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Add named field construction
The `named` library seems to make a lot of this look much prettier, so I'm very excited about this change!
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Tom Harding
parent
476d73a92e
commit
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66
README.md
66
README.md
@ -50,6 +50,8 @@ example data types:
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{-# LANGUAGE DataKinds #-}
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{-# LANGUAGE DeriveGeneric #-}
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{-# LANGUAGE TypeApplications #-}
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{-# LANGUAGE OverloadedLabels #-}
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{-# LANGUAGE TypeOperators #-}
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module Main where
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import Control.Applicative (Alternative (empty))
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@ -61,6 +63,7 @@ import Data.Generic.HKD
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import Data.Maybe (isJust, isNothing)
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import Data.Monoid (Last (..))
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import GHC.Generics (Generic)
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import Named ((:!), (!))
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-- An example of a record (with named fields):
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data User
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@ -72,7 +75,7 @@ data User
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deriving (Generic, Show)
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user :: User
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user = User "Tom" 25 True
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user = User "Tom" 26 True
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-- An example of a product (without named fields):
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data Triple
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@ -145,7 +148,7 @@ eg3 :: Bare User
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eg3 = deconstruct user
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-- User
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-- { name = Identity "Tom"
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-- , age = Identity 25
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-- , age = Identity 26
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-- , likesDogs = Identity True
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-- }
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```
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@ -184,13 +187,30 @@ eg7 = eg6 [1] [] ["Tom", "Tim"]
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-- Triple [1] [] ["Tom","Tim"]
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```
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Should we need to work with records, we can exploit the label trickery of the
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[`named`](http://hackage.haskell.org/package/named) package. The `record`
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function behaves exactly as `build` does, but produces a function compatible
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with the `named` interface. After that, we can use the function with labels
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(and with no regard for the internal order):
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```haskell
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eg8 :: "name" :! f [Char]
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-> "age" :! f Int
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-> "likesDogs" :! f Bool
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-> HKD User f
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eg8 = record @User
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eg9 :: HKD User Maybe
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eg9 = eg8 ! #name (Just "Tom") ! #likesDogs (Just True) ! #age (Just 26)
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```
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If you're _still_ not satisfied, check out the
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[`buniq`](https://hackage.haskell.org/package/barbies-1.1.2.1/docs/Data-Barbie.html#v:buniq)
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method hiding in `barbies`:
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```haskell
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eg8 :: HKD Triple []
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eg8 = buniq empty
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eg10 :: HKD Triple []
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eg10 = buniq empty
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-- Triple [] [] []
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```
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@ -200,8 +220,8 @@ The `field` lens, when given a type-applied field name, allows us to focus on
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fields within a record:
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```haskell
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eg9 :: Last Int
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eg9 = eg0 ^. field @"age"
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eg11 :: Last Int
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eg11 = eg0 ^. field @"age"
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-- Last {getLast = Nothing}
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```
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@ -210,8 +230,8 @@ record (note that these set values will _also_ need to be in our functor of
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choice):
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```haskell
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eg10 :: Partial User
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eg10 = eg0 & field @"name" .~ pure "Evil Tom"
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eg12 :: Partial User
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eg12 = eg0 & field @"name" .~ pure "Evil Tom"
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& field @"likesDogs" .~ pure False
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-- User
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-- { name = Last {getLast = Just "Evil Tom"}
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@ -224,8 +244,8 @@ This also means, for example, we can check whether a particular value has been
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completed for a given partial type:
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```haskell
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eg11 :: Bool
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eg11 = anyOf (field @"name") (isJust . getLast) eg0
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eg13 :: Bool
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eg13 = anyOf (field @"name") (isJust . getLast) eg0
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-- False
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```
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@ -234,8 +254,8 @@ Finally, thanks to the fact that this library exploits some of the internals of
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doesn't exist in our type:
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```{haskell, ignore}
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eg12 :: Identity ()
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eg12 = eg3 ^. field @"oops"
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eg14 :: Identity ()
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eg14 = eg3 ^. field @"oops"
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-- error:
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-- • The type User does not contain a field named 'oops'.
