom data type with record syntax (or sometimes just tuples) and simply build a list of them, but the challenge in taking this to a higher level is reading in a user-supplied definition, perhaps translated as 'the first column should be indexed by the string "Purchase amount" and contains a Double', and then performing calculations on those doubles based on further user input. I'm trying to get over bad object-oriented habits of assigning attributes at runtime and inspecting types to determine which functions can be applied to which data, and I'm not sure what concepts of functional programming better address these requirements.
My intuition is to use some kind of initial algebra to create a list-like structure /for each record/ For example, with GADTs:.
data Field a = Field { name :: String }
data Value a = Value { value :: a }
> Presumably, your data definition will parse
> into:
data RecordScheme where
NoFields :: RecordScheme
AddField :: Field a -> RecordScheme -> RecordScheme
> And then, given a record scheme, you can
> construct a Table running the appropriate
> queries for the scheme and Populating its
> Records.
data Record where
EndOfRecord :: Record
Populate :: Value a -> Record -> Record
type Table = [Record]