minimal viable product

.gitignore

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+.stack-work/
+*~

CHANGELOG.md

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+# Changelog for `isomorphic-derive`
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to the
+[Haskell Package Versioning Policy](https://pvp.haskell.org/).
+
+## Unreleased
+
+### Generic instances for GenericCoerce
+
+- Derive Monoid and Semigroup instances via GenericCoerceTo
+
+## 0.1.0.0 - YYYY-MM-DD

Setup.hs

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+import Distribution.Simple
+main = defaultMain

generic-derive.cabal

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+cabal-version: 2.2
+
+-- This file has been generated from package.yaml by hpack version 0.38.0.
+--
+-- see: https://github.com/sol/hpack
+
+name:           generic-derive
+version:        0.1.0.0
+description:    Please see the README on GitHub at <https://github.com/githubuser/generic-derive#readme>
+homepage:       https://github.com/githubuser/isomorphic-derive#readme
+bug-reports:    https://github.com/githubuser/isomorphic-derive/issues
+author:         VegOwOtenks
+maintainer:     vegowotenks@jossco.de
+copyright:      AGPL3
+license:        BSD-3-Clause
+license-file:   LICENSE
+build-type:     Simple
+extra-source-files:
+    README.md
+    CHANGELOG.md
+
+source-repository head
+  type: git
+  location: https://github.com/githubuser/isomorphic-derive
+
+library
+  exposed-modules:
+      GHC.Generics.Coerce
+      GHC.Generics.Coerce.Example
+      GHC.Generics.CoerceTo
+      GHC.Generics.CoerceTo.Example
+  other-modules:
+      Paths_generic_derive
+  autogen-modules:
+      Paths_generic_derive
+  hs-source-dirs:
+      src
+  ghc-options: -Weverything
+  build-depends:
+      base >=4.7 && <5
+  default-language: Haskell2010
+
+test-suite isomorphic-derive-test
+  type: exitcode-stdio-1.0
+  main-is: Spec.hs
+  other-modules:
+      Paths_generic_derive
+  autogen-modules:
+      Paths_generic_derive
+  hs-source-dirs:
+      test
+  ghc-options: -Weverything -threaded -rtsopts -with-rtsopts=-N
+  build-depends:
+      base >=4.7 && <5
+    , isomorphic-derive
+  default-language: Haskell2010

package.yaml

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+name:                generic-derive
+version:             0.1.0.0
+github:              "githubuser/isomorphic-derive"
+license:             BSD-3-Clause
+author:              "VegOwOtenks"
+maintainer:          "vegowotenks@jossco.de"
+copyright:           "AGPL3"
+
+extra-source-files:
+- README.md
+- CHANGELOG.md
+
+# Metadata used when publishing your package
+# synopsis:            Short description of your package
+# category:            Web
+
+# To avoid duplicated efforts in documentation and dealing with the
+# complications of embedding Haddock markup inside cabal files, it is
+# common to point users to the README.md file.
+description:         Please see the README on GitHub at <https://github.com/githubuser/generic-derive#readme>
+
+dependencies:
+- base >= 4.7 && < 5
+
+ghc-options:
+- -Weverything
+
+library:
+  source-dirs: src
+
+tests:
+  isomorphic-derive-test:
+    main:                Spec.hs
+    source-dirs:         test
+    ghc-options:
+    - -threaded
+    - -rtsopts
+    - -with-rtsopts=-N
+    dependencies:
+    - isomorphic-derive

