content: {-# LANGUAGE DeriveDataTypeable #-}
  {-# LANGUAGE NamedFieldPuns #-}
  {-# LANGUAGE OverloadedStrings #-}
  {-# LANGUAGE PatternGuards #-}
  {-# LANGUAGE RecordWildCards #-}
  {-# LANGUAGE ViewPatterns #-}
  
  ------------------------------------------------------------------------------
  -- |
  -- Module:      Database.PostgreSQL.Simple
  -- Copyright:   (c) 2011 MailRank, Inc.
  --              (c) 2011-2012 Leon P Smith
  -- License:     BSD3
  -- Maintainer:  Leon P Smith <leon@melding-monads.com>
  -- Stability:   experimental
  --
  -- A mid-level client library for the PostgreSQL database, aimed at ease of
  -- use and high performance.
  --
  ------------------------------------------------------------------------------
  
  module Database.PostgreSQL.Simple
      (
      -- * Writing queries
      -- $use
  
      -- ** The Query type
      -- $querytype
  
      -- ** Parameter substitution
      -- $subst
  
      -- *** Type inference
      -- $inference
  
      -- ** Substituting a single parameter
      -- $only_param
  
      -- ** Representing a list of values
      -- $in
  
      -- ** Modifying multiple rows at once
      -- $many
  
      -- ** @RETURNING@: modifications that return results
      -- $returning
  
      -- * Extracting results
      -- $result
  
      -- ** Handling null values
      -- $null
  
      -- ** Type conversions
      -- $types
  
      -- * Types
        Connection
      , Query
      , ToRow
      , FromRow
      , In(..)
      , Binary(..)
      , Only(..)
      , (:.)(..)
      -- ** Exceptions
      , SqlError(..)
      , PQ.ExecStatus(..)
      , FormatError(..)
      , QueryError(..)
      , ResultError(..)
      -- * Connection management
      , Base.connectPostgreSQL
      , Base.close
      , Base.connect
      , Base.ConnectInfo(..)
      , Base.defaultConnectInfo
      , Base.postgreSQLConnectionString
      -- * Queries that return results
      , query
      , query_
      -- ** Queries taking parser as argument
      , queryWith
      , queryWith_
      -- * Queries that stream results
      , FoldOptions(..)
      , FetchQuantity(..)
      , defaultFoldOptions
      , fold
      , foldWithOptions
      , fold_
      , foldWithOptions_
      , forEach
      , forEach_
      , returning
      -- ** Queries that stream results taking a parser as an argument
      , foldWith
      , foldWithOptionsAndParser
      , foldWith_
      , foldWithOptionsAndParser_
      , forEachWith
      , forEachWith_
      , returningWith
      -- * Statements that do not return results
      , execute
      , execute_
      , executeMany
  --    , Base.insertID
      -- * Transaction handling
      , withTransaction
      , withSavepoint
  --    , Base.autocommit
      , begin
      , commit
      , rollback
      -- * Helper functions
      , formatMany
      , formatQuery
      ) where
  
  import           Data.ByteString.Builder (Builder, byteString, char8)
  import           Control.Applicative ((<$>))
  import           Control.Exception as E
  import           Data.ByteString (ByteString)
  import           Data.Int (Int64)
  import           Data.List (intersperse)
  import           Data.Monoid (mconcat)
  import           Database.PostgreSQL.Simple.Compat ((<>), toByteString)
  import           Database.PostgreSQL.Simple.Cursor
  import           Database.PostgreSQL.Simple.FromField (ResultError(..))
  import           Database.PostgreSQL.Simple.FromRow (FromRow(..))
  import           Database.PostgreSQL.Simple.ToField (Action(..))
  import           Database.PostgreSQL.Simple.ToRow (ToRow(..))
  import           Database.PostgreSQL.Simple.Types
                     ( Binary(..), In(..), Only(..), Query(..), (:.)(..) )
  import           Database.PostgreSQL.Simple.Internal as Base
  import           Database.PostgreSQL.Simple.Internal.PQResultUtils
  import           Database.PostgreSQL.Simple.Transaction
  import qualified Database.PostgreSQL.LibPQ as PQ
  import qualified Data.ByteString.Char8 as B
  
