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Changelog for beam-core-0.7.2.1

0.7.2.0

Add compatibility with GHC 8.4 and stack nightly

0.7.1.0

Note '0.7.1.0' was released because the signature of delete was too specific in '0.7.0.0' due to an error when uploading the package.

0.7.0.0

Weaker functional dependencies on MonadBeam

The functional dependency on MonadBeam is now just m -> syntax be handle. This allows us to define MonadBeam instances atop monad transformers (although we don't have any yet!).

Correct boolean handling

Previous versions of beam used the SQL = operator to compare potentially NULL values. This is incorrect, as NULL = NULL => UNKNOWN in ANSI-compliant implementations. Beam has changed its emitted SQL to produce proper comparisons, but this can dramatically affect performance in some backends. Particularly, proper JOIN index usage in Postgres requires an exact match on an equality constructor, which may not be what you get when using the proper boolean handling.

If you are okay using SQL null handling, you can use the new ==?. and /=?. operators which produce an expression with type SqlBool instead. SqlBool is a type that can represent the SQL BOOL type in all its gritty glory. Note however, that these operators do not compare for haskell equality, only SQL equality, so please understand what that means before using them.

Correspondingly, many functions that took Bool expressions now have corresponding versions that take SqlBool. For example, to use guard_ with a SqlBool expression use guard_' (note the prime).

(Note: I don't really like that we have to do this, but this is the only way unless we introspect user expressions. Beam's philosophy is to be as direct as possible. The ==. operator corresponds to haskell ==, and so produces the boolean we would expect as Haskell programmers. The ==?. operator is a new operator that users must explicitly opt in to. Both produce the most direct code possible on each backend.)

Aggregations return Maybe types

In previous versions of beam, aggregations such as avg_, sum_, etc returned the an expression of the same type as its inputs. However, this does not match standard SQL behavior, where these aggregates can return NULL if no rows are selected for the aggregation. This breaks older code, but is more correct. To restore the older behavior, use the fromMaybe_ function to supply a default value.

Miscellaneous name changes

The Database.Beam.Query.lookup function was renamed to lookup_ to avoid overlap with the Prelude function of the same name.

Reintroduce explicit backends to Database class

Some database entites only work on particular backends. For example, beam-postgres extension support only works in beam-postgres. The lack of a backend parameter on the Database type class essentially mandated that every database entity worked on every backend. By introducing a backend parameter to Database, we allow the user to restrict which backends a database can work on.

The old behavior is still easily recovered. Whereas before you'd write

instance Database MyDatabase

Now write

instance Database be MyDatabase

Require backends to explicitly declare types that can be compared for equality

Beam previously allowed any two types to be compared for SQL equality. This is no longer the case. Rather, only types that are instances of HasSqlEqualityCheck for the given expression syntax can be checked for equality. Correspondingly, only types that are instances of HasSqlQuantifiedEqualityCheck can be checked for quantified equality.

This change is somewhat invasive, as the relationship and join operators depend on the ability to check primary keys for equality. You may have to add appropriate class constraints to your queries. In order to assert that a table can be compared for equality, you can use the HasTableEquality constraint synonym.

For Backends

Backend implementors should establish instances of HasSqlEqualityCheck and HasSqlQuantifiedEqualityCheck for every type that can be compared in their syntax. You may choose to implement a custom equality and inequality operator. Alternatively, you can leave the instances empty to use the defaults, which match the old behavior.

Properly deal with NULL values in equality

Previous versions of Beam would use SQL = and <> operators to compare potentially NULL values. However, NULL = NULL is always false, according to the SQL standard, so this behavior is incorrect.

Now, Beam will generate a CASE .. WHEN .. statement to explicitly handle mismatching NULLs. This is the 'expected' behavior from the Haskell perspective, but does not match what one may expect in SQL. Note that it is always better to explicitly handle NULLs using maybe_, and beam recommends this approach in robust code.

Remove Auto for fields with default values

Auto was a convenience type for dealing with tables where some columns have been given a default value. Auto worked well enough but it was a very leaky abstraction. Moreover, it was unnecessary. Everything you can do with Auto can be done more safely with default_.

For example, instead of using

insertValues [ Table1 (Auto Nothing) "Field Value" "Another Field Value" ]

use

insertExpressions [ Table1 default_ (val_ "Field Value") (val_ "Another Field Value") ]

0.6.0.0

0.5.0.0