    {-# 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
        , SomePostgreSqlException(..)
        , SqlError(..)
        , PQ.ExecStatus(..)
        , FormatError(..)
        , QueryError(..)
        , ResultError(..)
        -- * Connection management
        , Base.connectPostgreSQL
        , Base.close
        , Base.connect
        , Base.withConnect
        , 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 :: forall q. ToRow q => Connection -> Query -> q -> IO ByteString
    formatQuery Connection
    conn q :: Query
    q@(Query ByteString
    template) q
    qs
        | [Action] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [Action]
    xs Bool -> Bool -> Bool
    && Char
    '?' Char -> ByteString -> Bool
    `B.notElem` ByteString
    template = ByteString -> IO ByteString
    forall a. a -> IO a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ByteString
    template
        | Bool
    otherwise = Builder -> ByteString
    toByteString (Builder -> ByteString) -> IO Builder -> IO ByteString
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> Connection -> Query -> ByteString -> [Action] -> IO Builder
    buildQuery Connection
    conn Query
    q ByteString
    template [Action]
    xs
      where xs :: [Action]
    xs = q -> [Action]
    forall a. ToRow a => a -> [Action]
    toRow q
    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 :: forall q. ToRow q => Connection -> Query -> [q] -> IO ByteString
    formatMany Connection
    _ Query
    q [] = String -> Query -> [Action] -> IO ByteString
    forall a. String -> Query -> [Action] -> a
    fmtError String
    "no rows supplied" Query
    q []
    formatMany Connection
    conn q :: Query
    q@(Query ByteString
    template) [q]
    qs = do
      case ByteString -> Maybe (ByteString, ByteString, ByteString)
    parseTemplate ByteString
    template of
        Just (ByteString
    before, ByteString
    qbits, ByteString
    after) -> do
          bs <- (q -> IO Builder) -> [q] -> IO [Builder]
    forall (t :: * -> *) (m :: * -> *) a b.
    (Traversable t, Monad m) =>
    (a -> m b) -> t a -> m (t b)
    forall (m :: * -> *) a b. Monad m => (a -> m b) -> [a] -> m [b]
    mapM (Connection -> Query -> ByteString -> [Action] -> IO Builder
    buildQuery Connection
    conn Query
    q ByteString
    qbits ([Action] -> IO Builder) -> (q -> [Action]) -> q -> IO Builder
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . q -> [Action]
    forall a. ToRow a => a -> [Action]
    toRow) [q]
    qs
          return . toByteString . mconcat $ byteString before :
                                            intersperse (char8 ',') bs ++
                                            [byteString after]
        Maybe (ByteString, ByteString, ByteString)
    Nothing -> String -> Query -> [Action] -> IO ByteString
    forall a. String -> Query -> [Action] -> a
    fmtError String
    "syntax error in multi-row template" Query
    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 :: ByteString -> Maybe (ByteString, ByteString, ByteString)
    parseTemplate ByteString
    template =
        -- Convert input string to uppercase, to facilitate searching.
        ByteString -> Maybe (ByteString, ByteString, ByteString)
    search (ByteString -> Maybe (ByteString, ByteString, ByteString))
    -> ByteString -> Maybe (ByteString, ByteString, ByteString)
    forall a b. (a -> b) -> a -> b
    $ (Char -> Char) -> ByteString -> ByteString
    B.map Char -> Char
    toUpper_ascii ByteString
    template
      where
        -- Search for the next occurrence of "VALUES"
        search :: ByteString -> Maybe (ByteString, ByteString, ByteString)
    search ByteString
    bs =
            case ByteString -> ByteString -> (ByteString, ByteString)
    B.breakSubstring ByteString
    "VALUES" ByteString
    bs of
                (ByteString
    x, ByteString
    y)
                    -- If "VALUES" is not present in the string, or any '?' characters
                    -- were encountered prior to it, fail.
                    | ByteString -> Bool
    B.null ByteString
    y Bool -> Bool -> Bool
    || (Char
    '?' Char -> ByteString -> Bool
    `B.elem` ByteString
    x)
                   -> Maybe (ByteString, ByteString, ByteString)
    forall a. Maybe a
    Nothing

