[Haskell-cafe] representing spreadsheets

[Alexander Solla]

On Fri, May 27, 2011 at 3:11 PM, Eric Rasmussen <ericrasmussen at 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]
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