  
  -- | Format a query string.
  --
  -- This function is exposed to help with debugging and logging. Do not
  -- use it to prepare queries for execution.
  --
  -- String parameters are escaped according to the character set in use
  -- on the 'Connection'.
  --
  -- Throws 'FormatError' if the query string could not be formatted
  -- correctly.
  formatQuery :: ToRow q => Connection -> Query -> q -> IO ByteString
  formatQuery conn q@(Query template) qs
      | null xs && '?' `B.notElem` template = return template
      | otherwise = toByteString <$> buildQuery conn q template xs
    where xs = toRow qs
  
  -- | Format a query string with a variable number of rows.
  --
  -- This function is exposed to help with debugging and logging. Do not
  -- use it to prepare queries for execution.
  --
  -- The query string must contain exactly one substitution group,
  -- identified by the SQL keyword \"@VALUES@\" (case insensitive)
  -- followed by an \"@(@\" character, a series of one or more \"@?@\"
  -- characters separated by commas, and a \"@)@\" character. White
  -- space in a substitution group is permitted.
  --
  -- Throws 'FormatError' if the query string could not be formatted
  -- correctly.
  formatMany :: (ToRow q) => Connection -> Query -> [q] -> IO ByteString
  formatMany _ q [] = fmtError "no rows supplied" q []
  formatMany conn q@(Query template) qs = do
    case parseTemplate template of
      Just (before, qbits, after) -> do
        bs <- mapM (buildQuery conn q qbits . toRow) qs
        return . toByteString . mconcat $ byteString before :
                                          intersperse (char8 ',') bs ++
                                          [byteString after]
      Nothing -> fmtError "syntax error in multi-row template" q []
  
  -- Split the input string into three pieces, @before@, @qbits@, and @after@,
  -- following this grammar:
  --
  -- start: ^ before qbits after $
  --     before: ([^?]* [^?\w])? 'VALUES' \s*
  --     qbits:  '(' \s* '?' \s* (',' \s* '?' \s*)* ')'
  --     after:  [^?]*
  --
  -- \s: [ \t\n\r\f]
  -- \w: [A-Z] | [a-z] | [\x80-\xFF] | '_' | '$' | [0-9]
  --
  -- This would be much more concise with some sort of regex engine.
  -- 'formatMany' used to use pcre-light instead of this hand-written parser,
  -- but pcre is a hassle to install on Windows.
  parseTemplate :: ByteString -> Maybe (ByteString, ByteString, ByteString)
  parseTemplate template =
      -- Convert input string to uppercase, to facilitate searching.
      search $ B.map toUpper_ascii template
    where
      -- Search for the next occurrence of "VALUES"
      search bs =
          case B.breakSubstring "VALUES" bs of
              (x, y)
                  -- If "VALUES" is not present in the string, or any '?' characters
                  -- were encountered prior to it, fail.
                  | B.null y || ('?' `B.elem` x)
                 -> Nothing
  
                  -- If "VALUES" is preceded by an identifier character (a.k.a. \w),
                  -- try the next occurrence.
                  | not (B.null x) && isIdent (B.last x)
                 -> search $ B.drop 6 y
  
                  -- Otherwise, we have a legitimate "VALUES" token.
                  | otherwise
                 -> parseQueryBits $ skipSpace $ B.drop 6 y
  
      -- Parse '(' \s* '?' \s* .  If this doesn't match
      -- (and we don't consume a '?'), look for another "VALUES".
      --
      -- qb points to the open paren (if present), meaning it points to the
      -- beginning of the "qbits" production described above.  This is why we
      -- pass it down to finishQueryBits.
      parseQueryBits qb
          | Just ('(', skipSpace -> bs1) <- B.uncons qb
          , Just ('?', skipSpace -> bs2) <- B.uncons bs1
          = finishQueryBits qb bs2
          | otherwise
          = search qb
  
      -- Parse (',' \s* '?' \s*)* ')' [^?]* .
      --
      -- Since we've already consumed at least one '?', there's no turning back.
      -- The parse has to succeed here, or the whole thing fails
      -- (because we don't allow '?' to appear outside of the VALUES list).
      finishQueryBits qb bs0
          | Just (')', bs1) <- B.uncons bs0
          = if '?' `B.elem` bs1
                then Nothing
                else Just $ slice3 template qb bs1
          | Just (',', skipSpace -> bs1) <- B.uncons bs0
          , Just ('?', skipSpace -> bs2) <- B.uncons bs1
          = finishQueryBits qb bs2
          | otherwise
          = Nothing
  