                    -- If "VALUES" is preceded by an identifier character (a.k.a. \w),
                    -- try the next occurrence.
                    | Bool -> Bool
    not (ByteString -> Bool
    B.null ByteString
    x) Bool -> Bool -> Bool
    && Char -> Bool
    isIdent (ByteString -> Char
    B.last ByteString
    x)
                   -> ByteString -> Maybe (ByteString, ByteString, ByteString)
    search (ByteString -> Maybe (ByteString, ByteString, ByteString))
    -> ByteString -> Maybe (ByteString, ByteString, ByteString)
    forall a b. (a -> b) -> a -> b
    $ Int -> ByteString -> ByteString
    B.drop Int
    6 ByteString
    y

                    -- Otherwise, we have a legitimate "VALUES" token.
                    | Bool
    otherwise
                   -> ByteString -> Maybe (ByteString, ByteString, ByteString)
    parseQueryBits (ByteString -> Maybe (ByteString, ByteString, ByteString))
    -> ByteString -> Maybe (ByteString, ByteString, ByteString)
    forall a b. (a -> b) -> a -> b
    $ ByteString -> ByteString
    skipSpace (ByteString -> ByteString) -> ByteString -> ByteString
    forall a b. (a -> b) -> a -> b
    $ Int -> ByteString -> ByteString
    B.drop Int
    6 ByteString
    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 :: ByteString -> Maybe (ByteString, ByteString, ByteString)
    parseQueryBits ByteString
    qb
            | Just (Char
    '(', ByteString -> ByteString
    skipSpace -> ByteString
    bs1) <- ByteString -> Maybe (Char, ByteString)
    B.uncons ByteString
    qb
            , Just (Char
    '?', ByteString -> ByteString
    skipSpace -> ByteString
    bs2) <- ByteString -> Maybe (Char, ByteString)
    B.uncons ByteString
    bs1
            = ByteString
    -> ByteString -> Maybe (ByteString, ByteString, ByteString)
    finishQueryBits ByteString
    qb ByteString
    bs2
            | Bool
    otherwise
            = ByteString -> Maybe (ByteString, ByteString, ByteString)
    search ByteString
    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 :: ByteString
    -> ByteString -> Maybe (ByteString, ByteString, ByteString)
    finishQueryBits ByteString
    qb ByteString
    bs0
            | Just (Char
    ')', ByteString
    bs1) <- ByteString -> Maybe (Char, ByteString)
    B.uncons ByteString
    bs0
            = if Char
    '?' Char -> ByteString -> Bool
    `B.elem` ByteString
    bs1
                  then Maybe (ByteString, ByteString, ByteString)
    forall a. Maybe a
    Nothing
                  else (ByteString, ByteString, ByteString)
    -> Maybe (ByteString, ByteString, ByteString)
    forall a. a -> Maybe a
    Just ((ByteString, ByteString, ByteString)
     -> Maybe (ByteString, ByteString, ByteString))
    -> (ByteString, ByteString, ByteString)
    -> Maybe (ByteString, ByteString, ByteString)
    forall a b. (a -> b) -> a -> b
    $ ByteString
    -> ByteString -> ByteString -> (ByteString, ByteString, ByteString)
    slice3 ByteString
    template ByteString
    qb ByteString
    bs1
            | Just (Char
    ',', ByteString -> ByteString
    skipSpace -> ByteString
    bs1) <- ByteString -> Maybe (Char, ByteString)
    B.uncons ByteString
    bs0
            , Just (Char
    '?', ByteString -> ByteString
    skipSpace -> ByteString
    bs2) <- ByteString -> Maybe (Char, ByteString)
    B.uncons ByteString
    bs1
            = ByteString
    -> ByteString -> Maybe (ByteString, ByteString, ByteString)
    finishQueryBits ByteString
    qb ByteString
    bs2
            | Bool
    otherwise
            = Maybe (ByteString, ByteString, ByteString)
    forall a. Maybe a
    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 :: ByteString
    -> ByteString -> ByteString -> (ByteString, ByteString, ByteString)
    slice3 ByteString
    source ByteString
    p1 ByteString
    p2 =
            (ByteString
    s1, ByteString
    s2, ByteString
    source'')
          where
            (ByteString
    s1, ByteString
    source')  = Int -> ByteString -> (ByteString, ByteString)
    B.splitAt (ByteString -> Int
    B.length ByteString
    source Int -> Int -> Int
    forall a. Num a => a -> a -> a
    - ByteString -> Int
    B.length ByteString
    p1) ByteString
    source
            (ByteString
    s2, ByteString
    source'') = Int -> ByteString -> (ByteString, ByteString)
    B.splitAt (ByteString -> Int
    B.length ByteString
    p1     Int -> Int -> Int
    forall a. Num a => a -> a -> a
    - ByteString -> Int
    B.length ByteString
    p2) ByteString
    source'

