1{-# language DuplicateRecordFields #-}
20 -- *** Deriving-via helpers
28 -- *** @TypeInformation@
31 , parseTypeInformation
33 -- ** The @DBType@ hierarchy
39 -- * Tables and higher-kinded tables
49 , EqTable, (==:), (/=:)
50 , OrdTable, ascTable, descTable
57 , maybeTable, nothingTable, justTable
58 , isNothingTable, isJustTable
66 , eitherTable, leftTable, rightTable
67 , isLeftTable, isRightTable
71 , bitraverseEitherTable
75 , theseTable, thisTable, thatTable, thoseTable
76 , isThisTable, isThatTable, isThoseTable
77 , hasHereTable, hasThereTable
78 , justHereTable, justThereTable
80 , keepHereTable, loseHereTable
81 , keepThereTable, loseThereTable
82 , keepThisTable, loseThisTable
83 , keepThatTable, loseThatTable
84 , keepThoseTable, loseThoseTable
86 , bitraverseTheseTable
113 -- ** Boolean operations
118 , (==.), (/=.), (==?), (/=?)
124 , (<.), (<=.), (>.), (>=.)
125 , (<?), (<=?), (>?), (>=?)
126 , leastExpr, greatestExpr
151 -- ** @LIMIT@/@OFFSET@
183 , DBString, stringAgg
202 -- * Running statements
226 -- These need organizing, but are reachable from Rel8's documentation so we
227 -- do need to export and document them.
241import Rel8.Expr.Aggregate
244import Rel8.Expr.Function
247import Rel8.Expr.Order
248import Rel8.Expr.Serialize
251import Rel8.Query.Aggregate
252import Rel8.Query.Distinct
253import Rel8.Query.Each
254import Rel8.Query.Either
255import Rel8.Query.Exists
256import Rel8.Query.Filter
257import Rel8.Query.Limit
258import Rel8.Query.List
259import Rel8.Query.Maybe
260import Rel8.Query.Null
261import Rel8.Query.Order
262import Rel8.Query.SQL (showQuery)
264import Rel8.Query.These
265import Rel8.Query.Values
266import Rel8.Schema.Column
267import Rel8.Schema.Context (Name)
268import Rel8.Schema.Context.Label
269import Rel8.Schema.Generic
270import Rel8.Schema.HTable
271import Rel8.Schema.Nullability
272import Rel8.Schema.Table
273import Rel8.Statement.Delete
274import Rel8.Statement.Insert
275import Rel8.Statement.Returning
276import Rel8.Statement.Select
277import Rel8.Statement.Update
279import Rel8.Table.Aggregate
280import Rel8.Table.Alternative
281import Rel8.Table.Bool
282import Rel8.Table.Either
284import Rel8.Table.Insert
285import Rel8.Table.List
286import Rel8.Table.Maybe
287import Rel8.Table.NonEmpty
289import Rel8.Table.Order
290import Rel8.Table.Recontextualize
291import Rel8.Table.Serialize
292import Rel8.Table.These
295import Rel8.Type.Information
296import Rel8.Type.JSONEncoded
297import Rel8.Type.JSONBEncoded
298import Rel8.Type.Monoid
301import Rel8.Type.ReadShow
302import Rel8.Type.Semigroup
303import Rel8.Type.String
309-- In this section, we'll take a look at using Rel8 to work with a small
310-- database for Haskell packages. We'll take a look at idiomatic usage of Rel8,
311-- mapping tables to Haskell, and then look at writing some simple queries.
313-- The documentation in this library uses the following language extensions:
315-- >>> :set -XBlockArguments
316-- >>> :set -XDeriveAnyClass
317-- >>> :set -XDeriveGeneric
318-- >>> :set -XDerivingStrategies
319-- >>> :set -XDerivingVia
320-- >>> :set -XDuplicateRecordFields
321-- >>> :set -XGeneralizedNewtypeDeriving
322-- >>> :set -XOverloadedStrings
323-- >>> :set -XStandaloneDeriving
324-- >>> :set -XTypeApplications
325-- >>> :set -XTypeFamilies
327-- Before we start writing any Haskell, let's take a look at the schema we'll
332-- > Column │ Type │ Nullable Column │ Type │ Nullable
333-- > ═══════════╪═════════╪══════════ ═══════════╪═════════╪══════════
334-- > author_id │ integer │ not null author_id │ integer │ not null
335-- > name │ text │ not null name │ text │ not null
338-- Our schema consists of two tables - @author@ and @project@. A @project@
339-- always has an @author@, but not all @author@s have projects. Each @author@
340-- has a name and (maybe) an associated website, and each project has a name.