      -- Slice a string into three pieces, given the start offset of the second
      -- and third pieces.  Each "offset" is actually a tail of the uppercase
      -- version of the template string.  Its length is used to infer the offset.
      --
      -- It is important to note that we only slice the original template.
      -- We don't want our all-caps trick messing up the actual query string.
      slice3 source p1 p2 =
          (s1, s2, source'')
        where
          (s1, source')  = B.splitAt (B.length source - B.length p1) source
          (s2, source'') = B.splitAt (B.length p1     - B.length p2) source'
  
      toUpper_ascii c | c >= 'a' && c <= 'z' = toEnum (fromEnum c - 32)
                      | otherwise            = c
  
      -- Based on the definition of {ident_cont} in src/backend/parser/scan.l
      -- in the PostgreSQL source.  No need to check [a-z], since we converted
      -- the whole string to uppercase.
      isIdent c = (c >= '0'    && c <= '9')
               || (c >= 'A'    && c <= 'Z')
               || (c >= '\x80' && c <= '\xFF')
               || c == '_'
               || c == '$'
  
      -- Based on {space} in scan.l
      isSpace_ascii c = (c == ' ') || (c >= '\t' && c <= '\r')
  
      skipSpace = B.dropWhile isSpace_ascii
  
  
  
  buildQuery :: Connection -> Query -> ByteString -> [Action] -> IO Builder
  buildQuery conn q template xs =
      zipParams (split template) <$> mapM (buildAction conn q xs) xs
    where split s =
              let (h,t) = B.break (=='?') s
              in byteString h
                 : if B.null t
                   then []
                   else split (B.tail t)
          zipParams (t:ts) (p:ps) = t <> p <> zipParams ts ps
          zipParams [t] []        = t
          zipParams _ _ = fmtError (show (B.count '?' template) ++
                                    " '?' characters, but " ++
                                    show (length xs) ++ " parameters") q xs
  
  -- | Execute an @INSERT@, @UPDATE@, or other SQL query that is not
  -- expected to return results.
  --
  -- Returns the number of rows affected.
  --
  -- Throws 'FormatError' if the query could not be formatted correctly, or
  -- a 'SqlError' exception if the backend returns an error.
  execute :: (ToRow q) => Connection -> Query -> q -> IO Int64
  execute conn template qs = do
    result <- exec conn =<< formatQuery conn template qs
    finishExecute conn template result
  
  -- | Execute a multi-row @INSERT@, @UPDATE@, or other SQL query that is not
  -- expected to return results.
  --
  -- Returns the number of rows affected.   If the list of parameters is empty,
  -- this function will simply return 0 without issuing the query to the backend.
  -- If this is not desired, consider using the 'Values' constructor instead.
  --
  -- Throws 'FormatError' if the query could not be formatted correctly, or
  -- a 'SqlError' exception if the backend returns an error.
  --
  -- For example,  here's a command that inserts two rows into a table
  -- with two columns:
  --
  -- @
  -- executeMany c [sql|
  --     INSERT INTO sometable VALUES (?,?)
  --  |] [(1, \"hello\"),(2, \"world\")]
  -- @
  --
  -- Here's an canonical example of a multi-row update command:
  --
  -- @
  -- executeMany c [sql|
  --     UPDATE sometable
  --        SET sometable.y = upd.y
  --       FROM (VALUES (?,?)) as upd(x,y)
  --      WHERE sometable.x = upd.x
  --  |] [(1, \"hello\"),(2, \"world\")]
  -- @
  
  executeMany :: (ToRow q) => Connection -> Query -> [q] -> IO Int64
  executeMany _ _ [] = return 0
  executeMany conn q qs = do
    result <- exec conn =<< formatMany conn q qs
    finishExecute conn q result
  
