Generics.Linear.TH

This module contains Template Haskell code that can be used to automatically generate the boilerplate code for the generic deriving library.

To use these functions, pass the name of a data type as an argument:

{-# LANGUAGE TemplateHaskell #-}

data Example a = Example Int Char a
$(deriveGeneric     ''Example) -- Derives Generic instance
$(deriveGeneric1     ''Example) -- Derives Generic1 instance
$(deriveGenericAnd1 ''Example) -- Derives Generic and Generic1 instances

This code can also be used with data families. To derive for a data family instance, pass the name of one of the instance's constructors:

{-# LANGUAGE FlexibleInstances, TemplateHaskell, TypeFamilies #-}

data family Family a b
newtype instance Family Char x = FamilyChar Char
data    instance Family Bool x = FamilyTrue | FamilyFalse

$(deriveGeneric 'FamilyChar) -- instance Generic (Family Char b) where ...
$(deriveGeneric1 'FamilyTrue) -- instance Generic1 (Family Bool) where ...
-- Alternatively, one could type $(deriveGeneric1 'FamilyFalse)

General usage notes

Template Haskell imposes some fairly harsh limitations on ordering and visibility within a module. In most cases, classes derived generically will need to be derived using StandaloneDeriving after the deriveGeneric* invocation. For example, if Generically is a class that uses a Generic constraint for its instances, then you cannot write

data Fish = Fish
  deriving Show via (Generically Fish)

$(deriveGeneric 'Fish)

You must instead write

data Fish = Fish

$(deriveGeneric 'Fish)

deriving via Generically Fish
  instance Show Fish

Furthermore, types defined after a deriveGeneric* invocation are not visible before that invocation. This may require some careful ordering, especially in the case of mutually recursive types. For example, the following will not compile:

data Foo = Foo | Bar Baz
$(deriveGeneric 'Foo)

data Baz = Baz Int Foo
$(deriveGeneric 'Baz)

Instead, you must write

data Foo = Foo | Bar Baz
data Baz = Baz Int Foo

$(deriveGeneric 'Foo)
$(deriveGeneric 'Baz)