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```
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@ -246,8 +266,8 @@ Just as with field names, we can use positions when working with non-record
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product types:
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```haskell
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eg13 :: Labels Triple
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eg13 = mempty & position @1 .~ Const "hello"
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eg15 :: Labels Triple
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eg15 = mempty & position @1 .~ Const "hello"
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& position @2 .~ Const "world"
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-- Triple
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-- Const "hello"
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@ -258,11 +278,11 @@ eg13 = mempty & position @1 .~ Const "hello"
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Again, this is a `Lens`, so we can just as easily _set_ values:
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```haskell
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eg14 :: Partial User
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eg14 = eg10 & position @2 .~ pure 25
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eg16 :: Partial User
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eg16 = eg12 & position @2 .~ pure 26
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-- User
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-- { name = Last {getLast = Just "Evil Tom"}
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-- , age = Last {getLast = Just 25}
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-- , age = Last {getLast = Just 26}
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-- , likesDogs = Last {getLast = Just False}
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-- }
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```
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@ -271,8 +291,8 @@ Similarly, the internals here come to us courtesy of `generic-lens`, so the
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type errors are a delight:
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```{haskell, ignore}
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eg15 :: Identity ()
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eg15 = deconstruct @Identity triple ^. position @4
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eg17 :: Identity ()
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eg17 = deconstruct @Identity triple ^. position @4
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-- error:
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-- • The type Triple does not contain a field at position 4
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```
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@ -284,8 +304,8 @@ names of the fields into the functor we're using. The `label` value gives us
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this interface:
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```haskell
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eg16 :: Labels User
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eg16 = label
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eg18 :: Labels User
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eg18 = label
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-- User
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-- { name = Const "name"
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-- , age = Const "age"
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@ -300,8 +320,8 @@ can implement functions such as `labelsWhere`, which returns the names of all
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fields whose values satisfy some predicate:
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```haskell
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eg17 :: [String]
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eg17 = labelsWhere (isNothing . getLast) eg10
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eg19 :: [String]
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eg19 = labelsWhere (isNothing . getLast) eg12
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-- ["age"]
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```
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@ -20,6 +20,7 @@ library
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Data.Generic.HKD.Build
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Data.Generic.HKD.Construction
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Data.Generic.HKD.Labels
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Data.Generic.HKD.Named
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Data.Generic.HKD.Types
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-- other-modules:
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-- other-extensions:
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@ -27,6 +28,7 @@ library
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, barbies ^>= 1.1.0
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, generic-lens ^>= 1.1.0
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, QuickCheck >= 2.12.6 && < 2.14
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, named ^>= 0.3.0.0
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hs-source-dirs: src
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default-language: Haskell2010
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@ -48,6 +50,7 @@ test-suite readme
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, barbies ^>= 1.1.0
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, lens ^>= 4.17
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, higgledy
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, named ^>= 0.3.0.0
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main-is: README.lhs
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type: exitcode-stdio-1.0
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default-language: Haskell2010
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@ -27,6 +27,7 @@ module Data.Generic.HKD
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import Data.Generic.HKD.Build as Exports
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import Data.Generic.HKD.Construction as Exports
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import Data.Generic.HKD.Labels as Exports
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import Data.Generic.HKD.Named as Exports
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import Data.Generic.HKD.Types as Exports
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import qualified Data.Barbie as Barbie
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106
src/Data/Generic/HKD/Named.hs
Normal file
106
src/Data/Generic/HKD/Named.hs
Normal file
@ -0,0 +1,106 @@
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{-# OPTIONS_HADDOCK not-home #-}
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{-# LANGUAGE AllowAmbiguousTypes #-}
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{-# LANGUAGE BlockArguments #-}
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{-# LANGUAGE DataKinds #-}
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{-# LANGUAGE FlexibleContexts #-}
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{-# LANGUAGE FlexibleInstances #-}
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{-# LANGUAGE FunctionalDependencies #-}
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{-# LANGUAGE KindSignatures #-}
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{-# LANGUAGE RankNTypes #-}
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{-# LANGUAGE ScopedTypeVariables #-}
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{-# LANGUAGE TypeApplications #-}
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{-# LANGUAGE TypeFamilies #-}
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{-# LANGUAGE TypeOperators #-}
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{-# LANGUAGE UndecidableInstances #-}
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{-|
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Module : Data.Generic.HKD.Named
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Description : Construct an HKD record with named parameters.
