[Haskell-cafe] Handling Postgresql array types

Tom Ellis tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk
Sat Dec 27 11:23:36 UTC 2014

On Fri, Dec 26, 2014 at 09:31:02PM -0800, info at rotnetix.com wrote:
> I would actually like my data IIM to be:
> data IIM = IIM {key :: String
>                ,itype :: String
>                ,idet :: Maybe String
>                ,imat :: [[Double]]} deriving (Read, Show, Eq)
> But I can't figure out how to get a fromRow instance for that

instance FromRow IIM where 
  fromRow = IIM <$> field
                <*> field
                <*> field
                <*> fmap (T.fromPGArray . fmap T.fromPGArray) field

> PS. I started looking into Opaleye as a possible DSL and would appreciate 
> any help you can give me in figuring out how to represent arrays in that. 
>  I am assuming I need to do something with queryRunnerColumn but I was not 
> able to understand the example.

This is indeed somewhat fiddly to do because it requires fiddling with the
implementation of postgresql-simple instances.  Unfortunately that library
does not provide us with enough primitives to do this directly.

I will work on this.


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