Database.HaskellDB

HaskellDB is a Haskell library for expressing database queries and operations in a type safe and declarative way. HaskellDB compiles a relational algebra-like syntax into SQL, submits the operations to the database for processing, and returns the results as ordinary Haskell values.

This is the main module that the user should import. Beside this module, the user should import a particular database binding (ie. Database.HaskellDB.HSQL.ODBC) and database definitions.

HaskellDB was originally written by Daan Leijen and it's design is described in the paper Domain Specific Embedded Compilers, Daan Leijen and Erik Meijer. 2nd USENIX Conference on Domain-Specific Languages (DSL), Austin, USA, October 1999 (http://www.usenix.org/events/dsl99/).

This new version of HaskellDB was produced as a student project at Chalmers University of Technology in Gothenburg, Sweden. The aim of the project was to make HaskellDB a practically useful database library. That work is described in Student Paper: HaskellDB Improved, Bjrn Bringert, Anders Hckersten, Conny Andersson, Martin Andersson, Mary Bergman, Victor Blomqvist and Torbjrn Martin. In Proceedings of the ACM SIGPLAN 2004 Haskell Workshop, Snowbird, Utah, USA, September 22, 2004. (http://haskelldb.sourceforge.net/haskelldb.pdf)

Documentation

data Rel r

Type of relations, contains the attributes of the relation and an Alias to which the attributes are renamed in the PrimQuery.

Instances

Show (Query (Rel r))

Shows the optimized SQL for the query.

ExprTypes r => ExprTypes (Rel r)

ExprType a => ExprType (Rel a)

HasField f r => Select (Attr f a) (Rel r) (Expr a)

Field selection operator. It is overloaded to work for both relations in a query and the result of a query. That is, it corresponds to both ! and !. from the original HaskellDB. An overloaded operator was selected because users (and the developers) always forgot to use !. instead of ! on query results.

data Attr f a

Typed attributes

Instances

HasField f r => Select (Attr f a) (Rel r) (Expr a)

Field selection operator. It is overloaded to work for both relations in a query and the result of a query. That is, it corresponds to both ! and !. from the original HaskellDB. An overloaded operator was selected because users (and the developers) always forgot to use !. instead of ! on query results.

data Expr a

Type of normal expressions, contains the untyped PrimExpr.

Instances

ProjectExpr Expr

InsertExpr Expr

ExprC Expr

Read (Expr a)

Show (Expr a)

IsExpr (Expr a)

Args (Expr a)

ExprType a => ExprType (Expr a)

BStrToStr (Expr String) (Expr String)

BStrToStr (Expr (Maybe String)) (Expr (Maybe String))

Size n => BStrToStr (Expr (Maybe (BoundedString n))) (Expr (Maybe String))

Size n => BStrToStr (Expr (BoundedString n)) (Expr String)

IsExpr tail => IsExpr (Expr a -> tail)

Args (Expr a -> ExprAggr c)

(IsExpr tail, Args tail) => Args (Expr a -> tail)

HasField f r => Select (Attr f a) (Rel r) (Expr a)

Field selection operator. It is overloaded to work for both relations in a query and the result of a query. That is, it corresponds to both ! and !. from the original HaskellDB. An overloaded operator was selected because users (and the developers) always forgot to use !. instead of ! on query results.

(RelToRec rest, FieldTag f) => RelToRec (RecCons f (Expr a) rest)

(ShowConstant a, ConstantRecord r cr) => ConstantRecord (RecCons f a r) (RecCons f (Expr a) cr)

(ProjectExpr e, ProjectRec r er) => ProjectRec (RecCons f (e a) r) (RecCons f (Expr a) er)

(InsertExpr e, InsertRec r er) => InsertRec (RecCons f (e a) r) (RecCons f (Expr a) er)

(GetValue a, GetRec er vr) => GetRec (RecCons f (Expr a) er) (RecCons f a vr)

data ExprAggr a

Type of aggregate expressions.

