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khibino / haskell-relational-record

This repository includes a joined query generator based on typefull relational algebra, and mapping tools between SQL values list and Haskell record type.
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Provide table name at runtime when building relations #57

Open bssstudio opened 6 years ago

bssstudio commented 6 years ago

Hi! In my use case, I have multiple tables with schema that is exactly the same. What I would like to do is to provide the table name at runtime. I would use one table to generate the definitions with template haskell, but when building a relation, I would provide a different name.

nRel :: String -> Relation () MyData
nRel name = relationName name $ do
  query $ MyData.myData
  ...

Is this currently possible, or might this be a feature request?

igrep commented 6 years ago

Providing a table name as a String can break the type safety of this library. So you should not do that.

I have multiple tables with schema that is exactly the same.

I guess you should make use of HasProjection type class. Given tables with columns foo :: String and bar :: Int, you can write the query like this.

(Updated due to a type error)

nRel
  :: (HasProjection "foo" a String, HasProjection "bar" a Int)
  => Relation () a -> Relation () a
nRel tbl = relation $ do
  t <- query tbl
  wheres $ t ! #bar .=. value "some bar value"
  ...

(You may have to enable DataKinds, OverloadedLabels, and the other related extensions).