  -- | Execute @INSERT ... RETURNING@, @UPDATE ... RETURNING@, or other SQL
  -- query that accepts multi-row input and is expected to return results.
  -- Note that it is possible to write
  --    @'query' conn "INSERT ... RETURNING ..." ...@
  -- in cases where you are only inserting a single row,  and do not need
  -- functionality analogous to 'executeMany'.
  --
  -- If the list of parameters is empty,  this function will simply return @[]@
  -- without issuing the query to the backend.   If this is not desired,
  -- consider using the 'Values' constructor instead.
  --
  -- Throws 'FormatError' if the query could not be formatted correctly.
  returning :: (ToRow q, FromRow r) => Connection -> Query -> [q] -> IO [r]
  returning = returningWith fromRow
  
  returningWith :: (ToRow q) => RowParser r -> Connection -> Query -> [q] -> IO [r]
  returningWith _ _ _ [] = return []
  returningWith parser conn q qs = do
    result <- exec conn =<< formatMany conn q qs
    finishQueryWith parser conn q result
  
  -- | Perform a @SELECT@ or other SQL query that is expected to return
  -- results. All results are retrieved and converted before this
  -- function returns.
  --
  -- When processing large results, this function will consume a lot of
  -- client-side memory.  Consider using 'fold' instead.
  --
  -- Exceptions that may be thrown:
  --
  -- * 'FormatError': the query string could not be formatted correctly.
  --
  -- * 'QueryError': the result contains no columns (i.e. you should be
  --   using 'execute' instead of 'query').
  --
  -- * 'ResultError': result conversion failed.
  --
  -- * 'SqlError':  the postgresql backend returned an error,  e.g.
  --   a syntax or type error,  or an incorrect table or column name.
  query :: (ToRow q, FromRow r) => Connection -> Query -> q -> IO [r]
  query = queryWith fromRow
  
  -- | A version of 'query' that does not perform query substitution.
  query_ :: (FromRow r) => Connection -> Query -> IO [r]
  query_ = queryWith_ fromRow
  
  -- | A version of 'query' taking parser as argument
  queryWith :: ToRow q => RowParser r -> Connection -> Query -> q -> IO [r]
  queryWith parser conn template qs = do
    result <- exec conn =<< formatQuery conn template qs
    finishQueryWith parser conn template result
  
  -- | A version of 'query_' taking parser as argument
  queryWith_ :: RowParser r -> Connection -> Query -> IO [r]
  queryWith_ parser conn q@(Query que) = do
    result <- exec conn que
    finishQueryWith parser conn q result
  
  -- | Perform a @SELECT@ or other SQL query that is expected to return
  -- results. Results are streamed incrementally from the server, and
  -- consumed via a left fold.
  --
  -- When dealing with small results, it may be simpler (and perhaps
  -- faster) to use 'query' instead.
  --
  -- This fold is /not/ strict. The stream consumer is responsible for
  -- forcing the evaluation of its result to avoid space leaks.
  --
  -- This is implemented using a database cursor.    As such,  this requires
  -- a transaction.   This function will detect whether or not there is a
  -- transaction in progress,  and will create a 'ReadCommitted' 'ReadOnly'
  -- transaction if needed.   The cursor is given a unique temporary name,
  -- so the consumer may itself call fold.
  --
  -- Exceptions that may be thrown:
  --
  -- * 'FormatError': the query string could not be formatted correctly.
  --
  -- * 'QueryError': the result contains no columns (i.e. you should be
  --   using 'execute' instead of 'query').
  --
  -- * 'ResultError': result conversion failed.
  --
  -- * 'SqlError':  the postgresql backend returned an error,  e.g.
  --   a syntax or type error,  or an incorrect table or column name.
  fold            :: ( FromRow row, ToRow params )
                  => Connection
                  -> Query
                  -> params
                  -> a
                  -> (a -> row -> IO a)
                  -> IO a
  fold = foldWithOptions defaultFoldOptions
  
  -- | A version of 'fold' taking a parser as an argument
  foldWith        :: ( ToRow params )
                  => RowParser row
                  -> Connection
                  -> Query
                  -> params
                  -> a
                  -> (a -> row -> IO a)
                  -> IO a
  foldWith = foldWithOptionsAndParser defaultFoldOptions
  
  -- | Number of rows to fetch at a time.   'Automatic' currently defaults
  --   to 256 rows,  although it might be nice to make this more intelligent
  --   based on e.g. the average size of the rows.
  data FetchQuantity
     = Automatic
     | Fixed !Int
  
  data FoldOptions
     = FoldOptions {
         fetchQuantity   :: !FetchQuantity,
         transactionMode :: !TransactionMode
       }
  