Instances

ProjectExpr ExprAggr

ExprC ExprAggr

Read (ExprAggr a)

Show (ExprAggr a)

Args (Expr a -> ExprAggr c)

data Table r

Basic tables, contains table name and an association from attributes to attribute names in the real table.

data Query a

Instances

Monad Query

Functor Query

Show (Query (Rel r))

Shows the optimized SQL for the query.

data OrderExpr

Instances

Show OrderExpr

Records

class HasField f r

The record r has the field f if there is an instance of HasField f r.

Instances

HasField f r => HasField f (Record r)

HasField f r => HasField f (RecCons g a r)

HasField f (RecCons f a r)

type Record r = RecNil -> r

The type used for records. This is a function that takes a RecNil so that the user does not have to put a RecNil at the end of every record.

class Select f r a | f r -> a where

Methods

(!) :: r -> f -> a

Field selection operator. It is overloaded so that users (read HaskellDB) can redefine it for things with phantom record types.

Instances

SelectField f r a => Select (l f a) (Record r) a

HasField f r => Select (Attr f a) (Rel r) (Expr a)

Field selection operator. It is overloaded to work for both relations in a query and the result of a query. That is, it corresponds to both ! and !. from the original HaskellDB. An overloaded operator was selected because users (and the developers) always forgot to use !. instead of ! on query results.

(#)

Arguments

:: Record (RecCons f a RecNil)

Field to add

-> (b -> c)

Rest of record

-> b -> RecCons f a c

New record

Adds the field from a one-field record to another record.

(<<)

Arguments

:: Attr f a

Label

-> e a

Expression

-> Record (RecCons f (e a) RecNil)

New record

Creates a record field. Similar to '(.=.)', but gets the field label from an Attr.

(<<-)

Arguments

:: ShowConstant a

=> Attr f a

Field label

-> a

Field value

-> Record (RecCons f (Expr a) RecNil)

New record

Convenience operator for constructing records of constants. Useful primarily with insert. f <<- x is the same as f << constant x

(!.) :: Select f r a => r -> f -> a

The (!.) operator selects over returned records from the database (= rows) Non-overloaded version of !. For backwards compatibility.

Relational operators

restrict :: Expr Bool -> Query ()

Restricts the records to only those who evaluates the expression to True.

table :: ShowRecRow r => Table r -> Query (Rel r)

Return all records from a specific table.

project :: (ShowLabels r, ToPrimExprs r, ProjectRec r er) => Record r -> Query (Rel er)

Specifies a subset of the columns in the table.

unique :: Query ()

Restricts the relation given to only return unique records. Upshot is all projected attributes will be grouped.

union :: Query (Rel r) -> Query (Rel r) -> Query (Rel r)

Return all records which are present in at least one of the relations.

intersect :: Query (Rel r) -> Query (Rel r) -> Query (Rel r)

Return all records which are present in both relations.

divide :: Query (Rel r) -> Query (Rel r) -> Query (Rel r)

Not in SQL92.

minus :: Query (Rel r) -> Query (Rel r) -> Query (Rel r)

Return all records from the first relation that are not present in the second relation.

copy :: HasField f r => Attr f a -> Rel r -> Record (RecCons f (Expr a) RecNil)

Creates a single-field record from an attribute and a table. Useful for building projections that will re-use the same attribute name. copy attr tbl is equivalent to:

attr .=. (tbl .!. attr)

copyAll :: RelToRec r => Rel r -> Record r

Copies all columns in the relation given. Useful for appending the remaining columns in a table to a projection. For example:

   query = do
     tbl <- table some_table
     project $ copyAll tbl

will add all columns in some_table to the query.

subQuery :: Query (Rel r) -> Query (Rel r)

Allows a subquery to be created between another query and this query. Normally query definition is associative and query definition is interleaved. This combinator ensures the given query is added as a whole piece.

Query expressions

(.==.) :: Eq a => Expr a -> Expr a -> Expr Bool

Equality comparison on Exprs, = in SQL.

(.<>.) :: Eq a => Expr a -> Expr a -> Expr Bool

Inequality on Exprs, in SQL.

