Database.Esqueleto.Internal.Language

This is an internal module, anything exported by this module may change without a major version bump. Please use only Database.Esqueleto if possible.

The pretty face

class (Functor query, Applicative query, Monad query) => Esqueleto query expr backend | query -> expr backend, expr -> query backend where

Finally tagless representation of esqueleto's EDSL.

Methods

fromStart :: (PersistEntity a, PersistEntityBackend a ~ backend) => query (expr (PreprocessedFrom (expr (Entity a))))

(Internal) Start a from query with an entity. from does two kinds of magic using fromStart, fromJoin and fromFinish:

1. The simple but tedious magic of allowing tuples to be used.
2. The more advanced magic of creating JOINs. The JOIN is processed from right to left. The rightmost entity of the JOIN is created with fromStart. Each JOIN step is then translated into a call to fromJoin. In the end, fromFinish is called to materialize the JOIN.

fromStartMaybe :: (PersistEntity a, PersistEntityBackend a ~ backend) => query (expr (PreprocessedFrom (expr (Maybe (Entity a)))))

(Internal) Same as fromStart, but entity may be missing.

fromJoin :: IsJoinKind join => expr (PreprocessedFrom a) -> expr (PreprocessedFrom b) -> query (expr (PreprocessedFrom (join a b)))

(Internal) Do a JOIN.

fromFinish :: expr (PreprocessedFrom a) -> query a

(Internal) Finish a JOIN.

where_ :: expr (Value Bool) -> query ()

WHERE clause: restrict the query's result.

on :: expr (Value Bool) -> query ()

ON clause: restrict the a JOIN's result. The ON clause will be applied to the last JOIN that does not have an ON clause yet. If there are no JOINs without ON clauses (either because you didn't do any JOIN, or because all JOINs already have their own ON clauses), a runtime exception OnClauseWithoutMatchingJoinException is thrown. ON clauses are optional when doing JOINs.

On the simple case of doing just one JOIN, for example

 select $
 from $ \(foo `InnerJoin` bar) -> do
   on (foo ^. FooId ==. bar ^. BarFooId)
   ...

there's no ambiguity and the rules above just mean that you're allowed to call on only once (as in SQL). If you have many joins, then the ons are applied on the reverse order that the JOINs appear. For example:

 select $
 from $ \(foo `InnerJoin` bar `InnerJoin` baz) -> do
   on (baz ^. BazId ==. bar ^. BarBazId)
   on (foo ^. FooId ==. bar ^. BarFooId)
   ...

The order is reversed in order to improve composability. For example, consider query1 and query2 below:

 let query1 =
       from $ \(foo `InnerJoin` bar) -> do
         on (foo ^. FooId ==. bar ^. BarFooId)
     query2 =
       from $ \(mbaz `LeftOuterJoin` quux) -> do
         return (mbaz ?. BazName, quux)
     test1 =      (,) <$> query1 <*> query2
     test2 = flip (,) <$> query2 <*> query1

If the order was not reversed, then test2 would be broken: query1's on would refer to query2's LeftOuterJoin.

orderBy :: [expr OrderBy] -> query ()

ORDER BY clause. See also asc and desc.

asc :: PersistField a => expr (Value a) -> expr OrderBy

Ascending order of this field or expression.

desc :: PersistField a => expr (Value a) -> expr OrderBy

Descending order of this field or expression.

sub_select :: PersistField a => query (expr (Value a)) -> expr (Value a)

Execute a subquery SELECT in an expression.

sub_selectDistinct :: PersistField a => query (expr (Value a)) -> expr (Value a)

Execute a subquery SELECT DISTINCT in an expression.

(^.) :: (PersistEntity val, PersistField typ) => expr (Entity val) -> EntityField val typ -> expr (Value typ)

Project a field of an entity.

(?.) :: (PersistEntity val, PersistField typ) => expr (Maybe (Entity val)) -> EntityField val typ -> expr (Value (Maybe typ))

Project a field of an entity that may be null.

val :: PersistField typ => typ -> expr (Value typ)

Lift a constant value from Haskell-land to the query.

isNothing :: PersistField typ => expr (Value (Maybe typ)) -> expr (Value Bool)

IS NULL comparison.

just :: expr (Value typ) -> expr (Value (Maybe typ))

Analogous to Just, promotes a value of type typ into one of type Maybe typ. It should hold that val . Just === just . val.

nothing :: expr (Value (Maybe typ))

NULL value.

countRows :: Num a => expr (Value a)

COUNT(*) value.

not_ :: expr (Value Bool) -> expr (Value Bool)

(==.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)

(>=.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)

(>.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)

(<=.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)

(<.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)

(!=.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)

(&&.) :: expr (Value Bool) -> expr (Value Bool) -> expr (Value Bool)

(||.) :: expr (Value Bool) -> expr (Value Bool) -> expr (Value Bool)

(+.) :: PersistField a => expr (Value a) -> expr (Value a) -> expr (Value a)

(-.) :: PersistField a => expr (Value a) -> expr (Value a) -> expr (Value a)

(/.) :: PersistField a => expr (Value a) -> expr (Value a) -> expr (Value a)

(*.) :: PersistField a => expr (Value a) -> expr (Value a) -> expr (Value a)

set :: PersistEntity val => expr (Entity val) -> [expr (Update val)] -> query ()

SET clause used on UPDATEs. Note that while it's not a type error to use this function on a SELECT, it will most certainly result in a runtime error.

