Paul Brinkmeier
4bf489c554
Remove LocalTime decoding because it requires IO to convert from Postgres time.
77 lines
2.3 KiB
Markdown
77 lines
2.3 KiB
Markdown
# opium
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> An opionated Haskell Postgres library.
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## Quick Start
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We assume that our database contains this table:
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```sql
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CREATE TABLE person (
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name TEXT NOT NULL,
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age INT NOT NULL,
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score DOUBLE PRECISION NOT NULL,
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motto TEXT
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)
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```
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We can use `opium` to decode query results into Haskell data types:
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```haskell
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{-# LANGUAGE DeriveGeneric #-}
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{-# LANGUAGE OverloadedStrings #-}
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import GHC.Generics (Generic)
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import Database.PostgreSQL.LibPQ (Connection)
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import qualified Database.PostgreSQL.Opium as Opium
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data User = User
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{ name :: String
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, age :: Int
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, score :: Double
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} deriving (Eq, Generic, Show)
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instance Opium.FromRow User where
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getUsers :: Connection -> IO (Either Opium.Error [Users])
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getUsers conn = Opium.fetch_ conn "SELECT * FROM user"
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```
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The `Opium.FromRow` instance is implemented generically for all product types ("records"). It looks up the field name in the query result and decodes the column value using `Opium.FromField`.
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The intended use case for this library is to enable us to write ad-hoc types for query results without having to manually write instances.
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For example, if we wanted to figure out how user age influences their score:
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```haskell
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data ScoreByAge = ScoreByAge { t :: Double, m :: Double }
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deriving (Eq, Generic, Show)
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instance Opium.FromRow ScoreByAge where
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getScoreByAge :: Connection -> IO ScoreByAge
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getScoreByAge conn = do
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let query = "SELECT regr_intercept(score, age) AS t, regr_slope(score, age) AS m FROM user"
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Right [x] <- Opium.fetch_ conn query
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pure x
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```
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## TO DO
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- [x] Implement `String` and `Text` decoding
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- [x] Implement `Int` decoding
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- [x] Implement error reporting i.e. use `Either OpiumError` instead of `Maybe`
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- [x] Implement `Float` and `Double` decoding
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- [x] Clean up and document column table stuff
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- [x] Decode `LibPQ.Binary`
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- [x] Implement `date -> Day` decoding
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- [x] Implement `UTCTime`
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- [x] Implement `ByteString` decoding (`bytea`)
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- [x] Test negative integer decoding, especially for `Integer`
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- [ ] Implement time intervals
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- [ ] and zoned time decoding
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- [ ] Implement `fetch` (`fetch_` but with parameter passing)
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- [ ] Implement JSON decoding
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- [ ] Implement (anonymous) composite types
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- [ ] Catch [UnicodeException](https://hackage.haskell.org/package/text-2.1/docs/Data-Text-Encoding-Error.html#t:UnicodeException) when decoding text
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- [ ] Implement array decoding
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