        toUpper_ascii :: Char -> Char
    toUpper_ascii Char
    c | Char
    c Char -> Char -> Bool
    forall a. Ord a => a -> a -> Bool
    >= Char
    'a' Bool -> Bool -> Bool
    && Char
    c Char -> Char -> Bool
    forall a. Ord a => a -> a -> Bool
    <= Char
    'z' = Int -> Char
    forall a. Enum a => Int -> a
    toEnum (Char -> Int
    forall a. Enum a => a -> Int
    fromEnum Char
    c Int -> Int -> Int
    forall a. Num a => a -> a -> a
    - Int
    32)
                        | Bool
    otherwise            = Char
    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 :: Char -> Bool
    isIdent Char
    c = (Char
    c Char -> Char -> Bool
    forall a. Ord a => a -> a -> Bool
    >= Char
    '0'    Bool -> Bool -> Bool
    && Char
    c Char -> Char -> Bool
    forall a. Ord a => a -> a -> Bool
    <= Char
    '9')
                 Bool -> Bool -> Bool
    || (Char
    c Char -> Char -> Bool
    forall a. Ord a => a -> a -> Bool
    >= Char
    'A'    Bool -> Bool -> Bool
    && Char
    c Char -> Char -> Bool
    forall a. Ord a => a -> a -> Bool
    <= Char
    'Z')
                 Bool -> Bool -> Bool
    || (Char
    c Char -> Char -> Bool
    forall a. Ord a => a -> a -> Bool
    >= Char
    '\x80' Bool -> Bool -> Bool
    && Char
    c Char -> Char -> Bool
    forall a. Ord a => a -> a -> Bool
    <= Char
    '\xFF')
                 Bool -> Bool -> Bool
    || Char
    c Char -> Char -> Bool
    forall a. Eq a => a -> a -> Bool
    == Char
    '_'
                 Bool -> Bool -> Bool
    || Char
    c Char -> Char -> Bool
    forall a. Eq a => a -> a -> Bool
    == Char
    '$'

        -- Based on {space} in scan.l
        isSpace_ascii :: Char -> Bool
    isSpace_ascii Char
    c = (Char
    c Char -> Char -> Bool
    forall a. Eq a => a -> a -> Bool
    == Char
    ' ') Bool -> Bool -> Bool
    || (Char
    c Char -> Char -> Bool
    forall a. Ord a => a -> a -> Bool
    >= Char
    '\t' Bool -> Bool -> Bool
    && Char
    c Char -> Char -> Bool
    forall a. Ord a => a -> a -> Bool
    <= Char
    '\r')

        skipSpace :: ByteString -> ByteString
    skipSpace = (Char -> Bool) -> ByteString -> ByteString
    B.dropWhile Char -> Bool
    isSpace_ascii

