deriving-compat

Backports of GHC deriving extensions

https://github.com/haskell-compat/deriving-compat

LTS Haskell 24.17:0.6.7@rev:1
Stackage Nightly 2025-10-31:0.6.7@rev:1
Latest on Hackage:0.6.7@rev:1

See all snapshots deriving-compat appears in

BSD-3-Clause licensed by Ryan Scott
Maintained by Ryan Scott
This version can be pinned in stack with:deriving-compat-0.6.7@sha256:b4edb909e6ecc7e0c88dda5f7f25579ed682ab3a97388e9a9e1976aa9ca0b772,7986

Module documentation for 0.6.7

Used by 4 packages in nightly-2025-10-31(full list with versions):
elm-syntax, functor-combinators, hedn, what4

deriving-compat

Hackage Hackage Dependencies Haskell Programming Language BSD3 License Build Status

deriving-compat provides Template Haskell functions that mimic deriving extensions that were introduced or modified in recent versions of GHC. Currently, the following typeclasses/extensions are covered:

  • Deriving Bounded
  • Deriving Enum
  • Deriving Ix
  • Deriving Eq
  • Deriving Ord
  • Deriving Read
  • Deriving Show
  • DeriveFoldable
  • DeriveFunctor
  • DeriveTraversable
  • GeneralizedNewtypeDeriving (with GHC 8.2 or later)
  • DerivingVia (with GHC 8.2 or later)

See the Data.Deriving module for a full list of backported changes.

In addition, deriving-compat also provides some additional deriving functionality that has not yet been merged into upstream GHC. Aside from the GHC deriving extensions mentioned above, deriving-compat also permits deriving instances of classes in the Data.Functor.Classes module, covering the Eq1, Eq2, Ord1, Ord2, Read1, Read2, Show1, and Show2 classes. This extra functionality is outside of the main scope of deriving-compat, as it does not backport extensions that exist in today’s GHC. Nevertheless, the underlying Template Haskell machinery needed to derive Eq and friends extends very naturally to Eq1 and friends, so this extra functionality is included in deriving-compat as a convenience.

Note that some recent GHC typeclasses/extensions are not covered by this package:

  • DeriveDataTypeable
  • DeriveGeneric, which was introducted in GHC 7.2 for deriving Generic instances, and modified in GHC 7.6 to allow derivation of Generic1 instances. Use Generics.Deriving.TH from generic-deriving to derive Generic(1) using Template Haskell.
  • DeriveLift, which was introduced in GHC 8.0 for deriving Lift instances. Use Language.Haskell.TH.Lift from th-lift to derive Lift using Template Haskell.
  • The Bifunctor typeclass, which was introduced in GHC 7.10, as well as the Bifoldable and Bitraversable typeclasses, which were introduced in GHC 8.2. Use Data.Bifunctor.TH from bifunctors to derive these typeclasses using Template Haskell.

Changes

0.6.7 [2024.12.05]

  • Drop support for pre-8.0 versions of GHC.

0.6.6 [2024.03.19]

  • Support building with template-haskell-2.22.* (GHC 9.10).

0.6.5 [2023.08.06]

  • When generating Show(1)(2) instances with Text.Show.Deriving using GHC 9.8 or later, data types that have fields of type Int{8,16,32,64}# or Word{8,16,32,64}# will be printed using extended literal syntax, mirroring corresponding changes introduced in GHC 9.8 (see https://github.com/ghc-proposals/ghc-proposals/pull/596).

0.6.4 [2023.08.06]

  • Support building with template-haskell-2.21.* (GHC 9.8).
  • The Template Haskell machinery now uses TemplateHaskellQuotes when building with GHC 8.0+ instead of manually constructing each Template Haskell Name. A consequence of this is that deriving-compat will now build with GHC 9.8, as TemplateHaskellQuotes abstracts over some internal Template Haskell changes introduced in 9.8.

0.6.3 [2023.02.27]

  • Support th-abstraction-0.5.*.

0.6.2 [2022.12.07]

  • Make the test suite build with GHC 9.6 or later.

0.6.1 [2022.05.07]

  • Backport GHC!6955, which makes derived Eq instances check data constructor tags, which can improve runtime performance for data types with nullary constructors.
  • Allow building the test suite with transformers-0.6.*.

0.6 [2021.08.29]

  • Allow building with template-haskell-2.18.0.0 (GHC 9.2).
  • Using deriveEnum and deriveIx on poly-kinded data family instances may now require the use of the TypeInType extension if using GHC 8.0, 8.2, or 8.4. (On later versions of GHC, TypeInType’s functionality has been folded into PolyKinds.)
  • Support deriving Eq, Ord, and Show instances for data types with fields of type Int32# or Word32# on GHC 9.2 or later.
  • deriveVia now instantiates “floating” via type variables (i.e., type variables mentioned in the via type that are not mentioned in the instance context or the first argument to Via) to Any in the generated code. As a result, deriveVia no longer generates code that produces -Wunused-foralls warnings.

0.5.10 [2020.09.30]

  • Allow building with template-haskell-2.17.0.0 (GHC 9.0).

0.5.9 [2019.06.08]

  • Have deriveFunctor and deriveFoldable derive implementations of (<$) and null, which GHC starting doing in 8.2 and 8.4, respectively.
  • Fix a bug in which deriveOrd{,1,2} could generate incorrect code for data types with a combination of nullary and non-nullary constructors.
  • Fix a bug in which deriveFunctor would fail on sufficiently complex uses of rank-n types in constructor fields.
  • Fix a bug in which deriveFunctor and related functions would needlessly reject data types whose last type parameters appear as oversaturated arguments to a type family.

