[Haskell-beginners] Static records (CSV tables as modules)

[Mark Fredrickson]

Thanks for this solution. I think I could pair this with a data type
generated at runtime to index the vector and I'd be in great shape.

Related question: Does anyone know example code that creates data types at
runtime via TH?

-M



On Fri, May 30, 2014 at 6:40 PM, Ben Gamari <bgamari.foss at gmail.com> wrote:

> Mark Fredrickson <mark.m.fredrickson at gmail.com> writes:
>
> > Hello,
> >
> > I writing a program that operates on some static data, currently saved in
> > CSV files. Right now I parse the files at run time and then generate
> > hashmap tables to connect the different data.
> >
> > Since I'm only ever operating on static data, I'm wondering if I can
> > generate module files that capture the records as a sum type. To access
> the
> > fields of the records, I could then imagine functions that exhaustively
> map
> > the constructors to the data.
> >
> > Do any tools to generate .hs files from CSV or other formats exist?
> Insofar
> > as this question has been asked before, the recommendation is "use
> Template
> > Haskell", which makes sense, but is a less complete solution than I'm
> > hoping for.
> >
> How does the TH hack below look?
>
> See this Gist for this code and a test-case. Unfortunately there are a
> few gotchas here,
>
>   1. The record type needs a `Lift` instance[2]. There are a pain to
>      write but can be derived[3]
>   2. The type of your data can't be defined in the same module as the TH
>      splice due to GHC's stage restriction
>
> Cheers,
>
> - Ben
>
>
> [1] https://gist.github.com/bgamari/efad8560ab7dd38e9407
> [2]
> http://hackage.haskell.org/package/template-haskell-2.9.0.0/docs/Language-Haskell-TH-Syntax.html#t:Lift
> [3] http://hackage.haskell.org/package/th-lift
>
>
> {-# LANGUAGE TemplateHaskell, FlexibleContexts #-}
>
> module StaticCSV (staticCSV) where
>
> import Control.Applicative
> import Data.Csv as Csv hiding (Name)
> import Data.Proxy
> import Data.Data
> import Language.Haskell.TH
> import Language.Haskell.TH.Syntax (Lift, lift)
> import qualified Data.ByteString.Lazy as BSL
> import qualified Data.Vector as V
>
> staticCSV :: (FromRecord a, Lift (V.Vector a)) => FilePath -> Proxy a ->
> ExpQ
> staticCSV fileName ty = do
>     contents <- runIO $ BSL.readFile fileName
>     csv <- case decode NoHeader contents of
>       Right a  -> return $ fmap (flip asProxyTypeOf ty) a
>       Left err -> fail err
>     [| csv |]
>
> instance Lift a => Lift (V.Vector a) where
>     lift v = do
>         list <- ListE <$> mapM lift (V.toList v)
>         return $ AppE (VarE 'V.fromList) list
>
>
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