    buildQuery :: Connection -> Query -> ByteString -> [Action] -> IO Builder
    buildQuery :: Connection -> Query -> ByteString -> [Action] -> IO Builder
    buildQuery Connection
    conn Query
    q ByteString
    template [Action]
    xs =
        [Builder] -> [Builder] -> Builder
    forall {t}. Semigroup t => [t] -> [t] -> t
    zipParams (ByteString -> [Builder]
    split ByteString
    template) ([Builder] -> Builder) -> IO [Builder] -> IO Builder
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> (Action -> IO Builder) -> [Action] -> IO [Builder]
    forall (t :: * -> *) (m :: * -> *) a b.
    (Traversable t, Monad m) =>
    (a -> m b) -> t a -> m (t b)
    forall (m :: * -> *) a b. Monad m => (a -> m b) -> [a] -> m [b]
    mapM (Connection -> Query -> [Action] -> Action -> IO Builder
    buildAction Connection
    conn Query
    q [Action]
    xs) [Action]
    xs
      where split :: ByteString -> [Builder]
    split ByteString
    s =
                -- This part escapes double '??'s to make literal '?'s possible
                -- in PostgreSQL queries using the JSON operators: @?@, @?|@ and @?&@
                let (ByteString
    h,ByteString
    t) = ByteString -> (ByteString, ByteString)
    breakOnSingleQuestionMark ByteString
    s
                in ByteString -> Builder
    byteString ByteString
    h
                   Builder -> [Builder] -> [Builder]
    forall a. a -> [a] -> [a]
    : if ByteString -> Bool
    B.null ByteString
    t
                     then []
                     else ByteString -> [Builder]
    split (HasCallStack => ByteString -> ByteString
    ByteString -> ByteString
    B.tail ByteString
    t)
            zipParams :: [t] -> [t] -> t
    zipParams (t
    t:[t]
    ts) (t
    p:[t]
    ps) = t
    t t -> t -> t
    forall a. Semigroup a => a -> a -> a
    <> t
    p t -> t -> t
    forall a. Semigroup a => a -> a -> a
    <> [t] -> [t] -> t
    zipParams [t]
    ts [t]
    ps
            zipParams [t
    t] []        = t
    t
            zipParams [t]
    _ [t]
    _ = String -> Query -> [Action] -> t
    forall a. String -> Query -> [Action] -> a
    fmtError (Int -> String
    forall a. Show a => a -> String
    show Int
    countSingleQs String -> String -> String
    forall a. [a] -> [a] -> [a]
    ++
                                      String
    " single '?' characters, but " String -> String -> String
    forall a. [a] -> [a] -> [a]
    ++
                                      Int -> String
    forall a. Show a => a -> String
    show ([Action] -> Int
    forall a. [a] -> Int
    forall (t :: * -> *) a. Foldable t => t a -> Int
    length [Action]
    xs) String -> String -> String
    forall a. [a] -> [a] -> [a]
    ++ String
    " parameters") Query
    q [Action]
    xs
            countSingleQs :: Int
    countSingleQs = Int -> ByteString -> Int
    go Int
    0 ByteString
    template
              where go :: Int -> ByteString -> Int
    go Int
    i ByteString
    "" = (Int
    i :: Int)
                    go Int
    i ByteString
    bs = case (ByteString, ByteString)
    qms of
                                (ByteString
    "?",ByteString
    "?") -> Int -> ByteString -> Int
    go Int
    i ByteString
    nextQMBS
                                (ByteString
    "?",ByteString
    _) -> Int -> ByteString -> Int
    go (Int
    iInt -> Int -> Int
    forall a. Num a => a -> a -> a
    +Int
    1) ByteString
    nextQMBS
                                (ByteString, ByteString)
    _ -> Int
    i
                      where qms :: (ByteString, ByteString)
    qms = Int -> ByteString -> (ByteString, ByteString)
    B.splitAt Int
    1 ByteString
    qmBS
                            (ByteString
    qmBS,ByteString
    nextQMBS) = Int -> ByteString -> (ByteString, ByteString)
    B.splitAt Int
    2 ByteString
    qmBS'
                            qmBS' :: ByteString
    qmBS' = (Char -> Bool) -> ByteString -> ByteString
    B.dropWhile (Char -> Char -> Bool
    forall a. Eq a => a -> a -> Bool
    /= Char
    '?') ByteString
    bs

