Database.Beam.Query.CustomSQL

Allows the creation of custom SQL expressions from arbitrary string-like values.

Simply write a polymorphic function with an arbitrary number of arguments, all of the same type, and returns a value of the same type. The type will have instances of Monoid and IsString.

For example, to implement a function MYFUNC that takes three arguments

myFuncImpl :: (Monoid a, IsString a) => a -> a -> a -> a

Then, apply customExpr_ to your function. This will result in a function with the same arity, that takes in and returns QGenExprs instead of generic as.

The returned function is polymorphic in the types of SQL expressions it will accept, but you can give it a more specific signature. For example, to mandate that we receive two Ints and a Text and return a Bool.

myFunc_ :: QGenExpr e ctxt s Int -> QGenExpr e ctxt s Int -> QGenExpr e ctxt s T.Text -> QGenExpr e ctxt s Bool
myFunc_ = customExpr_ myFuncImpl

Semantically, the expression builder function (myFuncImpl in this case) is called with arguments representing SQL expressions, that, when evaluated, will evaluate to the result of the expressions supplied as arguments to customExpr_. See the section on extensibility in the user guide for example usage.

The customExpr_ function

class IsCustomExprFn fn res | res -> fn where

Methods

customExpr_ :: fn -> res

Type-inference help

valueExpr_ :: QExpr be s a -> QExpr be s a

Force a QGenExpr to be typed as a value expression (a QGenExpr). Useful for getting around type-inference errors with supplying the entire type.

agg_ :: QAgg be s a -> QAgg be s a

Force a QGenExpr to be typed as an aggregate. Useful for defining custom aggregates for use in aggregate_.

For backends

class (Monoid (CustomSqlSyntax syntax), Semigroup (CustomSqlSyntax syntax), IsString (CustomSqlSyntax syntax)) => IsCustomSqlSyntax syntax where

A type-class for expression syntaxes that can embed custom expressions.

Associated Types

data CustomSqlSyntax syntax :: *

Methods

customExprSyntax :: CustomSqlSyntax syntax -> syntax

Given an arbitrary string-like expression, produce a syntax that represents the ByteString as a SQL expression.

renderSyntax :: syntax -> CustomSqlSyntax syntax

Given an arbitrary syntax, produce a string-like value that corresponds to how that syntax would look when rendered in the backend.

Instances

IsCustomSqlSyntax SqlSyntaxBuilder

Defined in Database.Beam.Query.CustomSQL

Associated Types

data CustomSqlSyntax SqlSyntaxBuilder :: Type

Methods

customExprSyntax :: CustomSqlSyntax SqlSyntaxBuilder -> SqlSyntaxBuilder

renderSyntax :: SqlSyntaxBuilder -> CustomSqlSyntax SqlSyntaxBuilder