342-- Now that we've seen our schema, we can begin writing a mapping in Rel8. The
343-- idiomatic way to map a table is to use a record that is parameterised what
344-- Rel8 calls an /interpretation functor/, and to define each field with
345-- 'Column'. For this type to be usable with Rel8 we need it to be an instance
346-- of 'HigherKindedTable', which can be derived with a combination of
347-- @DeriveAnyClass@ and @DeriveGeneric@.
349-- Following these steps for @author@, we have:
351-- > data Author f = Author
352-- > { authorId :: Column f Int64
353-- > , name :: Column f Text
354-- > , url :: Column f (Maybe Text)
355-- > } deriving (Generic, HigherKindedTable)
357-- This is a perfectly reasonable definition, but cautious readers might notice
358-- a problem - in particular, with the type of the @authorId@ field. While
359-- @Int64@ is correct, it's not the best type. If we had other identifier types
360-- in our project, it would be too easy to accidentally mix them up and create
361-- nonsensical joins. As Haskell programmers, we often solve this problem by
362-- creating @newtype@ wrappers, and we can also use this technique with Rel8:
365-- newtype AuthorId = AuthorId { toInt64 :: Int64 }
366-- deriving newtype (DBEq, DBType, Eq, Show)
369-- Now we can write our final schema mapping. First, the @author@ table:
372-- data Author f = Author
373-- { authorId :: Column f AuthorId
374-- , authorName :: Column f Text
375-- , authorUrl :: Column f (Maybe Text)
377-- deriving stock Generic
378-- deriving anyclass HigherKindedTable
381-- And similarly, the @project@ table:
384-- data Project f = Project
385-- { projectAuthorId :: Column f AuthorId
386-- , projectName :: Column f Text
388-- deriving stock Generic
389-- deriving anyclass HigherKindedTable
392-- To show query results in this documentation, we'll also need @Show@
393-- instances: Unfortunately these definitions look a bit scary, but they are
394-- essentially just @deriving (Show)@:
396-- >>> deriving stock instance f ~ Identity => Show (Author f)
397-- >>> deriving stock instance f ~ Identity => Show (Project f)
399-- These data types describe the structural mapping of the tables, but we also
400-- need to specify a 'TableSchema' for each table. A @TableSchema@ contains the
401-- name of the table and the name of all columns in the table, which will
402-- ultimately allow us to @SELECT@ and @INSERT@ rows for these tables.
404-- To define a @TableSchema@, we just need to fill construct appropriate
405-- @TableSchema@ values. When it comes to the @tableColumns@ field, we
406-- construct values of our data types above, and set each field to the name of
407-- the column that it maps to:
409-- First, @authorSchema@ describes the column names of the @author@ table when
410-- associated with the @Author@ type:
413-- authorSchema :: TableSchema (Author ColumnSchema)
414-- authorSchema = TableSchema
415-- { tableName = "author"
416-- , tableSchema = Nothing
417-- , tableColumns = Author
418-- { authorId = "author_id"
419-- , authorName = "name"
420-- , authorUrl = "url"
425-- And likewise for @project@ and @Project@:
428-- projectSchema :: TableSchema (Project ColumnSchema)
429-- projectSchema = TableSchema
430-- { tableName = "project"
431-- , tableSchema = Nothing
432-- , tableColumns = Project
433-- { projectAuthorId = "author_id"
434-- , projectName = "name"
439-- Aside: you might be wondering why this information isn't in the definitions
440-- of @Author@ and @Project@ above. Rel8 decouples @TableSchema@ from the data
441-- types themselves, as not all tables you define will necessarily have a
442-- schema. For example, Rel8 allows you to define helper types to simplify the
443-- types of queries - these tables only exist at query time, but there is no
444-- corresponding base table. We'll see more on this idea later!
