Repa — How to make a Read instance?

Question

What is the best way to make

type Configuration = Array DIM1 (Double, Double, Double)

an instance of Read? So later I could derive

data SimulationData = SD Configuration Double StdGen Int

to be an instance of Read too.

Answer 1

Such an instance will be an orphan instance, which you should generally avoid. However, it's fairly simple to write it:

{-# LANGUAGE TypeOperators #-}

import Data.Array.Repa (Array, Shape, Elt, Z(..), (:.)(..))
import qualified Data.Array.Repa as R

instance Read Z where
  readsPrec _ r = do
    ("Z", s) <- lex r
    return (Z, s)

instance (Read tail, Read head) => Read (tail :. head) where
  readsPrec d =
    readParen (d > prec) $ \r -> do
      (tl, s) <- readsPrec (prec + 1) r
      (":.", t) <- lex s
      (hd, u) <- readsPrec (prec + 1) t
      return (tl :. hd, u)
    where prec = 3

instance (Shape sh, Read sh, Elt a, Read a) => Read (Array sh a) where
  readsPrec d =
    readParen (d > app) $ \r -> do
      ("Array", s) <- lex r
      (sh, t) <- readsPrec (app + 1) s
      (xs, u) <- readsPrec (app + 1) t
      return (R.fromList sh xs, u)
    where app = 10

If you use the StandaloneDeriving extension, the first two instances can be simplified:

deriving instance Read Z
deriving instance (Read tail, Read head) => Read (tail :. head)

These instances should probably be in repa itself; I just based them on the example instance given in Text.Show and repa's show output. I suggest making a feature request on repa's bug tracker, and putting these instances into a module of your program for now (unless you want to avoid orphan instances entirely, in which case you'll have to solve the problem another way altogether).

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That said, you should probably consider simply converting your data to a list (with toList) and using that; it avoids the orphan instance, and shouldn't have any downsides. You might also want to consider using a "real" serialisation library like cereal if you're more interested in processing the data with code than having it be human-readable; Read is generally considered to be of rather limited use.