(.<.) :: Ord a => Expr a -> Expr a -> Expr Bool

(.<=.) :: Ord a => Expr a -> Expr a -> Expr Bool

(.>.) :: Ord a => Expr a -> Expr a -> Expr Bool

(.>=.) :: Ord a => Expr a -> Expr a -> Expr Bool

(.&&.) :: Expr Bool -> Expr Bool -> Expr Bool

"Logical and" on Expr, AND in SQL.

(.||.) :: Expr Bool -> Expr Bool -> Expr Bool

"Logical or" on Expr. OR in SQL.

(.*.) :: Num a => Expr a -> Expr a -> Expr a

Multiplication

(./.) :: Num a => Expr a -> Expr a -> Expr a

Division

(.+.) :: Num a => Expr a -> Expr a -> Expr a

Addition

(.-.) :: Num a => Expr a -> Expr a -> Expr a

Subtraction

(.%.) :: Num a => Expr a -> Expr a -> Expr a

Modulo

(.++.) :: Expr String -> Expr String -> Expr String

Concatenates two String-expressions.

_not :: Expr Bool -> Expr Bool

The inverse of an Expr Bool.

like :: Expr String -> Expr String -> Expr Bool

The HaskellDB counterpart to the SQL LIKE keyword. In the expresions, % is a wildcard representing any characters in the same position relavtive to the given characters and _ is a wildcard representing one character e.g.

 like (constant "ABCDEFFF") (constant "AB%F_F")

is true while

 like (constant "ABCDEF") (constant "AC%F") 

is false.

Note that SQL92 does not specify whether LIKE is case-sensitive or not. Different database systems implement this differently.

_in :: Eq a => Expr a -> [Expr a] -> Expr Bool

Returns true if the value of the first operand is equal to the value of any of the expressions in the list operand.

cat :: Expr String -> Expr String -> Expr String

Produces the concatenation of two String-expressions.

_length :: Expr String -> Expr Int

Gets the length of a string.

isNull :: Expr a -> Expr Bool

Returns true if the expression is Null.

notNull :: Expr a -> Expr Bool

The inverse of isNull, returns false if the expression supplied is Null.

fromNull

Arguments

:: Expr a

Default value (to be returned for Nothing)

-> Expr (Maybe a)

A nullable expression

-> Expr a

Takes a default value a and a nullable value. If the value is NULL, the default value is returned, otherwise the value itself is returned. Simliar to fromMaybe

fromVal :: ShowConstant a => a -> Expr (Maybe a) -> Expr a

Similar to fromNull, but takes a value argument rather than an Expr.

constant :: ShowConstant a => a -> Expr a

Creates a constant expression from a haskell value.

constVal :: ShowConstant a => a -> Expr (Maybe a)

Turn constant data into a nullable expression. Same as constant . Just

constNull :: Expr (Maybe a)

Represents a null value.

constExpr :: Expr a -> Expr (Maybe a)

Turn constant data into a nullable expression. Same as constant . Just

param

Arguments

:: Expr a

Default value.

-> Expr a

Create an anonymous parameter with a default value.

namedParam

Arguments

:: Name

Name of the parameter.

-> Expr a

Default value for the parameter.

-> Expr a

Create a named parameter with a default value.

class Args a

Used to implement variable length arguments to func, below.

Instances

Args (Expr a)

Args (Expr a -> ExprAggr c)

(IsExpr tail, Args tail) => Args (Expr a -> tail)

func :: Args a => String -> a

Can be used to define SQL functions which will appear in queries. Each argument for the function is specified by its own Expr value. Examples include:

  lower :: Expr a -> Expr (Maybe String) 
  lower str = func "lower" str

The arguments to the function do not have to be Expr if they can be converted to Expr:

  data DatePart = Day | Century deriving Show 

  datePart :: DatePart -> Expr (Maybe CalendarTime) -> Expr (Maybe Int) 
  datePart date col = func "date_part" (constant $ show date) col

Aggregate functions can also be defined. For example:

  every :: Expr Bool -> ExprAggr Bool 
  every col = func "every" col

Aggregates are implemented to always take one argument, so any attempt to define an aggregate with any more or less arguments will result in an error.