  -- | defaults to 'Automatic',  and 'TransactionMode' 'ReadCommitted' 'ReadOnly'
  defaultFoldOptions :: FoldOptions
  defaultFoldOptions = FoldOptions {
        fetchQuantity   = Automatic,
        transactionMode = TransactionMode ReadCommitted ReadOnly
      }
  
  -- | The same as 'fold',  but this provides a bit more control over
  --   lower-level details.  Currently,  the number of rows fetched per
  --   round-trip to the server and the transaction mode may be adjusted
  --   accordingly.    If the connection is already in a transaction,
  --   then the existing transaction is used and thus the 'transactionMode'
  --   option is ignored.
  foldWithOptions :: ( FromRow row, ToRow params )
                  => FoldOptions
                  -> Connection
                  -> Query
                  -> params
                  -> a
                  -> (a -> row -> IO a)
                  -> IO a
  foldWithOptions opts = foldWithOptionsAndParser opts fromRow
  
  -- | A version of 'foldWithOptions' taking a parser as an argument
  foldWithOptionsAndParser :: (ToRow params)
                           => FoldOptions
                           -> RowParser row
                           -> Connection
                           -> Query
                           -> params
                           -> a
                           -> (a -> row -> IO a)
                           -> IO a
  foldWithOptionsAndParser opts parser conn template qs a f = do
      q <- formatQuery conn template qs
      doFold opts parser conn template (Query q) a f
  
  -- | A version of 'fold' that does not perform query substitution.
  fold_ :: (FromRow r) =>
           Connection
        -> Query                  -- ^ Query.
        -> a                      -- ^ Initial state for result consumer.
        -> (a -> r -> IO a)       -- ^ Result consumer.
        -> IO a
  fold_ = foldWithOptions_ defaultFoldOptions
  
  -- | A version of 'fold_' taking a parser as an argument
  foldWith_ :: RowParser r
            -> Connection
            -> Query
            -> a
            -> (a -> r -> IO a)
            -> IO a
  foldWith_ = foldWithOptionsAndParser_ defaultFoldOptions
  
  foldWithOptions_ :: (FromRow r) =>
                      FoldOptions
                   -> Connection
                   -> Query             -- ^ Query.
                   -> a                 -- ^ Initial state for result consumer.
                   -> (a -> r -> IO a)  -- ^ Result consumer.
                   -> IO a
  foldWithOptions_ opts conn query a f = doFold opts fromRow conn query query a f
  
  -- | A version of 'foldWithOptions_' taking a parser as an argument
  foldWithOptionsAndParser_ :: FoldOptions
                            -> RowParser r
                            -> Connection
                            -> Query             -- ^ Query.
                            -> a                 -- ^ Initial state for result consumer.
                            -> (a -> r -> IO a)  -- ^ Result consumer.
                            -> IO a
  foldWithOptionsAndParser_ opts parser conn query a f = doFold opts parser conn query query a f
  
  doFold :: FoldOptions
         -> RowParser row
         -> Connection
         -> Query
         -> Query
         -> a
         -> (a -> row -> IO a)
         -> IO a
  doFold FoldOptions{..} parser conn _template q a0 f = do
      stat <- withConnection conn PQ.transactionStatus
      case stat of
        PQ.TransIdle    -> withTransactionMode transactionMode conn go
        PQ.TransInTrans -> go
        PQ.TransActive  -> fail "foldWithOpts FIXME:  PQ.TransActive"
           -- This _shouldn't_ occur in the current incarnation of
           -- the library,  as we aren't using libpq asynchronously.
           -- However,  it could occur in future incarnations of
           -- this library or if client code uses the Internal module
           -- to use raw libpq commands on postgresql-simple connections.
        PQ.TransInError -> fail "foldWithOpts FIXME:  PQ.TransInError"
           -- This should be turned into a better error message.
           -- It is probably a bad idea to automatically roll
           -- back the transaction and start another.
        PQ.TransUnknown -> fail "foldWithOpts FIXME:  PQ.TransUnknown"
           -- Not sure what this means.
    where
      declare =
        declareCursor conn q
      fetch cursor a =
        foldForwardWithParser cursor parser chunkSize f a
  
      go = bracket declare closeCursor $ \cursor ->
               let loop a = fetch cursor a >>=
                              \r -> case r of
                                      Left a -> return a
                                      Right a -> loop a
                 in loop a0
  