446-- With these table definitions, we can now start writing some queries! To
447-- actually run a query, we'll need a database connection. Rel8 uses the
448-- @hasql@ library, and for this documentation we'll get the connection string
449-- from the @$TEST_DATABASE_URL@ environment variable.
451-- >>> connectionString <- System.Environment.getEnv "TEST_DATABASE_URL"
452-- >>> Right c <- Hasql.Connection.acquire (Data.ByteString.Char8.pack connectionString)
453-- >>> Control.Monad.void $ Hasql.Session.run (Hasql.Session.sql "BEGIN") c
457-- First, we'll take a look at @SELECT@ statements - usually the bulk of most
458-- database heavy applications.
460-- In Rel8, @SELECT@ statements are built using the 'Query' monad. You can
461-- think of this monad like the ordinary @[]@ (List) monad - but this isn't
462-- required knowledge.
464-- To start, we'll look at one of the simplest queries possible - a basic
465-- @SELECT * FROM@ statement. To select all rows from a table, we use 'each',
466-- and supply a 'TableSchema'. So to select all @project@ rows, we can write:
468-- >>> :t each projectSchema
469-- each projectSchema :: Query (Project Expr)
471-- Notice that @each@ gives us a @Query@ that yields @Project Expr@ rows. To
472-- see what this means, let's have a look at a single field of a
475-- >>> let aProjectExpr = undefined :: Project Expr
476-- >>> :t projectAuthorId aProjectExpr
477-- projectAuthorId aProjectExpr :: Expr AuthorId
479-- Recall we defined @projectAuthorId@ as @Column f AuthorId@. Now we have @f@
480-- is @Expr@, and @Column Expr AuthorId@ reduces to @Expr AuthorId@. We'll see
481-- more about @Expr@ soon, but you can think of @Expr a@ as "SQL expressions of
484-- To execute this @Query@, we pass it to 'select':
486-- >>> :t select c (each projectSchema)
487-- select c (each projectSchema) :: MonadIO m => m [Project Identity]
489-- When we @select@ things containing @Expr@s, Rel8 builds a new response table
490-- with the @Identity@ interpretation. This means you'll get back plain Haskell
491-- values. Studying @projectAuthorId@ again, we have:
493-- >>> let aProjectIdentity = undefined :: Project Identity
494-- >>> :t projectAuthorId aProjectIdentity
495-- projectAuthorId aProjectIdentity :: AuthorId
497-- Here @Column Identity AuthorId@ reduces to just @AuthorId@, with no
498-- wrappping type at all.
500-- Putting this all together, we can run our first query:
502-- >>> select c (each projectSchema) >>= mapM_ print
503-- Project {projectAuthorId = 1, projectName = "rel8"}
504-- Project {projectAuthorId = 2, projectName = "aeson"}
505-- Project {projectAuthorId = 2, projectName = "text"}
509-- We now know that 'each' is the equivalent of a @SELECT *@ query, but
510-- sometimes we're only interested in a subset of the columns of a table. To
511-- restrict the returned columns, we can specify a projection by using 'Query's
512-- @Functor@ instance:
514-- >>> select c $ projectName <$> each projectSchema
515-- ["rel8","aeson","text"]
519-- Another common operation in relational databases is to take the @JOIN@ of
520-- multiple tables. Rel8 doesn't have a specific join operation, but we can
521-- recover the functionality of a join by selecting all rows of two tables, and
522-- then using 'where_' to filter them.
524-- To see how this works, first let's look at taking the product of two tables.