Note that type signatures are usually required for each function defined, unless the arguments can be inferred.

queryParams :: Query (Rel r) -> [Param]

Get paramaters from a query in order.

type Param = Either Int String

Represents a query parameter. Left parameters are indexed by position, while right parameters are named.

cast

Arguments

:: String

Destination type.

-> Expr a

Source expression.

-> Expr b

Generates a CAST expression for the given expression, using the argument given as the destination type.

coerce

Arguments

:: Expr a

Source expression

-> Expr b

Destination type.

Coerce the type of an expression to another type. Does not affect the actual primitive value - only the phantom type.

literal :: String -> Expr a

Inserts the string literally - no escaping, no quoting.

toStr :: BStrToStr s d => s -> d

Convert a bounded string to a real string.

count :: Expr a -> ExprAggr Int

Returns the number of records (=rows) in a query.

_sum :: Num a => Expr a -> ExprAggr a

Returns the total sum of a column.

_max :: Ord a => Expr a -> ExprAggr a

Returns the highest value of a column.

_min :: Ord a => Expr a -> ExprAggr a

Returns the lowest value of a column.

avg :: Num a => Expr a -> ExprAggr a

Returns the average of a column.

stddev :: Num a => Expr a -> ExprAggr a

Returns the standard deviation of a column.

stddevP :: Num a => Expr a -> ExprAggr a

variance :: Num a => Expr a -> ExprAggr a

Returns the standard variance of a column.

varianceP :: Num a => Expr a -> ExprAggr a

asc :: HasField f r => Rel r -> Attr f a -> OrderExpr

Use this together with the function order to order the results of a query in ascending order. Takes a relation and an attribute of that relation, which is used for the ordering.

desc :: HasField f r => Rel r -> Attr f a -> OrderExpr

Use this together with the function order to order the results of a query in descending order. Takes a relation and an attribute of that relation, which is used for the ordering.

order :: [OrderExpr] -> Query ()

Order the results of a query. Use this with the asc or desc functions.

top :: Int -> Query ()

Return the n topmost records.

_case

Arguments

:: [(Expr Bool, Expr a)]

A list of conditions and expressions.

-> Expr a

Else-expression.

-> Expr a

Creates a conditional expression. Returns the value of the expression corresponding to the first true condition. If none of the conditions are true, the value of the else-expression is returned.

_default :: ExprDefault a

The default value of the column. Only works with insert.

Database operations

data Database

query :: GetRec er vr => Database -> Query (Rel er) -> IO [Record vr]

performs a query on a database

recCat :: RecCat r1 r2 r3 => r1 -> r2 -> r3

Concatenates two records.

insert :: (ToPrimExprs r, ShowRecRow r, InsertRec r er) => Database -> Table er -> Record r -> IO ()

Inserts a record into a table

delete

Arguments

:: ShowRecRow r

=> Database

The database

-> Table r

The table to delete records from

-> (Rel r -> Expr Bool)

Predicate used to select records to delete

-> IO ()

deletes a bunch of records

update

Arguments

:: (ShowLabels s, ToPrimExprs s)

=> Database

The database

-> Table r

The table to update

-> (Rel r -> Expr Bool)

Predicate used to select records to update

-> (Rel r -> Record s)

Function used to modify selected records

-> IO ()

Updates records

insertQuery :: ShowRecRow r => Database -> Table r -> Query (Rel r) -> IO ()

Inserts values from a query into a table

tables

Arguments

:: Database

Database

-> IO [TableName]

Names of all tables in the database

List all tables in the database

describe

Arguments

:: Database

Database

-> TableName

Name of the tables whose columns are to be listed

-> IO [(Attribute, FieldDesc)]

Name and type info for each column

List all columns in a table, along with their types

transaction

Arguments

:: Database

Database

-> IO a

Action to run

-> IO a

Performs some database action in a transaction. If no exception is thrown, the changes are committed.

Debugging

showQuery :: Query (Rel r) -> String

Shows the optimized PrimQuery.

showQueryUnOpt :: Query (Rel r) -> String

Shows the unoptimized PrimQuery.

showSql :: Query (Rel r) -> String

Shows the optimized SQL query.

showSqlUnOpt :: Query (Rel r) -> String

Shows the unoptimized SQL query.