(=.) :: (PersistEntity val, PersistField typ) => EntityField val typ -> expr (Value typ) -> expr (Update val)

(+=.) :: (PersistEntity val, PersistField a) => EntityField val a -> expr (Value a) -> expr (Update val)

(-=.) :: (PersistEntity val, PersistField a) => EntityField val a -> expr (Value a) -> expr (Update val)

(*=.) :: (PersistEntity val, PersistField a) => EntityField val a -> expr (Value a) -> expr (Update val)

(/=.) :: (PersistEntity val, PersistField a) => EntityField val a -> expr (Value a) -> expr (Update val)

Instances

Esqueleto SqlQuery SqlExpr SqlPersist

from :: From query expr backend a => (a -> query b) -> query b

FROM clause: bring entities into scope.

This function internally uses two type classes in order to provide some flexibility of how you may call it. Internally we refer to these type classes as the two different magics.

The innermost magic allows you to use from with the following types:

• expr (Entity val), which brings a single entity into scope.
• expr (Maybe (Entity val)), which brings a single entity that may be NULL into scope. Used for OUTER JOINs.
• A JOIN of any other two types allowed by the innermost magic, where a JOIN may be an InnerJoin, a CrossJoin, a LeftOuterJoin, a RightOuterJoin, or a FullOuterJoin. The JOINs have right fixity, the same as in SQL.

The outermost magic allows you to use from on any tuples of types supported by innermost magic (and also tuples of tuples, and so on), up to 8-tuples.

Note that using from for the same entity twice does work and corresponds to a self-join. You don't even need to use two different calls to from, you may use a JOIN or a tuple.

The following are valid examples of uses of from (the types of the arguments of the lambda are inside square brackets):

 from $ \person -> ...
 from $ \(person, blogPost) -> ...
 from $ \(p `LeftOuterJoin` mb) -> ...
 from $ \(p1 `InnerJoin` f `InnerJoin` p2) -> ...
 from $ \((p1 `InnerJoin` f) `InnerJoin` p2) -> ...

The types of the arguments to the lambdas above are, respectively:

 person
   :: ( Esqueleto query expr backend
      , PersistEntity Person
      , PersistEntityBackend Person ~ backend
      ) => expr (Entity Person)
 (person, blogPost)
   :: (...) => (expr (Entity Person), expr (Entity BlogPost))
 (p `LeftOuterJoin` mb)
   :: (...) => InnerJoin (expr (Entity Person)) (expr (Maybe (Entity BlogPost)))
 (p1 `InnerJoin` f `InnerJoin` p2)
   :: (...) => InnerJoin
                 (expr (Entity Person))
                 (InnerJoin (expr (Entity Follow))
                            (expr (Entity Person)))
 ((p1 `InnerJoin` f) `InnerJoin` p2) ::
   :: (...) => InnerJoin
                 (InnerJoin (expr (Entity Person))
                            (expr (Entity Follow)))
                 (expr (Entity Person))

Note that some backends may not support all kinds of JOINs. For example, when using the SQL backend with SQLite, it will not accept the last example above (which is associated to the left, instead of being to the right) and will not accept RightOuterJoins or FullOuterJoins.

data Value a

A single value (as opposed to a whole entity). You may use (^.) or (?.) to get a Value from an Entity.

Constructors

Value a

Instances

Typeable1 Value

Eq a => Eq (Value a)

Ord a => Ord (Value a)

Show a => Show (Value a)

PersistField a => SqlSelect (SqlExpr (Value a)) (Value a)

You may return any single value (i.e. a single column) from a select query.

data InnerJoin a b

Data type that represents an INNER JOIN (see LeftOuterJoin for an example).

Constructors

a InnerJoin b

Instances

IsJoinKind InnerJoin

(Esqueleto query expr backend, FromPreprocess query expr backend (InnerJoin a b)) => From query expr backend (InnerJoin a b)

data CrossJoin a b

Data type that represents a CROSS JOIN (see LeftOuterJoin for an example).

Constructors

a CrossJoin b

Instances

IsJoinKind CrossJoin

(Esqueleto query expr backend, FromPreprocess query expr backend (CrossJoin a b)) => From query expr backend (CrossJoin a b)

data LeftOuterJoin a b

Data type that represents a LEFT OUTER JOIN. For example,

 select $
 from $ \(person `LeftOuterJoin` pet) ->
   ...

is translated into

 SELECT ...
 FROM Person LEFT OUTER JOIN Pet
 ...

See also: from.

Constructors

a LeftOuterJoin b

Instances

IsJoinKind LeftOuterJoin

(Esqueleto query expr backend, FromPreprocess query expr backend (LeftOuterJoin a b)) => From query expr backend (LeftOuterJoin a b)

data RightOuterJoin a b

Data type that represents a RIGHT OUTER JOIN (see LeftOuterJoin for an example).