  -- FIXME: choose the Automatic chunkSize more intelligently
  --   One possibility is to use the type of the results,  although this
  --   still isn't a perfect solution, given that common types (e.g. text)
  --   are of highly variable size.
  --   A refinement of this technique is to pick this number adaptively
  --   as results are read in from the database.
      chunkSize = case fetchQuantity of
                   Automatic   -> 256
                   Fixed n     -> n
  
  -- | A version of 'fold' that does not transform a state value.
  forEach :: (ToRow q, FromRow r) =>
             Connection
          -> Query                -- ^ Query template.
          -> q                    -- ^ Query parameters.
          -> (r -> IO ())         -- ^ Result consumer.
          -> IO ()
  forEach = forEachWith fromRow
  {-# INLINE forEach #-}
  
  -- | A version of 'forEach' taking a parser as an argument
  forEachWith :: ( ToRow q )
              => RowParser r
              -> Connection
              -> Query
              -> q
              -> (r -> IO ())
              -> IO ()
  forEachWith parser conn template qs = foldWith parser conn template qs () . const
  {-# INLINE forEachWith #-}
  
  -- | A version of 'forEach' that does not perform query substitution.
  forEach_ :: (FromRow r) =>
              Connection
           -> Query                -- ^ Query template.
           -> (r -> IO ())         -- ^ Result consumer.
           -> IO ()
  forEach_ = forEachWith_ fromRow
  {-# INLINE forEach_ #-}
  
  forEachWith_ :: RowParser r
               -> Connection
               -> Query
               -> (r -> IO ())
               -> IO ()
  forEachWith_ parser conn template = foldWith_ parser conn template () . const
  {-# INLINE forEachWith_ #-}
  
  
  -- $use
  --
  -- SQL-based applications are somewhat notorious for their
  -- susceptibility to attacks through the injection of maliciously
  -- crafted data. The primary reason for widespread vulnerability to
  -- SQL injections is that many applications are sloppy in handling
  -- user data when constructing SQL queries.
  --
  -- This library provides a 'Query' type and a parameter substitution
  -- facility to address both ease of use and security.
  
  -- $querytype
  --
  -- A 'Query' is a @newtype@-wrapped 'ByteString'. It intentionally
  -- exposes a tiny API that is not compatible with the 'ByteString'
  -- API; this makes it difficult to construct queries from fragments of
  -- strings.  The 'query' and 'execute' functions require queries to be
  -- of type 'Query'.
  --
  -- To most easily construct a query, enable GHC's @OverloadedStrings@
  -- language extension and write your query as a normal literal string.
  --
  -- > {-# LANGUAGE OverloadedStrings #-}
  -- >
  -- > import Database.PostgreSQL.Simple
  -- >
  -- > hello :: IO Int
  -- > hello = do
  -- >   conn <- connectPostgreSQL ""
  -- >   [Only i] <- query_ conn "select 2 + 2"
  -- >   return i
  --
  -- A 'Query' value does not represent the actual query that will be
  -- executed, but is a template for constructing the final query.
  
  -- $subst
  --
  -- Since applications need to be able to construct queries with
  -- parameters that change, this library provides a query substitution
  -- capability.
  --
  -- The 'Query' template accepted by 'query' and 'execute' can contain
  -- any number of \"@?@\" characters.  Both 'query' and 'execute'
  -- accept a third argument, typically a tuple. When constructing the
  -- real query to execute, these functions replace the first \"@?@\" in
  -- the template with the first element of the tuple, the second
  -- \"@?@\" with the second element, and so on. If necessary, each
  -- tuple element will be quoted and escaped prior to substitution;
  -- this defeats the single most common injection vector for malicious
  -- data.
  --
  -- For example, given the following 'Query' template:
  --
  -- > select * from user where first_name = ? and age > ?
  --
  -- And a tuple of this form:
  --
  -- > ("Boris" :: String, 37 :: Int)
  --
  -- The query to be executed will look like this after substitution:
  --
  -- > select * from user where first_name = 'Boris' and age > 37
  --
  -- If there is a mismatch between the number of \"@?@\" characters in
  -- your template and the number of elements in your tuple, a
  -- 'FormatError' will be thrown.
  --
  -- Note that the substitution functions do not attempt to parse or
  -- validate your query. It's up to you to write syntactically valid
  -- SQL, and to ensure that each \"@?@\" in your query template is
  -- matched with the right tuple element.
  