    -- | 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 :: forall q. ToRow q => Connection -> Query -> q -> IO Int64
    execute Connection
    conn Query
    template q
    qs = do
      result <- Connection -> ByteString -> IO Result
    exec Connection
    conn (ByteString -> IO Result) -> IO ByteString -> IO Result
    forall (m :: * -> *) a b. Monad m => (a -> m b) -> m a -> m b
    =<< Connection -> Query -> q -> IO ByteString
    forall q. ToRow q => Connection -> Query -> q -> IO ByteString
    formatQuery Connection
    conn Query
    template q
    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 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 :: forall q. ToRow q => Connection -> Query -> [q] -> IO Int64
    executeMany Connection
    _ Query
    _ [] = Int64 -> IO Int64
    forall a. a -> IO a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Int64
    0
    executeMany Connection
    conn Query
    q [q]
    qs = do
      result <- Connection -> ByteString -> IO Result
    exec Connection
    conn (ByteString -> IO Result) -> IO ByteString -> IO Result
    forall (m :: * -> *) a b. Monad m => (a -> m b) -> m a -> m b
    =<< Connection -> Query -> [q] -> IO ByteString
    forall q. ToRow q => Connection -> Query -> [q] -> IO ByteString
    formatMany Connection
    conn Query
    q [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 :: forall q r.
    (ToRow q, FromRow r) =>
    Connection -> Query -> [q] -> IO [r]
    returning = RowParser r -> Connection -> Query -> [q] -> IO [r]
    forall q r.
    ToRow q =>
    RowParser r -> Connection -> Query -> [q] -> IO [r]
    returningWith RowParser r
    forall a. FromRow a => RowParser a
    fromRow

    -- | A version of 'returning' taking parser as argument
    returningWith :: (ToRow q) => RowParser r -> Connection -> Query -> [q] -> IO [r]
    returningWith :: forall q r.
    ToRow q =>
    RowParser r -> Connection -> Query -> [q] -> IO [r]
    returningWith RowParser r
    _ Connection
    _ Query
    _ [] = [r] -> IO [r]
    forall a. a -> IO a
    forall (m :: * -> *) a. Monad m => a -> m a
    return []
    returningWith RowParser r
    parser Connection
    conn Query
    q [q]
    qs = do
      result <- Connection -> ByteString -> IO Result
    exec Connection
    conn (ByteString -> IO Result) -> IO ByteString -> IO Result
    forall (m :: * -> *) a b. Monad m => (a -> m b) -> m a -> m b
    =<< Connection -> Query -> [q] -> IO ByteString
    forall q. ToRow q => Connection -> Query -> [q] -> IO ByteString
    formatMany Connection
    conn Query
    q [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 :: forall q r.
    (ToRow q, FromRow r) =>
    Connection -> Query -> q -> IO [r]
    query = RowParser r -> Connection -> Query -> q -> IO [r]
    forall q r.
    ToRow q =>
    RowParser r -> Connection -> Query -> q -> IO [r]
    queryWith RowParser r
    forall a. FromRow a => RowParser a
    fromRow

    -- | A version of 'query' that does not perform query substitution.
    query_ :: (FromRow r) => Connection -> Query -> IO [r]
    query_ :: forall r. FromRow r => Connection -> Query -> IO [r]
    query_ = RowParser r -> Connection -> Query -> IO [r]
    forall r. RowParser r -> Connection -> Query -> IO [r]
    queryWith_ RowParser r
    forall a. FromRow a => RowParser a
    fromRow

    -- | A version of 'query' taking parser as argument
    queryWith :: ToRow q => RowParser r -> Connection -> Query -> q -> IO [r]
    queryWith :: forall q r.
    ToRow q =>
    RowParser r -> Connection -> Query -> q -> IO [r]
    queryWith RowParser r
    parser Connection
    conn Query
    template q
    qs = do
      result <- Connection -> ByteString -> IO Result
    exec Connection
    conn (ByteString -> IO Result) -> IO ByteString -> IO Result
    forall (m :: * -> *) a b. Monad m => (a -> m b) -> m a -> m b
    =<< Connection -> Query -> q -> IO ByteString
    forall q. ToRow q => Connection -> Query -> q -> IO ByteString
    formatQuery Connection
    conn Query
    template q
    qs
      finishQueryWith parser conn template result

