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lpsmith / postgresql-simple

Mid-level client library for accessing PostgreSQL from Haskell
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Increse safety of parameter subsitution? #241

Open BardurArantsson opened 6 years ago

BardurArantsson commented 6 years ago

So, I've been notificing a pattern in some of my own code (and similarly in postgresql-simple code). There seems to be quite a bit of duplication of functions depending on whether or not query substitution is to be performed. For example, in postgresql-simple we have: 

query :: (ToRow q, FromRow r) => Connection -> Query -> q -> IO [r]
query_ :: FromRow r => Connection -> Query -> IO [r]

queryWith :: ToRow q => RowParser r -> Connection -> Query -> q -> IO [r]
queryWith_ :: RowParser r -> Connection -> Query -> IO [r]

I was just wondering: Do you know if there's any "deep" reason for this? It seems like it would be a straight up win to have separate data constructors, e.g.

data Query =
      ParameterizedQuery ByteString (exists p . ToRow p => [p])
    | RawQuery ByteString

(I'm not sure the type signature actually works, but the point is that a parameter list only requires a ToRow class. It could perhaps also be split out to decouple if from ToRow, but that's a side issue.)

This is obviously not a huge deal, but it would be nice to avoid the combinatorial "explosion".

Any ideas?

lpsmith commented 5 years ago

Yes, you have identified one of my dissatisfactions with the current API, but I haven't thought that much about it.