Bookkeeper is a new Haskell library that uses the new OverloadedLabels feature of GHC 8 to provide a new take on datatypes and records:
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE OverloadedLabels #-}
import Bookkeeper
jane :: Book '[ "name" :=> String, "age" :=> Int ]
jane = emptyBook
& #name =: "Jane"
& #age =: 30
-- >>> jane
-- Book {age = 30, name = "Jane"}
-- >>> jane ?: #name
-- "Jane"
It bears some similarities to Nikita Volkov's record library, but requires no Template Haskell.
Accesing a field that does not exist is a type error, made nicer with GHCs new custom type errors:
-- >>> jane ?: #address
-- • The provided Book does not contain the field "address"
-- Book type:
-- '["age" ':-> Int, "name" ':-> String]
The order in which fields are inserted or appear in types does not matter. That is, in:
-- type A = Book '[ "field1" :=> Int, "field2" :=> Bool]
-- type B = Book '[ "field2" :=> Bool, "field1" :=> Int ]
Types A and B are the same.
You can set, modify, or get fields. See the haddocks for more information.
main :: IO ()
main = return ()