src/GHC/Generics/Coerce.hs

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+{-# LANGUAGE Unsafe #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE StandaloneKindSignatures #-}
+{-# LANGUAGE InstanceSigs #-}
+{-# LANGUAGE TypeOperators #-}
+{-# LANGUAGE ImportQualifiedPost #-}
+{-# LANGUAGE FlexibleContexts #-}
+
+{-# OPTIONS_GHC -Wno-unsafe #-}
+
+module GHC.Generics.Coerce
+(GenericCoerce(..), genericCoerce)
+where
+
+import Prelude (id, ($), (.))
+
+import GHC.Generics (U1, (:*:) ((:*:)), (:+:) (L1, R1), M1 (M1), K1 (K1), Generic)
+import GHC.Generics qualified as Generics
+
+import Data.Coerce (coerce, Coercible)
+import Data.Kind (Type, Constraint)
+
+type GenericCoerce :: (Type -> Type) -> (Type -> Type) -> Constraint
+
+class GenericCoerce a b where
+  gfrom :: a i -> b i
+  gto :: b i -> a i
+
+-- unit type data
+instance GenericCoerce U1 U1 where
+  gfrom :: U1 i -> U1 i
+  gfrom = id
+  gto :: U1 i -> U1 i
+  gto = id
+
+-- product type combinations
+instance (GenericCoerce a c, GenericCoerce b d) => GenericCoerce (a :*: b) (c :*: d) where
+  gfrom :: (:*:) a b i -> (:*:) c d i
+  gfrom (a :*: b) = gfrom a :*: gfrom b
+  gto   :: (:*:) c d i -> (:*:) a b i
+  gto   (a :*: b) = gto a :*: gto b
+
+-- sum type options
+instance (GenericCoerce a c, GenericCoerce b d) => GenericCoerce (a :+: b) (c :+: d) where
+  gfrom :: (:+:) a b i -> (:+:) c d i
+  gfrom (L1 a) = L1 $ gfrom a
+  gfrom (R1 b) = R1 $ gfrom b
+  gto :: (:+:) c d i -> (:+:) a b i
+  gto (L1 a) = L1 $ gto a
+  gto (R1 b) = R1 $ gto b
+
+-- types and constructors
+instance GenericCoerce a a' => GenericCoerce (M1 i_ c a) (M1 i_' c' a') where
+  gfrom :: M1 i_ c a i -> M1 i_' c' a' i
+  gfrom (M1 x) = M1 $ gfrom x
+  gto :: M1 i_' c' a' i -> M1 i_ c a i
+  gto (M1 x) = M1 $ gto x
+
+-- constants and parameters
+instance Coercible a a' => GenericCoerce (K1 i_ a) (K1 i_' a') where
+  gfrom :: K1 i_ a i -> K1 i_' a' i
+  gfrom (K1 x) = K1 $ coerce x
+  gto :: K1 i_' a' i -> K1 i_ a i
+  gto (K1 x) = K1 $ coerce x
+
+
+-- convenience functions
+
+genericCoerce :: (Generic a, Generic b, GenericCoerce (Generics.Rep a) (Generics.Rep b)) => a -> b
+genericCoerce = Generics.to . gfrom . Generics.from

src/GHC/Generics/Coerce/Example.hs

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+{-# LANGUAGE StandaloneKindSignatures #-}
+{-# LANGUAGE RoleAnnotations #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE DerivingStrategies #-}
+
+{-# OPTIONS_GHC -Wno-unsafe #-}
+{-# OPTIONS_GHC -Wno-missing-safe-haskell-mode #-}
+{-# OPTIONS_GHC -Wno-unused-top-binds #-}
+
+module GHC.Generics.Coerce.Example
+()
+where
+
+import Prelude (Show)
+
+import GHC.Generics (Generic)
+import GHC.Generics.Coerce (genericCoerce)
+
+import Data.Int (Int)
+import Data.Kind (Type)
+
+
+type Pair :: Type -> Type -> Type
+type role Pair representational representational
+
+data Pair a b = Pair a b
+  deriving stock (Generic, Show)
+
+tupleCoerce :: Pair a b -> (a, b)
+tupleCoerce = genericCoerce
+
+-- >>> tupleCoerce (Pair 1 1)
+-- (1,1)
+
+
+type IntPair :: Type
+data IntPair = IntPair Int Int
+  deriving stock (Generic, Show)
+
+intTupleCoerce :: IntPair -> Pair Int Int
+intTupleCoerce = genericCoerce
+
+-- >>> intTupleCoerce (IntPair 1 2)
+-- Pair 1 2
+

src/GHC/Generics/CoerceTo.hs

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+{-# LANGUAGE Unsafe #-}
+{-# LANGUAGE RoleAnnotations #-}
+{-# LANGUAGE StandaloneKindSignatures #-}
+{-# LANGUAGE InstanceSigs #-}
+{-# OPTIONS_GHC -Wno-unsafe #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE ImportQualifiedPost #-}
+
+module GHC.Generics.CoerceTo
+(GenericCoerceTo(..))
+where
+
+import Prelude ((.), ($))
+
+import GHC.Generics (Generic(Rep))
+import GHC.Generics.Coerce (GenericCoerce())
+
+import GHC.Generics qualified as Generics
+import GHC.Generics.Coerce qualified as GenericCoerce
+
+import Data.Kind (Type)
+
+import Data.List qualified as List
+import Data.List.NonEmpty (NonEmpty)
+import Data.List.NonEmpty qualified as NonEmpty
+
+import Data.Semigroup (Semigroup ((<>), sconcat))
+import Data.Monoid (Monoid (mempty, mconcat))
+
+type role GenericCoerceTo representational phantom
+
+type GenericCoerceTo :: Type -> Type -> Type
+
+newtype GenericCoerceTo self other = GenericCoerceTo { get :: self }
+
+
+toOther :: (Generic b, Generic a, GenericCoerce (Rep a) (Rep b)) => GenericCoerceTo a b -> b
+toOther = Generics.to . GenericCoerce.gfrom . Generics.from . get
+
+fromOther :: (Generic a, Generic b, GenericCoerce (Rep a) (Rep b)) => b -> GenericCoerceTo a b
+fromOther = GenericCoerceTo . Generics.to . GenericCoerce.gto . Generics.from
+
+
+instance 
+  (Generic this
+  , Generic other
+  , GenericCoerce (Rep this) (Rep other)
+  , Semigroup other) 
+  => Semigroup (GenericCoerceTo this other) where
+
+  (<>) :: GenericCoerceTo this other -> GenericCoerceTo this other -> GenericCoerceTo this other
+  lhs <> rhs = fromOther $ toOther lhs <> toOther rhs
+
+  sconcat :: NonEmpty (GenericCoerceTo this other) -> GenericCoerceTo this other
+  sconcat = fromOther . sconcat . NonEmpty.map toOther
+
+instance
+  (Generic this
+  , Generic other
+  , GenericCoerce (Rep this) (Rep other)
+  , Monoid other)
+  => Monoid (GenericCoerceTo this other) where
+
+  mempty :: GenericCoerceTo this other
+  mempty = fromOther mempty
+
+  mconcat :: [GenericCoerceTo this other] -> GenericCoerceTo this other
+  mconcat = fromOther . mconcat . List.map toOther
+

