Opaleye.Experimental.Enum

Documentation

enumMapper

Arguments

:: String

The name of the ENUM type

-> (String -> Maybe haskellSum)

A function which converts from the string representation of the ENUM field

-> (haskellSum -> String)

A function which converts to the string representation of the ENUM field

-> EnumMapper sqlEnum haskellSum

The sqlEnum type variable is phantom. To protect yourself against type mismatches you should set it to the Haskell type that you use to represent the ENUM.

Create a mapping between a Postgres ENUM type and a Haskell type. Also works for DOMAIN types. For example, if you have the following ENUM

CREATE TYPE public.mpaa_rating AS ENUM (
   'G',
   'PG',
   'PG-13',
   'R',
   'NC-17'
);

then you can define data types to represent the enum on the SQL side and Haskell side respectively

data SqlRating
data Rating = G | PG | PG13 | R | NC17 deriving Show

and functions to map between them

toSqlRatingString :: Rating -> String
toSqlRatingString r = case r of
    G    -> "G"
    PG   -> "PG"
    PG13 -> "PG-13"
    R    -> "R"
    NC17 -> "NC-17"

fromSqlRatingString :: String -> Maybe Rating
fromSqlRatingString s = case s of
    "G"     -> Just G
    "PG"    -> Just PG
    "PG-13" -> Just PG13
    "R"     -> Just R
    "NC-17" -> Just NC17
    _       -> Nothing

Then you can use the mappings as follows

import qualified Opaleye as O
import qualified Data.Profunctor.Product.Default as D

sqlRatingMapper :: EnumMapper SqlRating Rating
sqlRatingMapper = enumMapper "mpaa_rating" fromSqlRatingString toSqlRatingString

instance O.DefaultFromField SqlRating Rating where
  defaultFromField = enumFromField sqlRatingMapper

instance rating ~ Rating
  => D.Default (Inferrable O.FromField) SqlRating rating where
  def = Inferrable D.def

instance D.Default O.ToFields Rating (O.Field SqlRating) where
  def = enumToFields sqlRatingMapper

data EnumMapper sqlEnum haskellSum

enumMapperWithSchema

Arguments

:: String

The schema of the ENUM type

-> String

The name of the ENUM type

-> (String -> Maybe haskellSum)

A function which converts from the string representation of the ENUM field

-> (haskellSum -> String)

A function which converts to the string representation of the ENUM field

-> EnumMapper sqlEnum haskellSum

The sqlEnum type variable is phantom. To protect yourself against type mismatches you should set it to the Haskell type that you use to represent the ENUM.

enumFromField :: EnumMapper sqlEnum haskellSum -> FromField sqlEnum haskellSum

enumToFields :: EnumMapper sqlEnum haskellSum -> ToFields haskellSum (Field sqlEnum)