skunk
package skunk
Skunk is a functional data access layer for Postgres.
Design principles:
- Skunk doesn't use JDBC. It speaks the Postgres wire protocol. It will not work with any other database back end.
- Skunk is asynchronous all the way down, via cats-effect, fs2, and ultimately nio. The
high-level network layers (
Protocol
andSession
) are safe to use concurrently. - Serialization to and from schema types is not typeclass-based, so there are no implicit derivations. Codecs are explicit, like parser combinators.
- I'm not sweating arity abstraction that much. Pass
a ~ b ~ c
for three args andVoid
if there are no args. This may change in the future but it's fine for now. - Skunk uses
Resource
for lifetime-managed objects, which means it takes some discipline to avoid leaks, especially when working concurrently. May or may not end up being problematic. - I'm trying to write good Scaladoc this time.
A minimal example follows. We construct a Resource
that yields a Session
, then use it.
package example import cats.effect._ import skunk._ import skunk.implicits._ import skunk.codec.numeric._ object Minimal extends IOApp { val session: Resource[IO, Session[IO]] = Session.single( host = "localhost", port = 5432, user = "postgres", database = "world", ) def run(args: List[String]): IO[ExitCode] = session.use { s => for { n <- s.unique(sql"select 42".query(int4)) _ <- IO(println(s"The answer is $n.")) } yield ExitCode.Success } }
Continue reading for an overview of the library. It's pretty small.
- Source
- package.scala
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Package Members
- package codec
- package data
- package exception
- package net
Skunk network stack, starting with
BitVectorSocket
at the bottom and ending withProtocol
at the top (Session
delegates all its work toProtocol
).Skunk network stack, starting with
BitVectorSocket
at the bottom and ending withProtocol
at the top (Session
delegates all its work toProtocol
). Everything is non-blocking. - package syntax
- package util
Type Members
- sealed trait AppliedFragment extends AnyRef
A fragment applied to its argument, yielding an existentially-typed fragment + argument pair that can be useful when constructing dynamic queries in application code.
A fragment applied to its argument, yielding an existentially-typed fragment + argument pair that can be useful when constructing dynamic queries in application code. Applied fragments must be deconstructed in order to prepare and execute.
- trait Channel[F[_], A, B] extends Pipe[F, A, Unit]
A channel that can be used for inter-process communication, implemented in terms of
LISTEN
andNOTIFY
.A channel that can be used for inter-process communication, implemented in terms of
LISTEN
andNOTIFY
. All instances start life as aChannel[F, String, Notification]
but can be mapped out to different input and output types. See the linked documentation for more information on the transactional semantics of these operations. - trait Codec[A] extends Encoder[A] with Decoder[A]
Symmetric encoder and decoder of Postgres text-format data to and from Scala types.
- final case class Command[A](sql: String, origin: Origin, encoder: Encoder[A]) extends Statement[A] with Product with Serializable
SQL and parameter encoder for a statement that returns no rows.
SQL and parameter encoder for a statement that returns no rows. We assume that
sql
has the same number of placeholders of the form$1
,$2
, etc., as the number of slots encoded byencoder
, and that the parameter types specified byencoder
are consistent with the schema. Thecheck
methods on Session provide a means to verify this assumption.You can construct a
Command
directly, although it is more typical to use thesql
interpolator.sql"INSERT INTO foo VALUES ($int2, $varchar)".command // Command[Short ~ String]
- sql
A SQL statement returning no rows.
- encoder
An encoder for all parameters
$1
,$2
, etc., insql
.
- See also
StringContextOps for information on the
sql
interpolator.Session for information on executing a
Command
.
- trait Cursor[F[_], A] extends AnyRef
An open cursor from which rows can be fetched, valid during the lifetime its defining
Session
.An open cursor from which rows can be fetched, valid during the lifetime its defining
Session
. You can use this mechanism to implement chunked reads and paged results, although it is ofen more pleasant to use aCursor
-backedStream
, as produced by PreparedQuery#stream. - trait Decoder[A] extends AnyRef
Decoder of Postgres text-format data into Scala types.
- trait Encoder[A] extends AnyRef
Encoder of Postgres text-format data from Scala types.
- final case class Fragment[A](parts: List[Either[String, State[Int, String]]], encoder: Encoder[A], origin: Origin) extends (A) => AppliedFragment with Product with Serializable
A composable, embeddable hunk of SQL and typed parameters (common precursor to
Command
andQuery
).A composable, embeddable hunk of SQL and typed parameters (common precursor to
Command
andQuery
). Although it is possible to construct aFragment
directly it is more typical to use thesql
interpolator. - trait PreparedCommand[F[_], A] extends AnyRef
A prepared command, valid for the life of its defining
Session
. - trait PreparedQuery[F[_], A, B] extends AnyRef
A prepared query, valid for the life of its originating
Session
. - final case class Query[A, B](sql: String, origin: Origin, encoder: Encoder[A], decoder: Decoder[B]) extends Statement[A] with Product with Serializable
SQL, parameter encoder, and row decoder for a statement that returns rows.