    -- | A version of 'query_' taking parser as argument
    queryWith_ :: RowParser r -> Connection -> Query -> IO [r]
    queryWith_ :: forall r. RowParser r -> Connection -> Query -> IO [r]
    queryWith_ RowParser r
    parser Connection
    conn q :: Query
    q@(Query ByteString
    que) = do
      result <- Connection -> ByteString -> IO Result
    exec Connection
    conn ByteString
    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 :: forall row params a.
    (FromRow row, ToRow params) =>
    Connection -> Query -> params -> a -> (a -> row -> IO a) -> IO a
    fold = FoldOptions
    -> Connection -> Query -> params -> a -> (a -> row -> IO a) -> IO a
    forall row params a.
    (FromRow row, ToRow params) =>
    FoldOptions
    -> Connection -> Query -> params -> a -> (a -> row -> IO a) -> IO a
    foldWithOptions FoldOptions
    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 :: forall params row a.
    ToRow params =>
    RowParser row
    -> Connection -> Query -> params -> a -> (a -> row -> IO a) -> IO a
    foldWith = FoldOptions
    -> RowParser row
    -> Connection
    -> Query
    -> params
    -> a
    -> (a -> row -> IO a)
    -> IO a
    forall params row a.
    ToRow params =>
    FoldOptions
    -> RowParser row
    -> Connection
    -> Query
    -> params
    -> a
    -> (a -> row -> IO a)
    -> IO a
    foldWithOptionsAndParser FoldOptions
    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 {
           FoldOptions -> FetchQuantity
    fetchQuantity   :: !FetchQuantity,
           FoldOptions -> TransactionMode
    transactionMode :: !TransactionMode
         }

    -- | defaults to 'Automatic',  and 'TransactionMode' 'ReadCommitted' 'ReadOnly'
    defaultFoldOptions :: FoldOptions
    defaultFoldOptions :: FoldOptions
    defaultFoldOptions = FoldOptions {
          fetchQuantity :: FetchQuantity
    fetchQuantity   = FetchQuantity
    Automatic,
          transactionMode :: TransactionMode
    transactionMode = IsolationLevel -> ReadWriteMode -> TransactionMode
    TransactionMode IsolationLevel
    ReadCommitted ReadWriteMode
    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 :: forall row params a.
    (FromRow row, ToRow params) =>
    FoldOptions
    -> Connection -> Query -> params -> a -> (a -> row -> IO a) -> IO a
    foldWithOptions FoldOptions
    opts = FoldOptions
    -> RowParser row
    -> Connection
    -> Query
    -> params
    -> a
    -> (a -> row -> IO a)
    -> IO a
    forall params row a.
    ToRow params =>
    FoldOptions
    -> RowParser row
    -> Connection
    -> Query
    -> params
    -> a
    -> (a -> row -> IO a)
    -> IO a
    foldWithOptionsAndParser FoldOptions
    opts RowParser row
    forall a. FromRow a => RowParser a
    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 :: forall params row a.
    ToRow params =>
    FoldOptions
    -> RowParser row
    -> Connection
    -> Query
    -> params
    -> a
    -> (a -> row -> IO a)
    -> IO a
    foldWithOptionsAndParser FoldOptions
    opts RowParser row
    parser Connection
    conn Query
    template params
    qs a
    a a -> row -> IO a
    f = do
        q <- Connection -> Query -> params -> IO ByteString
    forall q. ToRow q => Connection -> Query -> q -> IO ByteString
    formatQuery Connection
    conn Query
    template params
    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_ :: forall r a.
    FromRow r =>
    Connection -> Query -> a -> (a -> r -> IO a) -> IO a
    fold_ = FoldOptions -> Connection -> Query -> a -> (a -> r -> IO a) -> IO a
    forall r a.
    FromRow r =>
    FoldOptions -> Connection -> Query -> a -> (a -> r -> IO a) -> IO a
    foldWithOptions_ FoldOptions
    defaultFoldOptions

    -- | A version of 'fold_' taking a parser as an argument
    foldWith_ :: RowParser r
              -> Connection
              -> Query
              -> a
              -> (a -> r -> IO a)
              -> IO a
    foldWith_ :: forall r a.
    RowParser r -> Connection -> Query -> a -> (a -> r -> IO a) -> IO a
    foldWith_ = FoldOptions
    -> RowParser r
    -> Connection
    -> Query
    -> a
    -> (a -> r -> IO a)
    -> IO a
    forall r a.
    FoldOptions
    -> RowParser r
    -> Connection
    -> Query
    -> a
    -> (a -> r -> IO a)
    -> IO a
    foldWithOptionsAndParser_ FoldOptions
    defaultFoldOptions