0.5.8 [2019.11.26]

  • Allow building with GHC 8.10.

0.5.7 [2019.08.27]

  • Permit deriveVia to use “floating” via type variables, such as the a in:

    deriveVia [t| forall a. Show MyInt `Via` Const Int a |]

0.5.6 [2019.05.02]

  • Support deriving Eq, Ord, and Show instances for data types with fields of type Int8#, Int16#, Word8#, or Word16# on GHC 8.8 or later.

0.5.5 [2019.04.26]

  • Support th-abstraction-0.3 or later.

0.5.4 [2019.01.21]

  • Expose Internal modules.

0.5.3 [2019.01.20]

  • Fix a bug in which deriveEnum/deriveIx would generate ill-scoped code for certain poly-kinded data types.

0.5.2 [2018.09.13]

  • Fix a bug (on GHC 8.7 and above) in which deriveGND/deriveVia would generate ill-scoped code.

0.5.1 [2018.07.11]

  • Have deriveGND/deriveVia throw an error if an incorrect number of arguments are supplied to the type class.

0.5 [2018.07.01]

  • Backport the changes to GeneralizedNewtypeDeriving and DerivingVia code generation from Trac #15290.

    As a result, code generated by deriveGND or deriveVia now requires the InstanceSigs and ScopedTypeVariables language extensions. On the other hand, the generated code no longer requires the ImpredicativeTypes extension (unless any class methods use higher-rank types).

  • Allow building with containers-0.6 and template-haskell-2.14.

0.4.3 [2018.06.16]

  • Fix a bug that caused debug-enabled GHC builds to panic when generating code from this library (see Trac #15270). The fix only affects the library’s internals, so no changes are user-facing.

0.4.2 [2018.05.14]

  • Backport the fixes for GHC Trac #14364 and #14918, which significantly improve the compliation times of derived Read instances.

0.4.1 [2018.02.04]

  • Add Data.Deriving.Via, which allows emulating the behavior of the GeneralizedNewtypeDeriving and DerivingVia extensions.
  • Test suite fixes for GHC 8.4.

0.4 [2017.12.07]

  • Incorporate changes from the EmptyDataDeriving proposal (which is in GHC as of 8.4):
    • For derived Eq and Ord instances for empty data types, simply return True and EQ, respectively, without inspecting the arguments.
    • For derived Read instances for empty data types, simply return pfail (without parens).
    • For derived Show instances for empty data types, inspect the argument (instead of erroring). In addition, add showEmptyCaseBehavior to ShowOptions, which configures whether derived instances for empty data types should use the EmptyCase extension (this is disabled by default).
    • For derived Functor and Traversable instances for empty data types, make fmap and traverse strict in its argument.
    • For derived Foldable instances, do not error on empty data types. Instead, simply return the folded state (for foldr) or mempty (for foldMap), without inspecting the arguments.
    • Add FFTOptions (Functor/Foldable/Traversable options) to Data.Functor.Deriving, along with variants of existing functions that take FFTOptions as an argument. For now, the only configurable option is whether derived instances for empty data types should use the EmptyCase extension (this is disabled by default).
  • Backport the fix to #13328. That is, when deriving Functor or Traversable instances for data types where the last type variable is at phantom role, generated fmap/traverse implementations now use coerce for efficiency.
  • Rename emptyCaseBehavior from Data.Functor.Deriving to fftEmptyCaseBehavior.

0.3.6 [2017.04.10]

  • Make deriveTraversable use liftA2 in derived implementations of traverse when possible, now that liftA2 is a class method of Applicative (as of GHC 8.2)
  • Make deriveShow use showCommaSpace, a change introduced in GHC 8.2

0.3.5 [2016.12.12]

  • Fix bug in which derived Ord instances for datatypes with many constructors would fail to typecheck

0.3.4 [2016.10.20]

  • Fix bug in which infix record selectors weren’t shown with parentheses in derived Show instances
  • Fix bug in which record selectors weren’t parsed correctly in derived Read instances

0.3.3 [2016.09.11]

  • Add Data.Bounded.Deriving, which allows deriving Bounded with TH.
  • Add Data.Enum.Deriving, which allows deriving Enum with TH.
  • Add Data.Ix.Deriving, which allows deriving Ix with TH.
  • Fix bug in which derived Show instance would parenthesize the output too eagerly

0.3.2

  • Incorporate a fix to GHC Trac #10858, which will be introduced in GHC 8.2
  • Fix bug in which derived Ord instances accidentally swapped their less-than(-or-equal-to) and greater-than(-or-equal-to) methods
  • Fix GHC HEAD build

0.3.1

  • Allow deriving Functor and Foldable instances for datatypes containing unboxed tuples
  • Microoptimization in derived instances of higher-order versions of Eq, Ord, Read, and Show

0.3

  • Added Data.Eq.Deriving, which allows deriving Eq, Eq1, and Eq2 with TH.
  • Added Data.Ord.Deriving, which allows deriving Ord, Ord1, and Ord2 with TH.
  • Added Data.Read.Deriving, which allows deriving Read, Read1, and Eq2 with TH.
  • Renamed Text.Show.Deriving.Options to ShowOptions so as to disambiguate it from the options datatypes in other deriving-compat modules.

0.2.2

  • Fixed a bug in Text.Show.Deriving’s treatment of unlifted types

0.2.1

  • Added Text.Show.Deriving, which allows deriving Show, Show1, and Show2 with TH.

0.2

  • Added support for GHC 8.0
  • Added Data.Functor.Deriving and Data.Traversable.Deriving, which allow deriving Functor and Traversable with TH.
  • Added Data.Deriving, which reexports all other modules

0.1

  • Initial commit