Used to indicate whether a particular predicate is from the initial database state, or due to a sequence of actions we've committed too. Used to prevent runaway action generation based off of derived states.

Represents the state of a database as a migration is being generated

Represents an edge (or a path) in the database graph.

Given a particular starting point, the destination database is the database where each predicate in actionPreConditions has been removed and each predicate in actionPostConditions has been added.

Edge discovery mechanism. A newtype wrapper over ActionProviderFn.

An ActionProviderFn takes two arguments. The first is the set of predicates that exist in the current database.

The function should a set of edges from the database specified in the first argument to possible destination databases. For optimization purposes, the second argument is the set of predicates that ought to exist in the destination database. This can be used to eliminate edges that will not lead to a solution.

This second argument is just an optimization and doesn't change the final result, although it can significantly impact the time it takes to get there.

Both the current database set and the destination database set are given as polymorphic lists of predicates. When you instantiate the type, the current database predicate set is queried for predicates of that type.

For example, dropTableActionProvider provides a DROP TABLE action edge whenever it encounters a table that exists. In order to do this, it attempts to find all TableExistsPredicate that do not exist in the destination database. Its ActionProviderFn may be implemented like such:

dropTableActionProvider preConditions postConditions = do
    TableExistsPredicate srcTblNm <- preConditions
    ensuringNot_ $ $
      do TableExistsPredicate destTblNm <- postConditions
         guard (srcTblNm == destTblNm)

ensuringNot_ is a function that causes the action provider to return no results if there are any elements in the provided list. In this case, it's used to stop DROP TABLE action generation for tables which must be present in the final database.

See ActionProvider

Proceeds only if no predicate matches the given pattern. See the implementation of dropTableActionProvider for an example of usage.

Used to ensure that only one predicate matches the given pattern. See the implementation of createTableActionProvider for an example of usage.

Action provider for SQL92 CREATE TABLE actions.

Action provider for SQL92 DROP TABLE actions

Action provider for SQL92 ALTER TABLE ... ADD COLUMN ... actions

Action provider for SQL92 ALTER TABLE ... ALTER COLUMN ... SET NULL

Action provider for SQL92 ALTER TABLE ... ALTER COLUMN ... SET NOT NULL

Default action providers for any SQL92 compliant syntax.

In particular, this provides edges consisting of the following statements:

• CREATE TABLE
• DROP TABLE
• ALTER TABLE ... ADD COLUMN ...
• ALTER TABLE ... DROP COLUMN ...
• ALTER TABLE ... ALTER COLUMN ... SET [NOT] NULL

Represents current state of a database graph search.

If ProvideSolution, the destination database has been reached, and the given list of commands provides the path from the source database to the destination.

If SearchFailed, the search has failed. The provided DatabaseStates represent the closest we could make it to the destination database. By default, only the best 10 are kept around (to avoid unbounded memory growth).

If ChooseActions, we are still searching. The caller is provided with the current state as well as a list of actions, provided as an opaque type f. The getPotentialActionChoice function can be used to get the PotentialAction corresponding to any given f. The caller is free to cull the set of potential actions according however they'd like (for example, by prompting the user). The selected actions to explore should be passed to the continueSearch function.

Use of the f existential type may seem obtuse, but it prevents the caller from injecting arbitrary actions. Instead the caller is limited to choosing only valid actions as provided by the suppled ActionProvider.

Represents the final results of a search

An exhaustive solving strategy that simply continues the search, while exploring every possible action. If there is a solution, this will find it.

Conduct a breadth-first search of the database graph to find a path from the source database to the destination database, using the given ActionProvider to discovere "edges" (i.e., DDL commands) between the databases.

See the documentation on Solver for more information on how to consume the result.