Database.Beam

Run a query determined by the given syntax, providing an action that will be called to consume the results from the database (if any). The action will get a reader action that can be used to fetch the next row. When this reader action returns Nothing, there are no rows left to consume. When the reader action returns, the database result is freed.

Run the given command and don't consume any results. Useful for DML statements like INSERT, UPDATE, and DELETE, or DDL statements.

Run the given command and fetch the unique result. The result is Nothing if either no results are returned or more than one result is returned.

Run the given command and fetch the first result. The result is Nothing if no results are returned. This is not guaranteed to automatically limit the query to one result.

Run the given command, collect all the results, and return them as a list. May be more convenient than runReturningMany, but reads the entire result set into memory.

Parses a beam row. This should not fail, except in the case of an internal bug in beam deserialization code. If it does fail, this should throw a BeamRowParseError.

Lift a computation from the IO monad. This allows us to run IO computations in any monadic stack, so long as it supports these kinds of operations (i.e. IO is the base monad for the stack).

Example

import Control.Monad.Trans.State -- from the "transformers" library

printState :: Show s => StateT s IO ()
printState = do
  state <- get
  liftIO $ print state

Had we omitted liftIO, we would have ended up with this error:

• Couldn't match type ‘IO’ with ‘StateT s IO’
 Expected type: StateT s IO ()
   Actual type: IO ()

The important part here is the mismatch between StateT s IO () and IO ().

Luckily, we know of a function that takes an IO a and returns an (m a): liftIO, enabling us to run the program and see the expected results:

> evalStateT printState "hello"
"hello"

> evalStateT printState 3
3