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Copyright : (c) Tom Harding, 2019
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License : MIT
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Maintainer : tom.harding@habito.com
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Stability : experimental
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-}
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module Data.Generic.HKD.Named
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( Record (..)
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) where
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import Data.Functor.Contravariant (Contravariant (..))
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import Data.Generic.HKD.Types (HKD, HKD_)
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import Data.GenericLens.Internal (GUpcast (..))
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import Data.Kind (Type)
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import GHC.Generics
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import Named ((:!), NamedF (..))
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class Append (xs :: Type -> Type) (ys :: Type -> Type) (zs :: Type -> Type)
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| xs ys -> zs where
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instance Append (S1 m h) t (S1 m h :*: t)
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instance Append x y z => Append (w :*: x) y (w :*: z)
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class Rearrange (i :: Type -> Type) (o :: Type -> Type) | i -> o
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instance Rearrange i o => Rearrange (D1 m i) (D1 m o)
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instance Rearrange i o => Rearrange (C1 m i) (C1 m o)
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instance Rearrange i o => Rearrange (S1 m i) (S1 m o)
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instance Rearrange (Rec0 x) (Rec0 x)
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instance (Rearrange l l', Rearrange r r', Append l' r' o)
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=> Rearrange (l :*: r) o where
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-- | The 'Data.Generic.HKD.build' function lets us supply arguments to a type
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-- one by one, but can cause confusion when working with a record. If the
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-- record contains two fields of the same type, for example, we've introduced
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-- an opportunity for bugs and confusion. The @record@ function uses the
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-- wonderful @named@ package to help us:
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--
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-- >>> :set -XDeriveGeneric -XTypeApplications
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--
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-- >>> :{
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-- data User
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-- = User { name :: String, age :: Int, likesDogs :: Bool }
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-- deriving Generic
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-- :}
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--
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-- >>> :{
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-- test :: _
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-- test = record @User
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-- :}
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-- ...
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-- ... Found type wildcard ...
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-- ... standing for ...("name" :! f [Char])
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-- ... -> ("age" :! f Int) -> ("likesDogs" :! f Bool) -> HKD User f...
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-- ...
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class Record (structure :: Type) (f :: Type -> Type) (k :: Type)
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| f structure -> k where
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record :: k
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class GRecord (rep :: Type -> Type) (f :: Type -> Type) (structure :: Type) (k :: Type)
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| f structure rep -> k where
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grecord :: (forall p. rep p -> HKD structure f) -> k
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instance GRecord inner f structure k
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=> GRecord (D1 meta inner) f structure k where
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grecord rebuild = grecord (rebuild . M1)
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instance GRecord inner f structure k
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=> GRecord (C1 meta inner) f structure k where
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grecord rebuild = grecord (rebuild . M1)
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instance (rec ~ (Rec0 inner), k ~ (name :! inner -> HKD structure f))
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=> GRecord (S1 ('MetaSel ('Just name) i d c) rec) f structure k where
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grecord fill = \(Arg inner) -> fill (M1 (K1 inner))
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instance (GRecord right f structure k, rec ~ Rec0 x)
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=> GRecord (S1 ('MetaSel ('Just name) i d c) rec :*: right) f structure (name :! x -> k) where
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grecord fill = \(Arg left) -> grecord \right -> fill (M1 (K1 left) :*: right)
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instance
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( Contravariant (HKD_ f structure)
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, Functor (HKD_ f structure)
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, Rearrange (HKD_ f structure) list
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, GUpcast list (HKD_ f structure)
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, GRecord list f structure k
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)
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=> Record structure f k where
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record = grecord @_ @f @structure (to . gupcast @list @(HKD_ f structure))
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