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

[Ben Gamari]

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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