  -- $inference
  --
  -- Automated type inference means that you will often be able to avoid
  -- supplying explicit type signatures for the elements of a tuple.
  -- However, sometimes the compiler will not be able to infer your
  -- types. Consider a case where you write a numeric literal in a
  -- parameter tuple:
  --
  -- > query conn "select ? + ?" (40,2)
  --
  -- The above query will be rejected by the compiler, because it does
  -- not know the specific numeric types of the literals @40@ and @2@.
  -- This is easily fixed:
  --
  -- > query conn "select ? + ?" (40 :: Double, 2 :: Double)
  --
  -- The same kind of problem can arise with string literals if you have
  -- the @OverloadedStrings@ language extension enabled.  Again, just
  -- use an explicit type signature if this happens.
  --
  -- Finally, remember that the compiler must be able to infer the type
  -- of a query's /results/ as well as its parameters.  We might like
  -- the following example to work:
  --
  -- > print =<< query_ conn "select 2 + 2"
  --
  -- Unfortunately, while a quick glance tells us that the result type
  -- should be a single row containing a single numeric column, the
  -- compiler has no way to infer what the types are.  We can easily fix
  -- this by providing an explicit type annotation:
  --
  -- > xs <- query_ conn "select 2 + 2"
  -- > print (xs :: [Only Int])
  
  -- $only_param
  --
  -- Haskell lacks a single-element tuple type, so if you have just one
  -- value you want substituted into a query or a single-column result,
  -- what should you do?
  --
  -- The obvious approach would appear to be something like this:
  --
  -- > instance (ToField a) => ToRow a where
  -- >     ...
  --
  -- Unfortunately, this wreaks havoc with type inference, so we take a
  -- different tack. To represent a single value @val@ as a parameter, write
  -- a singleton list @[val]@, use 'Just' @val@, or use 'Only' @val@.
  --
  -- Here's an example using a singleton list:
  --
  -- > execute conn "insert into users (first_name) values (?)"
  -- >              ["Nuala"]
  --
  -- A row of /n/ query results is represented using an /n/-tuple, so
  -- you should use 'Only' to represent a single-column result.
  
  -- $in
  --
  -- Suppose you want to write a query using an @IN@ clause:
  --
  -- > select * from users where first_name in ('Anna', 'Boris', 'Carla')
  --
  -- In such cases, it's common for both the elements and length of the
  -- list after the @IN@ keyword to vary from query to query.
  --
  -- To address this case, use the 'In' type wrapper, and use a single
  -- \"@?@\" character to represent the list.  Omit the parentheses
  -- around the list; these will be added for you.
  --
  -- Here's an example:
  --
  -- > query conn "select * from users where first_name in ?" $
  -- >       Only $ In ["Anna", "Boris", "Carla"]
  --
  -- If your 'In'-wrapped list is empty, the string @\"(null)\"@ will be
  -- substituted instead, to ensure that your clause remains
  -- syntactically valid.
  
  -- $many
  --
  -- If you know that you have many rows of data to insert into a table,
  -- it is much more efficient to perform all the insertions in a single
  -- multi-row @INSERT@ statement than individually.
  --
  -- The 'executeMany' function is intended specifically for helping
  -- with multi-row @INSERT@ and @UPDATE@ statements. Its rules for
  -- query substitution are different than those for 'execute'.
  --
  -- What 'executeMany' searches for in your 'Query' template is a
  -- single substring of the form:
  --
  -- > values (?,?,?)
  --
  -- The rules are as follows:
  --
  -- * The keyword @VALUES@ is matched case insensitively.
  --
  -- * There must be no other \"@?@\" characters anywhere in your
  --   template.
  --
  -- * There must be one or more \"@?@\" in the parentheses.
  --
  -- * Extra white space is fine.
  --
  -- The last argument to 'executeMany' is a list of parameter
  -- tuples. These will be substituted into the query where the @(?,?)@
  -- string appears, in a form suitable for use in a multi-row @INSERT@
  -- or @UPDATE@.
  --
  -- Here is an example:
  --
  -- > executeMany conn
  -- >   "insert into users (first_name,last_name) values (?,?)"
  -- >   [("Boris","Karloff"),("Ed","Wood")]
  --
  -- The query that will be executed here will look like this
  -- (reformatted for tidiness):
  --
  -- > insert into users (first_name,last_name) values
  -- >   ('Boris','Karloff'),('Ed','Wood')
  
