Documentation

Type encoding for entity identifiers.

This encoding allows us to provide a phantom type for distinguishing between identifiers of varying types and an underlying identifier type.

For example:

>>> data A = A
>>> data B = B
>>> data C = C
>>> type IdA = Id A Int
>>> type IdB = Id B Int
>>> type IdC = Id C String
>>> let idA = Id 1 :: IdA
>>> let idB = Id 1 :: IdB
>>> let idC = Id "C1" :: IdC
>>> idA
1
>>> idB
1
>>> idC
"C1"
>>> idA == idA
True
>>> -- idA == idB  -- Compile error as: Couldn't match type ‘B’ with ‘A’

Hashes, on the otherhand, can be compared:

>>> import Data.Hashable
>>> hash idA == hash idB
True

Type encoding for a lookup table from entity Ids to corresponding entities.

>>> data A = A Int String deriving Show
>>> type IdA = Id A Int
>>> let a1 = A 1 "a1"
>>> let a2 = A 2 "a2"
>>> let a3 = A 3 "a3"
>>> let table = HM.fromList [(Id 1, a1), (Id 2, a2), (Id 3, a3)] :: IdLookup A Int
>>> HM.lookup (Id 1) table
Just (A 1 "a1")