haskell-relational-record/helloworld-9.md

Composing relations (for LTS-9)

Copy the following to "helloworld.hs" for LTS-9:

{% highlight haskell %} import Database.Relational.Query import Data.Int (Int32)

hello :: Relation () (Int32, String) hello = relation $ return (value 0 >< value "Hello")

world :: Relation () (Int32, String) world = relation $ return (value 0 >< value "World!")

helloWorld :: Relation () (Int32, String, String) helloWorld = relation $ do h <- query hello w <- query world on $ h ! fst' .=. w ! fst' return $ (,,) |$| h ! fst' || h ! snd' || w ! snd'

main :: IO () main = putStrLn $ show helloWorld ++ ";" {% endhighlight %}