  -- $returning
  --
  -- PostgreSQL supports returning values from data manipulation statements
  -- such as @INSERT@ and @UPDATE@.   You can use these statements by
  -- using 'query' instead of 'execute'.   For multi-tuple inserts,
  -- use 'returning' instead of 'executeMany'.
  --
  -- For example, were there an auto-incrementing @id@ column and
  -- timestamp column @t@ that defaulted to the present time for the
  -- @sales@ table, then the following query would insert two new
  -- sales records and also return their new @id@s and timestamps.
  --
  -- > let q = "insert into sales (amount, label) values (?,?) returning id, t"
  -- > xs :: [(Int, UTCTime)] <- query conn q (15,"Sawdust")
  -- > ys :: [(Int, UTCTime)] <- returning conn q [(20,"Chips"),(300,"Wood")]
  
  -- $result
  --
  -- The 'query' and 'query_' functions return a list of values in the
  -- 'FromRow' typeclass. This class performs automatic extraction
  -- and type conversion of rows from a query result.
  --
  -- Here is a simple example of how to extract results:
  --
  -- > import qualified Data.Text as Text
  -- >
  -- > xs <- query_ conn "select name,age from users"
  -- > forM_ xs $ \(name,age) ->
  -- >   putStrLn $ Text.unpack name ++ " is " ++ show (age :: Int)
  --
  -- Notice two important details about this code:
  --
  -- * The number of columns we ask for in the query template must
  --   exactly match the number of elements we specify in a row of the
  --   result tuple.  If they do not match, a 'ResultError' exception
  --   will be thrown.
  --
  -- * Sometimes, the compiler needs our help in specifying types. It
  --   can infer that @name@ must be a 'Text', due to our use of the
  --   @unpack@ function. However, we have to tell it the type of @age@,
  --   as it has no other information to determine the exact type.
  
  -- $null
  --
  -- The type of a result tuple will look something like this:
  --
  -- > (Text, Int, Int)
  --
  -- Although SQL can accommodate @NULL@ as a value for any of these
  -- types, Haskell cannot. If your result contains columns that may be
  -- @NULL@, be sure that you use 'Maybe' in those positions of your
  -- tuple.
  --
  -- > (Text, Maybe Int, Int)
  --
  -- If 'query' encounters a @NULL@ in a row where the corresponding
  -- Haskell type is not 'Maybe', it will throw a 'ResultError'
  -- exception.
  
  -- $only_result
  --
  -- To specify that a query returns a single-column result, use the
  -- 'Only' type.
  --
  -- > xs <- query_ conn "select id from users"
  -- > forM_ xs $ \(Only dbid) -> {- ... -}
  
  -- $types
  --
  -- Conversion of SQL values to Haskell values is somewhat
  -- permissive. Here are the rules.
  --
  -- * For numeric types, any Haskell type that can accurately represent
  --   all values of the given PostgreSQL type is considered \"compatible\".
  --   For instance, you can always extract a PostgreSQL 16-bit @SMALLINT@
  --   column to a Haskell 'Int'.  The Haskell 'Float' type can accurately
  --   represent a @SMALLINT@, so it is considered compatble with those types.
  --
  -- * A numeric compatibility check is based only on the type of a
  --   column, /not/ on its values. For instance, a PostgreSQL 64-bit
  --   @BIGINT@ column will be considered incompatible with a Haskell
  --   'Int16', even if it contains the value @1@.
  --
  -- * If a numeric incompatibility is found, 'query' will throw a
  --   'ResultError'.
  --
  -- * The 'String' and 'Text' types are assumed to be encoded as
  --   UTF-8. If you use some other encoding, decoding may fail or give
  --   wrong results. In such cases, write a @newtype@ wrapper and a
  --   custom 'Result' instance to handle your encoding.
  