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squeal-postgresql v0.6.0.0 Release Notes

Release Date: 2020-04-16 // about 4 years ago

  • 🔖 Version 0.6

    🔖 Version 0.6 makes a number of large changes and additions to Squeal.
    I want to thank folks who contributed issues and pull requests;
    ilyakooo0, tuomohopia, league, Raveline, Sciencei, mwotton, and more.

    I particularly would like to thank my employer SimSpace and colleagues.
    We are actively using Squeal at SimSpace which has pushed its development.

    My colleague Mark Wotton has also created a project
    squealgen to generate
    a Squeal schema directly from the database which is awesome.

    Module hierarchy

    Squeal had been growing some rather large modules, whereas I prefer
    sub-thousand line modules. Accordingly, I split up the module
    hierarchy further. This means there's 60 modules which looks a little
    overwhelming, but I think it makes it easier to locate functionality.
    It also makes working in a single module less overwhelming.
    All relevant functionality is still being exported by Squeal.PostgreSQL.

    Statement Profunctors

    Squeal's top level queries and manipulations left something to be desired.
    Because Query_ and Manipulation_ were type families, they could be
    a bit confusing to use. For instance,

    \>\>\> :{selectUser :: Query\_ DB UserId UserselectUser = select\_ (#id `as` #userId :\* #name `as` #userName) (from (table #users) & where\_ (#id .== param @1)):}\>\>\> :t selectUser selectUser :: Query'[]'[] '["public" ::: '["users" ::: 'Table ('[] :=\> UsersColumns)]] '['NotNull 'PGint4] '["userId" ::: 'NotNull 'PGint4, "userName" ::: 'NotNull 'PGtext]

    So the UserId and User types are completely replaced by corresponding
    Postgres types. This means that the query can be run, for instance,
    with any parameter that is a generic singleton container of Int32.
    We've lost apparent type safety. You could accidentally run selectUser
    with a WidgetId parameter instead of a UserId and it could typecheck.

    That's because Query is a pure SQL construct, with no knowledge for
    how to encode or decode Haskell values.

    Another annoyance of Query_ and Manipulation_ is that they must
    be applied to Haskell types which exactly match their corresponding
    Postgres types. So, in practice, you often end up with one-off
    data type definitions just to have a type that exactly matches,
    having the same field names, and the same ordering, etc. as the
    returned row.

    Both of these issues are solved with the new Statement type. Let's
    👀 see its definition.

    data Statement db x y whereManipulation:: (SOP.All (OidOfNull db) params, SOP.SListI row) =\> EncodeParams db params x-\> DecodeRow row y-\> Manipulation '[] db params row-\> Statement db x yQuery:: (SOP.All (OidOfNull db) params, SOP.SListI row) =\> EncodeParams db params x-\> DecodeRow row y-\> Query '[] '[] db params row-\> Statement db x y

    👀 You can see that a Statement bundles either a Query or a Manipulation
    together with a way to EncodeParams and a way to DecodeRow. This
    ties the statement to actual Haskell types. Going back to the example,

    \>\>\> :{selectUser :: Statement DB UserId UserselectUser = query $ select\_ (#id `as` #userId :\* #name `as` #userName) (from (table #users) & where\_ (#id .== param @1)):}

    Now we really do have the type safety of only being able to executeParams
    selectUser with a UserId parameter. Here we've used the smart
    constructor query which automatically uses the generic instances of
    UserId and User to construct a way to EncodeParams and a way to
    DecodeRow. We can use the Query constructor to do custom encodings
    and decodings.

    \>\>\> :{selectUser :: Statement DB UserId (UserId, Text) selectUser = Query enc dec sql where enc = contramap getUserId aParam dec = do uid \<- #id uname \<- #name return (uid, uname) sql = select Star (from (table #users) & where\_ (#id .== param @1)):}

    EncodeParams and DecodeRow both have convenient APIs. EncodeParams
    is Contravariant and can be composed with combinators. DecodeRow
    is a Monad and has IsLabel instances. Since Statements bundle
    both together, they form Profunctors, where you can lmap over
    parameters and rmap over rows.

    The Statement Profunctor is heavily influenced by
    the Statement Profunctor from Nikita Volkov's excellent hasql library,
    🏗 building on the use of postgresql-binary for encoding and decoding.

    Deriving

    Many Haskell types have corresponding Postgres types like Double
    corresponds to float8. Squeal makes this an open relationship with the
    PG type family. Squeal 0.6 makes it easy to generate PG of your
    Haskell types, though you might have to turn on -XUndecidableInstances,
    by deriving an IsPG instance.
    In addition to having a corresponding Postgres type,
    to fully embed your Haskell type you want instances of ToPG db to
    encode your type as an out-of-line parameter, FromPG to
    decode your type from a result value, and Inline to inline
    values of your type directly in SQL statements.