525-- We can do this by simply calling 'each' twice, and then returning a tuple of
529-- mapM_ print =<< select c do
530-- author <- each authorSchema
531-- project <- each projectSchema
532-- return (projectName project, authorName author)
535-- ("rel8","Bryan O'Sullivan")
536-- ("rel8","Emily Pillmore")
538-- ("aeson","Bryan O'Sullivan")
539-- ("aeson","Emily Pillmore")
541-- ("text","Bryan O'Sullivan")
542-- ("text","Emily Pillmore")
544-- This isn't quite right, though, as we have ended up pairing up the wrong
545-- projects and authors. To fix this, we can use 'where_' to restrict the
546-- returned rows. We could write:
549-- > author <- each authorSchema
550-- > project <- each projectSchema
551-- > where_ $ projectAuthorId project ==. authorId author
552-- > return (project, author)
554-- but doing this every time you need a join can obscure the meaning of the
555-- query you're writing. A good practice is to introduce specialised functions
556-- for the particular joins in your database. In our case, this would be:
559-- projectsForAuthor :: Author Expr -> Query (Project Expr)
560-- projectsForAuthor a = each projectSchema >>= filter \p ->
561-- projectAuthorId p ==. authorId a
564-- Our final query is then:
567-- mapM_ print =<< select c do
568-- author <- each authorSchema
569-- project <- projectsForAuthor author
570-- return (projectName project, authorName author)
573-- ("aeson","Bryan O'Sullivan")
574-- ("text","Bryan O'Sullivan")
578-- Rel8 is also capable of performing @LEFT JOIN@s. To perform @LEFT JOIN@s, we
579-- follow the same approach as before, but use the 'optional' query transformer
580-- to allow for the possibility of the join to fail.
582-- In our test database, we can see that there's another author who we haven't
585-- >>> select c $ authorName <$> each authorSchema
586-- ["Ollie","Bryan O'Sullivan","Emily Pillmore"]
588-- Emily wasn't returned in our earlier query because - in our database - she
589-- doesn't have any registered projects. We can account for this partiality in
590-- our original query by wrapping the @projectsForAuthor@ call with 'optional':
593-- mapM_ print =<< select c do
594-- author <- each authorSchema
595-- mproject <- optional $ projectsForAuthor author
596-- return (authorName author, projectName <$> mproject)
598-- ("Ollie",Just "rel8")
599-- ("Bryan O'Sullivan",Just "aeson")
600-- ("Bryan O'Sullivan",Just "text")
601-- ("Emily Pillmore",Nothing)
605-- Aggregations are operations like @sum@ and @count@ - operations that reduce
606-- multiple rows to single values. To perform aggregations in Rel8, we can use
607-- the 'aggregate' function, which takes a 'Query' of aggregated expressions,
608-- runs the aggregation, and returns aggregated rows.
610-- To start, let's look at a simple aggregation that tells us how many projects
615-- Rel8 is also capable of aggregating multiple rows into a single row by
616-- concatenating all rows as a list. This aggregation allows us to break free
617-- of the row-orientated nature of SQL and write queries that return tree-like
618-- structures. Earlier we saw an example of returning authors with their
619-- projects, but the query didn't do a great job of describing the one-to-many
620-- relationship between authors and their projects.
622-- Let's look again at a query that returns authors and their projects, and
623-- focus on the /type/ of that query.
626-- projectsForAuthor a = each projectSchema >>= filter \p ->
627-- projectAuthorId p ==. authorId a
631-- let authorsAndProjects = do
632-- author <- each authorSchema
633-- project <- projectsForAuthor author
634-- return (author, project)
638-- >>> :t select c authorsAndProjects
639-- select c authorsAndProjects
640-- :: MonadIO m => m [(Author Identity, Project Identity)]
642-- Our query gives us a single list of pairs of authors and projects. However,
643-- with our domain knowledge of the schema, this isn't a great type - what we'd
644-- rather have is a list of pairs of authors and /lists/ of projects. That is,
647-- > [(Author Identity, [Project Identity])]
649-- This would be a much better type! Rel8 can produce a query with this type by
650-- simply wrapping the call to @projectsForAuthor@ with either 'some' or
651-- 'many'. Here we'll use 'many', which allows for the possibility of an author
652-- to have no projects:
655-- mapM_ print =<< select c do
656-- author <- each authorSchema
657-- projectNames <- many $ projectName <$> projectsForAuthor author
658-- return (authorName author, projectNames)
661-- ("Bryan O'Sullivan",["aeson","text"])
662-- ("Emily Pillmore",[])
keysesc to close
- j k
- move down, up
- enter
- open
- esc
- back, or up a level
- 1 2 3 4
- readme, files, commits, refs
- /
- filter
- y
- copy the clone command
- c
- compare the chosen commit with another (or shift-click both)
- ?
- show or hide this list