    foldWithOptions_ :: (FromRow r) =>
                        FoldOptions
                     -> Connection
                     -> Query             -- ^ Query.
                     -> a                 -- ^ Initial state for result consumer.
                     -> (a -> r -> IO a)  -- ^ Result consumer.
                     -> IO a
    foldWithOptions_ :: forall r a.
    FromRow r =>
    FoldOptions -> Connection -> Query -> a -> (a -> r -> IO a) -> IO a
    foldWithOptions_ FoldOptions
    opts Connection
    conn Query
    query' a
    a a -> r -> IO a
    f = FoldOptions
    -> RowParser r
    -> Connection
    -> Query
    -> Query
    -> a
    -> (a -> r -> IO a)
    -> IO a
    forall row a.
    FoldOptions
    -> RowParser row
    -> Connection
    -> Query
    -> Query
    -> a
    -> (a -> row -> IO a)
    -> IO a
    doFold FoldOptions
    opts RowParser r
    forall a. FromRow a => RowParser a
    fromRow Connection
    conn Query
    query' Query
    query' a
    a a -> r -> IO 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_ :: forall r a.
    FoldOptions
    -> RowParser r
    -> Connection
    -> Query
    -> a
    -> (a -> r -> IO a)
    -> IO a
    foldWithOptionsAndParser_ FoldOptions
    opts RowParser r
    parser Connection
    conn Query
    query' a
    a a -> r -> IO a
    f = FoldOptions
    -> RowParser r
    -> Connection
    -> Query
    -> Query
    -> a
    -> (a -> r -> IO a)
    -> IO a
    forall row a.
    FoldOptions
    -> RowParser row
    -> Connection
    -> Query
    -> Query
    -> a
    -> (a -> row -> IO a)
    -> IO a
    doFold FoldOptions
    opts RowParser r
    parser Connection
    conn Query
    query' Query
    query' a
    a a -> r -> IO a
    f

    doFold :: FoldOptions
           -> RowParser row
           -> Connection
           -> Query
           -> Query
           -> a
           -> (a -> row -> IO a)
           -> IO a
    doFold :: forall row a.
    FoldOptions
    -> RowParser row
    -> Connection
    -> Query
    -> Query
    -> a
    -> (a -> row -> IO a)
    -> IO a
    doFold FoldOptions{TransactionMode
    FetchQuantity
    fetchQuantity :: FoldOptions -> FetchQuantity
    transactionMode :: FoldOptions -> TransactionMode
    fetchQuantity :: FetchQuantity
    transactionMode :: TransactionMode
    ..} RowParser row
    parser Connection
    conn Query
    _template Query
    q a
    a0 a -> row -> IO a
    f = do
        stat <- Connection
    -> (Connection -> IO TransactionStatus) -> IO TransactionStatus
    forall a. Connection -> (Connection -> IO a) -> IO a
    withConnection Connection
    conn Connection -> IO TransactionStatus
    PQ.transactionStatus
        case stat of
          TransactionStatus
    PQ.TransIdle    -> TransactionMode -> Connection -> IO a -> IO a
    forall a. TransactionMode -> Connection -> IO a -> IO a
    withTransactionMode TransactionMode
    transactionMode Connection
    conn IO a
    go
          TransactionStatus
    PQ.TransInTrans -> IO a
    go
          TransactionStatus
    PQ.TransActive  -> String -> IO a
    forall a. String -> IO a
    forall (m :: * -> *) a. MonadFail m => String -> m a
    fail String
    "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.
          TransactionStatus
    PQ.TransInError -> String -> IO a
    forall a. String -> IO a
    forall (m :: * -> *) a. MonadFail m => String -> m a
    fail String
    "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.
          TransactionStatus
    PQ.TransUnknown -> String -> IO a
    forall a. String -> IO a
    forall (m :: * -> *) a. MonadFail m => String -> m a
    fail String
    "foldWithOpts FIXME:  PQ.TransUnknown"
             -- Not sure what this means.
      where
        declare :: IO Cursor
    declare =
          Connection -> Query -> IO Cursor
    declareCursor Connection
    conn Query
    q
        fetch :: Cursor -> a -> IO (Either a a)
    fetch Cursor
    cursor a
    a =
          Cursor
    -> RowParser row
    -> Int
    -> (a -> row -> IO a)
    -> a
    -> IO (Either a a)
    forall r a.
    Cursor
    -> RowParser r -> Int -> (a -> r -> IO a) -> a -> IO (Either a a)
    foldForwardWithParser Cursor
    cursor RowParser row
    parser Int
    chunkSize a -> row -> IO a
    f a
    a