Constructors

a RightOuterJoin b

Instances

IsJoinKind RightOuterJoin

(Esqueleto query expr backend, FromPreprocess query expr backend (RightOuterJoin a b)) => From query expr backend (RightOuterJoin a b)

data FullOuterJoin a b

Data type that represents a FULL OUTER JOIN (see LeftOuterJoin for an example).

Constructors

a FullOuterJoin b

Instances

IsJoinKind FullOuterJoin

(Esqueleto query expr backend, FromPreprocess query expr backend (FullOuterJoin a b)) => From query expr backend (FullOuterJoin a b)

data OnClauseWithoutMatchingJoinException

Exception thrown whenever on is used to create an ON clause but no matching JOIN is found.

Constructors

OnClauseWithoutMatchingJoinException String

Instances

Eq OnClauseWithoutMatchingJoinException

Ord OnClauseWithoutMatchingJoinException

Show OnClauseWithoutMatchingJoinException

Typeable OnClauseWithoutMatchingJoinException

Exception OnClauseWithoutMatchingJoinException

data OrderBy

Phantom type used by orderBy, asc and desc.

data Update typ

Phantom type for a SET operation on an entity of the given type (see set and '(=.)').

The guts

data JoinKind

(Internal) A kind of JOIN.

Constructors

InnerJoinKind

INNER JOIN

CrossJoinKind

CROSS JOIN

LeftOuterJoinKind

LEFT OUTER JOIN

RightOuterJoinKind

RIGHT OUTER JOIN

FullOuterJoinKind

FULL OUTER JOIN

class IsJoinKind join where

(Internal) Functions that operate on types (that should be) of kind JoinKind.

Methods

smartJoin :: a -> b -> join a b

(Internal) smartJoin a b is a JOIN of the correct kind.

reifyJoinKind :: join a b -> JoinKind

(Internal) Reify a JoinKind from a JOIN. This function is non-strict.

Instances

IsJoinKind FullOuterJoin

IsJoinKind RightOuterJoin

IsJoinKind LeftOuterJoin

IsJoinKind CrossJoin

IsJoinKind InnerJoin

data PreprocessedFrom a

(Internal) Phantom type used to process from (see fromStart).

class Esqueleto query expr backend => From query expr backend a

(Internal) Class that implements the tuple from magic (see fromStart).

Instances

(Esqueleto query expr backend, FromPreprocess query expr backend (expr (Maybe (Entity val)))) => From query expr backend (expr (Maybe (Entity val)))

(Esqueleto query expr backend, FromPreprocess query expr backend (expr (Entity val))) => From query expr backend (expr (Entity val))

(From query expr backend a, From query expr backend b) => From query expr backend (a, b)

(Esqueleto query expr backend, FromPreprocess query expr backend (FullOuterJoin a b)) => From query expr backend (FullOuterJoin a b)

(Esqueleto query expr backend, FromPreprocess query expr backend (RightOuterJoin a b)) => From query expr backend (RightOuterJoin a b)

(Esqueleto query expr backend, FromPreprocess query expr backend (LeftOuterJoin a b)) => From query expr backend (LeftOuterJoin a b)

(Esqueleto query expr backend, FromPreprocess query expr backend (CrossJoin a b)) => From query expr backend (CrossJoin a b)

(Esqueleto query expr backend, FromPreprocess query expr backend (InnerJoin a b)) => From query expr backend (InnerJoin a b)

(From query expr backend a, From query expr backend b, From query expr backend c) => From query expr backend (a, b, c)

(From query expr backend a, From query expr backend b, From query expr backend c, From query expr backend d) => From query expr backend (a, b, c, d)

(From query expr backend a, From query expr backend b, From query expr backend c, From query expr backend d, From query expr backend e) => From query expr backend (a, b, c, d, e)

(From query expr backend a, From query expr backend b, From query expr backend c, From query expr backend d, From query expr backend e, From query expr backend f) => From query expr backend (a, b, c, d, e, f)

(From query expr backend a, From query expr backend b, From query expr backend c, From query expr backend d, From query expr backend e, From query expr backend f, From query expr backend g) => From query expr backend (a, b, c, d, e, f, g)

(From query expr backend a, From query expr backend b, From query expr backend c, From query expr backend d, From query expr backend e, From query expr backend f, From query expr backend g, From query expr backend h) => From query expr backend (a, b, c, d, e, f, g, h)

class Esqueleto query expr backend => FromPreprocess query expr backend a

(Internal) Class that implements the JOIN from magic (see fromStart).

Instances

(Esqueleto query expr backend, PersistEntity val, ~ ((* -> *) -> * -> *) (PersistEntityBackend val) backend) => FromPreprocess query expr backend (expr (Maybe (Entity val)))

(Esqueleto query expr backend, PersistEntity val, ~ ((* -> *) -> * -> *) (PersistEntityBackend val) backend) => FromPreprocess query expr backend (expr (Entity val))

(Esqueleto query expr backend, FromPreprocess query expr backend a, FromPreprocess query expr backend b, IsJoinKind join) => FromPreprocess query expr backend (join a b)