    \>\>\> :{newtype CustomerId = CustomerId {getCustomerId :: Int32}deriving newtype (IsPG, ToPG db, FromPG, Inline):}\>\>\> :kind! PG CustomerIdPG CustomerId :: PGType= 'PGint4

    You can even embed your Haskell records and enum types using
    deriving via.

    \>\>\> :{data Complex = Complex {real :: Double, imaginary :: Double}deriving stock (GHC.Generic) deriving anyclass (SOP.Generic, SOP.HasDatatypeInfo) deriving (IsPG, ToPG db, FromPG, Inline) via Composite Complex:}\>\>\> :kind! PG ComplexPG Complex :: PGType= 'PGcomposite '["real" ::: 'NotNull 'PGfloat8, "imaginary" ::: 'NotNull 'PGfloat8]\>\>\> printSQL (inline (Complex 0 1))ROW((0.0 :: float8), (1.0 :: float8))\>\>\> :{data Answer = Yes | Noderiving stock (GHC.Generic) deriving anyclass (SOP.Generic, SOP.HasDatatypeInfo) deriving (IsPG, ToPG db, FromPG, Inline) via Enumerated Answer:}\>\>\> :kind! PG AnswerPG Answer :: PGType= 'PGenum '["Yes", "No"]\>\>\> printSQL (inline Yes) 'Yes'

    You can also embed your types encoded as Json or Jsonb.

    \>\>\> :{data Foo = Bar Int | Baz Char Textderiving stock (GHC.Generic) deriving anyclass (ToJSON, FromJSON) deriving (IsPG, ToPG db, FromPG, Inline) via Jsonb Foo:}\>\>\> :kind! PG FooPG Foo :: PGType= 'PGjsonb\>\>\> printSQL (inline (Baz 'a' "aaa")) ('{"tag":"Baz","contents":["a","aaa"]}' :: jsonb)

    One thing to notice about ToParam db is that it has an
    extra parameter db that the other classes don't have. That's
    because for some types, such as arrays and composites, you
    need to know the OID of the element types in order to unambiguously
    encode those types. And if the element types are user defined,
    then they have to be looked up in the database. The extra parameter
    lets us look through the schema for a matching type, and then
    look up that type's OID.

    Migrations

    Previously Squeal migrations could be either pure, involving only
    data definitions, or impure, allowing arbitrary IO. But, they
    had to be rewindable; that is, every migration step had to have
    an inverse. Squeal 0.6 generalizes to allow both invertible and
    🆓 one-way migrations. The core datatype for migrations, the free
    📦 category Path has been moved to its own package free-categories.

    Aggregation

    Squeal 0.6 enables filtering and ordering for aggregate
    arguments and filtering for window function arguments.

    arrayAgg (All #col & orderBy [AscNullsFirst #col] & filterWhere (#col .\< 100))

    ⬆️ To upgrade existing code, if you have an aggregate with multiple arguments,
    👉 use Alls instead of All or Distincts instead of Distinct
    🏁 and if you have a window function, apply either Window or Windows
    to its argument(s). Additionally, convenient functions allNotNull and
    distinctNotNull safely filter out NULL.

    Ranges

    Squeal 0.6 adds both Haskell and corresponding Postgres range types.

    data Bound x= Infinite -- ^ unbounded | Closed x -- ^ inclusive | Open x -- ^ exclusivedata Range x = Empty | NonEmpty (Bound x) (Bound x)(\<=..\<=), (\<..\<), (\<=..\<), (\<..\<=) :: x -\> x -\> Range xmoreThan, atLeast, lessThan, atMost :: x -\> Range xsingleton :: x -\> Range xwhole :: Range x

    Indexes and functions

    👍 Squeal 0.6 adds support for creating and dropping user defined
    indexes and functions to your schema, which can then be used
    in statements.

    Lateral joins

    👍 Squeal 0.6 adds support for lateral joins, which may reference previous
    items.

    Null handling

    Some null handling functions were added such as monoNotNull
    and unsafeNotNull. Because Squeal is aggressively NULL polymorphic,
    sometimes inference errors can occur. You can apply monoNotNull
    to fix something to be not NULL. You can apply unsafeNotNull
    when you know that something can't be NULL, for instance if you've
    filtered NULL out of a column.

    Other changes

    👍 Lots of other things changed. Literal and literal are now called Inline and inline. ColumnConstraint is called Optionality. NullityTypes are called NullTypes. Squeal 0.6 adds support for domain types. It more carefully types
    CREATE _ IF NOT EXISTS and DROP _ IF EXISTS definitions. The
    🔨 Exception type was refactored to remove Maybes and new pattern
    synonyms were defined to easily match on a few common SQL errors.
    VarChar and FixChar types were added with smart constructors.
    ✅ Many bugs were fixed. Also, many more tests were added and
    a new benchmark suite. A lot more things were changed that I've
    probably forgotten about.