        go :: IO a
    go = IO Cursor -> (Cursor -> IO ()) -> (Cursor -> IO a) -> IO a
    forall a b c. IO a -> (a -> IO b) -> (a -> IO c) -> IO c
    bracket IO Cursor
    declare Cursor -> IO ()
    closeCursor ((Cursor -> IO a) -> IO a) -> (Cursor -> IO a) -> IO a
    forall a b. (a -> b) -> a -> b
    $ \Cursor
    cursor ->
                 let loop :: a -> IO a
    loop a
    a = Cursor -> a -> IO (Either a a)
    fetch Cursor
    cursor a
    a IO (Either a a) -> (Either a a -> IO a) -> IO a
    forall a b. IO a -> (a -> IO b) -> IO b
    forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
    >>=
                                \Either a a
    r -> case Either a a
    r of
                                        Left a
    x -> a -> IO a
    forall a. a -> IO a
    forall (m :: * -> *) a. Monad m => a -> m a
    return a
    x
                                        Right a
    x -> a -> IO a
    loop a
    x
                   in a -> IO a
    loop a
    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 :: Int
    chunkSize = case FetchQuantity
    fetchQuantity of
                     FetchQuantity
    Automatic   -> Int
    256
                     Fixed Int
    n     -> Int
    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 :: forall q r.
    (ToRow q, FromRow r) =>
    Connection -> Query -> q -> (r -> IO ()) -> IO ()
    forEach = RowParser r -> Connection -> Query -> q -> (r -> IO ()) -> IO ()
    forall q r.
    ToRow q =>
    RowParser r -> Connection -> Query -> q -> (r -> IO ()) -> IO ()
    forEachWith RowParser r
    forall a. FromRow a => RowParser a
    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 :: forall q r.
    ToRow q =>
    RowParser r -> Connection -> Query -> q -> (r -> IO ()) -> IO ()
    forEachWith RowParser r
    parser Connection
    conn Query
    template q
    qs = RowParser r
    -> Connection -> Query -> q -> () -> (() -> r -> IO ()) -> IO ()
    forall params row a.
    ToRow params =>
    RowParser row
    -> Connection -> Query -> params -> a -> (a -> row -> IO a) -> IO a
    foldWith RowParser r
    parser Connection
    conn Query
    template q
    qs () ((() -> r -> IO ()) -> IO ())
    -> ((r -> IO ()) -> () -> r -> IO ()) -> (r -> IO ()) -> IO ()
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . (r -> IO ()) -> () -> r -> IO ()
    forall a b. a -> b -> a
    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_ :: forall r. FromRow r => Connection -> Query -> (r -> IO ()) -> IO ()
    forEach_ = RowParser r -> Connection -> Query -> (r -> IO ()) -> IO ()
    forall r.
    RowParser r -> Connection -> Query -> (r -> IO ()) -> IO ()
    forEachWith_ RowParser r
    forall a. FromRow a => RowParser a
    fromRow
    {-# INLINE forEach_ #-}

    forEachWith_ :: RowParser r
                 -> Connection
                 -> Query
                 -> (r -> IO ())
                 -> IO ()
    forEachWith_ :: forall r.
    RowParser r -> Connection -> Query -> (r -> IO ()) -> IO ()
    forEachWith_ RowParser r
    parser Connection
    conn Query
    template = RowParser r
    -> Connection -> Query -> () -> (() -> r -> IO ()) -> IO ()
    forall r a.
    RowParser r -> Connection -> Query -> a -> (a -> r -> IO a) -> IO a
    foldWith_ RowParser r
    parser Connection
    conn Query
    template () ((() -> r -> IO ()) -> IO ())
    -> ((r -> IO ()) -> () -> r -> IO ()) -> (r -> IO ()) -> IO ()
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . (r -> IO ()) -> () -> r -> IO ()
    forall a b. a -> b -> a
    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 compatible 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.