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120 lines
4.6 KiB
Haskell
120 lines
4.6 KiB
Haskell
module Hasql.Statement
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( Statement (..),
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refineResult,
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-- * Recipies
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-- ** Insert many
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-- $insertMany
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-- ** IN and NOT IN
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-- $inAndNotIn
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)
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where
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import qualified Hasql.Decoders as Decoders
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import qualified Hasql.Encoders as Encoders
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import qualified Hasql.Private.Decoders as Decoders
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import Hasql.Private.Prelude
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-- |
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-- Specification of a strictly single-statement query, which can be parameterized and prepared.
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--
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-- Consists of the following:
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--
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-- * SQL template,
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-- * params encoder,
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-- * result decoder,
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-- * a flag, determining whether it should be prepared.
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--
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-- The SQL template must be formatted according to Postgres' standard,
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-- with any non-ASCII characters of the template encoded using UTF-8.
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-- According to the format,
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-- parameters must be referred to using a positional notation, as in the following:
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-- @$1@, @$2@, @$3@ and etc.
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-- Those references must be used in accordance with the order in which
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-- the value encoders are specified in 'Encoders.Params'.
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--
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-- Following is an example of a declaration of a prepared statement with its associated codecs.
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--
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-- @
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-- selectSum :: 'Statement' (Int64, Int64) Int64
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-- selectSum = 'Statement' sql encoder decoder True where
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-- sql = "select ($1 + $2)"
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-- encoder =
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-- ('fst' '>$<' Encoders.'Hasql.Encoders.param' (Encoders.'Hasql.Encoders.nonNullable' Encoders.'Hasql.Encoders.int8')) '<>'
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-- ('snd' '>$<' Encoders.'Hasql.Encoders.param' (Encoders.'Hasql.Encoders.nonNullable' Encoders.'Hasql.Encoders.int8'))
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-- decoder = Decoders.'Hasql.Decoders.singleRow' (Decoders.'Hasql.Decoders.column' (Decoders.'Hasql.Decoders.nonNullable' Decoders.'Hasql.Decoders.int8'))
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-- @
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--
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-- The statement above accepts a product of two parameters of type 'Int64'
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-- and produces a single result of type 'Int64'.
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data Statement a b
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= Statement ByteString (Encoders.Params a) (Decoders.Result b) Bool
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instance Functor (Statement a) where
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{-# INLINE fmap #-}
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fmap = rmap
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instance Profunctor Statement where
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{-# INLINE dimap #-}
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dimap f1 f2 (Statement template encoder decoder preparable) =
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Statement template (contramap f1 encoder) (fmap f2 decoder) preparable
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-- |
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-- Refine a result of a statement,
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-- causing the running session to fail with the `UnexpectedResult` error in case of refinement failure.
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--
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-- This function is especially useful for refining the results of statements produced with
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-- <http://hackage.haskell.org/package/hasql-th the \"hasql-th\" library>.
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refineResult :: (a -> Either Text b) -> Statement params a -> Statement params b
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refineResult refiner (Statement template encoder decoder preparable) =
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Statement template encoder (Decoders.refineResult refiner decoder) preparable
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-- $insertMany
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--
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-- It is not currently possible to pass in an array of encodable values
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-- to use in an insert many statement. Instead, PostgreSQL's
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-- (9.4 or later) @unnest@ function can be used in an analogous way
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-- to haskell's `zip` function by passing in multiple arrays of values
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-- to be zipped into the rows we want to insert:
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--
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-- @
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-- insertMultipleLocations :: 'Statement' (Vector (UUID, Double, Double)) ()
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-- insertMultipleLocations = 'Statement' sql encoder decoder True where
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-- sql = "insert into location (id, x, y) select * from unnest ($1, $2, $3)"
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-- encoder =
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-- contramap Vector.'Data.Vector.unzip3' $
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-- contrazip3 (vector Encoders.'Encoders.uuid') (vector Encoders.'Encoders.float8') (vector Encoders.'Encoders.float8')
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-- where
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-- vector =
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-- Encoders.'Encoders.param' .
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-- Encoders.'Encoders.nonNullable' .
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-- Encoders.'Encoders.array' .
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-- Encoders.'Encoders.dimension' 'foldl'' .
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-- Encoders.'Encoders.element' .
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-- Encoders.'Encoders.nonNullable'
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-- decoder = Decoders.'Decoders.noResult'
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-- @
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--
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-- This approach is much more efficient than executing a single-row Insert
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-- statement multiple times.
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-- $inAndNotIn
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--
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-- There is a common misconception that Postgresql supports array
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-- as a parameter for the @IN@ operator.
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-- However Postgres only supports a syntactical list of values with it,
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-- i.e., you have to specify each option as an individual parameter
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-- (@something IN ($1, $2, $3)@).
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--
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-- Clearly it would be much more convenient to provide an array as a single parameter,
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-- but the @IN@ operator does not support that.
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-- Fortunately, Postgres does provide such functionality with other operators:
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--
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-- * Use @something = ANY($1)@ instead of @something IN ($1)@
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-- * Use @something <> ALL($1)@ instead of @something NOT IN ($1)@
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--
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-- For details see
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-- <https://www.postgresql.org/docs/9.6/static/functions-comparisons.html#AEN20944 the Postgresql docs>.
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