src/GHC/Generics/CoerceTo/Example.hs

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+{-# LANGUAGE Unsafe #-}
+{-# LANGUAGE DerivingStrategies #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE DerivingVia #-}
+{-# LANGUAGE TypeOperators #-}
+
+{-# OPTIONS_GHC -Wno-unsafe #-}
+{-# LANGUAGE StandaloneKindSignatures #-}
+
+module GHC.Generics.CoerceTo.Example () where
+
+import Prelude ()
+
+import GHC.Generics (Generic)
+import GHC.Generics.CoerceTo ( GenericCoerceTo(GenericCoerceTo) )
+
+import Data.Kind (Type)
+
+import Data.Semigroup (Semigroup)
+import Data.Monoid (Sum(Sum))
+import Data.Int (Int)
+import Text.Show (Show)
+
+type IntPair :: Type
+
+data IntPair = IntPair Int Int
+  deriving stock (Generic, Show)
+  deriving (Semigroup) via (IntPair `GenericCoerceTo` (Sum Int, Sum Int))
+
+-- >>> IntPair 1 1 <> IntPair 1 1
+-- IntPair 2 2
+

stack.yaml

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+# This file was automatically generated by 'stack init'
+#
+# Some commonly used options have been documented as comments in this file.
+# For advanced use and comprehensive documentation of the format, please see:
+# https://docs.haskellstack.org/en/stable/configure/yaml/
+
+# A 'specific' Stackage snapshot or a compiler version.
+# A snapshot resolver dictates the compiler version and the set of packages
+# to be used for project dependencies. For example:
+#
+# snapshot: lts-23.14
+# snapshot: nightly-2025-02-15
+# snapshot: ghc-9.8.4
+#
+# The location of a snapshot can be provided as a file or url. Stack assumes
+# a snapshot provided as a file might change, whereas a url resource does not.
+#
+# snapshot: ./custom-snapshot.yaml
+# snapshot: https://example.com/snapshots/2024-01-01.yaml
+snapshot: lts-23.19
+compiler: ghc-9.10.1
+
+# User packages to be built.
+# Various formats can be used as shown in the example below.
+#
+# packages:
+# - some-directory
+# - https://example.com/foo/bar/baz-0.0.2.tar.gz
+#   subdirs:
+#   - auto-update
+#   - wai
+packages:
+- .
+# Dependency packages to be pulled from upstream that are not in the snapshot.
+# These entries can reference officially published versions as well as
+# forks / in-progress versions pinned to a git hash. For example:
+#
+# extra-deps:
+# - acme-missiles-0.3
+# - git: https://github.com/commercialhaskell/stack.git
+#   commit: e7b331f14bcffb8367cd58fbfc8b40ec7642100a
+#
+# extra-deps: []
+
+# Override default flag values for project packages and extra-deps
+# flags: {}
+
+# Extra package databases containing global packages
+# extra-package-dbs: []
+
+# Control whether we use the GHC we find on the path
+# system-ghc: true
+#
+# Require a specific version of Stack, using version ranges
+# require-stack-version: -any # Default
+# require-stack-version: ">=3.5"
+#
+# Override the architecture used by Stack, especially useful on Windows
+# arch: i386
+# arch: x86_64
+#
+# Extra directories used by Stack for building
+# extra-include-dirs: [/path/to/dir]
+# extra-lib-dirs: [/path/to/dir]
+#
+# Allow a newer minor version of GHC than the snapshot specifies
+# compiler-check: newer-minor

stack.yaml.lock

@@ -0,0 +1,12 @@
+# This file was autogenerated by Stack.
+# You should not edit this file by hand.
+# For more information, please see the documentation at:
+#   https://docs.haskellstack.org/en/stable/topics/lock_files
+
+packages: []
+snapshots:
+- completed:
+    sha256: 296a7960c37efa382432ab497161a092684191815eb92a608c5d6ea5f894ace3
+    size: 683835
+    url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/23/19.yaml
+  original: lts-23.19

test/Spec.hs

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+main :: IO ()
+main = putStrLn "Test suite not yet implemented"