SQL, parameter encoder, and row decoder for a statement that returns rows. We assume that
sql
has the same number of placeholders of the form$1
,$2
, etc., as the number of slots encoded byencoder
, thatsql
selects the same number of columns are the number of slots decoded bydecoder
, and that the parameter and column types specified byencoder
anddecoder
are consistent with the schema. Thecheck
methods on Session provide a means to verify this assumption.You can construct a
Query
directly, although it is more typical to use thesql
interpolator.sql"SELECT name, age FROM person WHERE age > $int2".query(varchar ~ int2) // Query[Short, String ~ Short]
- sql
A SQL statement returning no rows.
- origin
The
Origin
where the sql was defined, if any.- encoder
An encoder for all parameters
$1
,$2
, etc., insql
.- decoder
A decoder for selected columns.
- See also
StringContextOps for information on the
sql
interpolator.Session for information on executing a
Query
.
- sealed abstract class SSL extends AnyRef
- trait Session[F[_]] extends AnyRef
Represents a live connection to a Postgres database.
Represents a live connection to a Postgres database. Operations provided here are safe to use concurrently. Note that this is a lifetime-managed resource and as such is invalid outside the scope of its owning
Resource
, as are any streams constructed here. If youstart
an operation be sure tojoin
itsFiber
before releasing the resource.See the companion object for information on obtaining a pooled or single-use instance.
- type SessionPool[F[_]] = Resource[F, Resource[F, Session[F]]]
- sealed abstract class SqlState extends EnumEntry
Enumerated type of Postgres error codes.
Enumerated type of Postgres error codes. See the companion object for more information.
- trait Statement[A] extends AnyRef
- type Strategy = skunk.util.Typer.Strategy
- trait Transaction[F[_]] extends AnyRef
Control methods for use within a
transaction
block.Control methods for use within a
transaction
block. An instance is provided when you callSession.transaction(...).use
.- See also
Session#transaction for information on default commit/rollback behavior
- sealed trait Void extends AnyRef
A singly-inhabited type representing arguments to a parameterless statement.
- type ~[+A, +B] = (A, B)
Infix alias for
(A, B)
that provides convenient syntax for left-associated HLists.
Value Members
- val Strategy: skunk.util.Typer.Strategy.type
- object AppliedFragment
- object Channel
- object Codec
- object Command extends Serializable
- object Cursor
- object Decoder
- object Encoder
- object Fragment extends Serializable
- object PreparedCommand
- object PreparedQuery
- object Query extends Serializable
- object SSL
- object Session
- object SqlState extends Enum[SqlState]
Enumerated type of Postgres error codes.
Enumerated type of Postgres error codes. These can be used as extractors for error handling, for example:
doSomething.recoverWith { case SqlState.ForeignKeyViolation(ex) => ... }
- See also
- object Transaction
- case object Void extends Void with Product with Serializable
- object implicits extends ToAllOps
- object ~
Companion providing unapply for
~
such that(x ~ y ~ z) match { case a ~ b ~ c => ... }
.
Inherited from AnyRef
Inherited from Any
Queries and Commands
Skunk recognizes two classes of statements: Query
, for statements
that return rows; and Command
, for statements that do not return rows. These values can be
constructed directly but typically arise via the sql
interpolator.
val q = sql""" SELECT id, name FROM employee WHERE age > $int2 """.query(int4 ~ varchar) // Query[Short, Long ~ String]
In the above example note that query parameters are specified by interpolated Encoder
s and
column types are specified by a Decoder
passed to .query
. The ~
syntax constructs
left-associated HLists of types and values via nested pairs. These are all described in more
detail a bit further down.
Commands are constructed in a similar way but have no output columns and thus no Decoder
is
needed.
val c = sql""" UPDATE employee SET salary = salary * 1.05 WHERE id = $int8 """.command // Command[Long]
The interpolator also permits nested Fragment
s and interpolated constants like table names.
See StringContextOps
for more information on the interpolator.
Session Values
Skunk's central abstraction is the Session
, which represents a connection
to Postgres. From the Session
we can produce prepared statements, cursors, and other
resources that ultimately depend on their originating Session
.
Codecs
When you construct a statement each parameter is specified via an Encoder
, and
row data is specified via a Decoder
. In some cases encoders and decoders are symmetric and
are defined together, as a Codec
. There are many variants of this pattern in functional
Scala libraries; this is closest in spirit to the strategy adopted by scodec.
HLists
This idea was borrowed from scodec. We use ~
to build left-associated nested
pairs of values and types, and can destructure with ~
the same way.
val a: Int ~ String ~ Boolean = 1 ~ "foo" ~ true a match { case n ~ s ~ b => ... }
Note that the ~
operation for Codec
, Encoder
, and Decoder
is lifted. This is usually
what you want. If you do need an HList of encoders you can use Tuple2
.
val c: Encoder[Int ~ String ~ Boolean] int4 ~ bpchar ~ bit // Unusual, but for completeness you can do it thus: val d: Encoder[Int] ~ Encoder[String] ~ Encoder[Boolean] = ((int4, bpchar), bit)
It is possible that we will end up switching to shapeless.HList
but this is good for now.
Companion Objects
Companion objects for the traits and classes above.