Data.Functor.Invariant.TH

deriveInvariant automatically generates an Invariant instance declaration for a data type, newtype, or data family instance that has at least one type variable. This emulates what would (hypothetically) happen if you could attach a deriving Invariant clause to the end of a data declaration. Examples:

{-# LANGUAGE TemplateHaskell #-}
import Data.Functor.Invariant.TH

data Pair a = Pair a a
$(deriveInvariant ''Pair) -- instance Invariant Pair where ...

newtype Alt f a = Alt (f a)
$(deriveInvariant ''Alt) -- instance Invariant f => Invariant (Alt f) where ...

If you are using template-haskell-2.7.0.0 or later (i.e., GHC 7.4 or later), deriveInvariant can also be used to derive Invariant instances for data family instances (which requires the -XTypeFamilies extension). To do so, pass the name of a data or newtype instance constructor to deriveInvariant. Note that the generated code may require the -XFlexibleInstances extension. Some examples:

{-# LANGUAGE FlexibleInstances, TemplateHaskell, TypeFamilies #-}
import Data.Functor.Invariant.TH

class AssocClass a b where
    data AssocData a b
instance AssocClass Int b where
    data AssocData Int b = AssocDataInt1 Int | AssocDataInt2 b Int
$(deriveInvariant 'AssocDataInt1) -- instance Invariant (AssocData Int) where ...
-- Alternatively, one could use $(deriveInvariant 'AssocDataInt2)

data family DataFam a b
newtype instance DataFam () b = DataFamB b
$(deriveInvariant 'DataFamB) -- instance Invariant (DataFam ())

Note that there are some limitations:

• The Name argument to deriveInvariant must not be a type synonym.

• With deriveInvariant, the argument's last type variable must be of kind *. For other ones, type variables of kind * -> * are assumed to require an Invariant context. For more complicated scenarios, use makeInvmap.

• If using the -XDatatypeContexts, -XExistentialQuantification, or -XGADTs extensions, a constraint cannot mention the last type variable. For example, data Illegal a where I :: Ord a => a -> Illegal a cannot have a derived Invariant instance.

• If the last type variable is used within a data field of a constructor, it must only be used in the last argument of the data type constructor. For example, data Legal a = Legal (Either Int a) can have a derived Invariant instance, but data Illegal a = Illegal (Either a a) cannot.

• Data family instances must be able to eta-reduce the last type variable. In other words, if you have a instance of the form:

 data family Family a1 ... an t
 data instance Family e1 ... e2 v = ...
 

Then the following conditions must hold:

1. v must be a type variable.

2. v must not be mentioned in any of e1, ..., e2.

deriveInvariant2 automatically generates an Invariant2 instance declaration for a data type, newtype, or data family instance that has at least two type variables. This emulates what would (hypothetically) happen if you could attach a deriving Invariant2 clause to the end of a data declaration. Examples:

{-# LANGUAGE TemplateHaskell #-}
import Data.Functor.Invariant.TH

data OneOrNone a b = OneL a | OneR b | None
$(deriveInvariant2 ''OneOrNone) -- instance Invariant2 OneOrNone where ...

newtype Alt2 f a b = Alt2 (f a b)
$(deriveInvariant2 ''Alt2) -- instance Invariant2 f => Invariant2 (Alt2 f) where ...

The same restrictions that apply to deriveInvariant also apply to deriveInvariant2, with some caveats:

• With deriveInvariant2, the last type variables must both be of kind *. For other ones, type variables of kind * -> * are assumed to require an Invariant constraint, and type variables of kind * -> * -> * are assumed to require an Invariant2 constraint. For more complicated scenarios, use makeInvmap2.

• If using the -XDatatypeContexts, -XExistentialQuantification, or -XGADTs extensions, a constraint cannot mention either of the last two type variables. For example, data Illegal2 a b where I2 :: Ord a => a -> b -> Illegal2 a b cannot have a derived Invariant2 instance.

• If either of the last two type variables is used within a data field of a constructor, it must only be used in the last two arguments of the data type constructor. For example, data Legal a b = Legal (Int, Int, a, b) can have a derived Invariant2 instance, but data Illegal a b = Illegal (a, b, a, b) cannot.

• Data family instances must be able to eta-reduce the last two type variables. In other words, if you have a instance of the form:

 data family Family a1 ... an t1 t2
 data instance Family e1 ... e2 v1 v2 = ...
 

Then the following conditions must hold:

1. v1 and v2 must be distinct type variables.

2. Neither v1 not v2 must be mentioned in any of e1, ..., e2.

There may be scenarios in which you want to invmap over an arbitrary data type or data family instance without having to make the type an instance of Invariant. For these cases, this module provides several functions (all prefixed with make-) that splice the appropriate lambda expression into your source code. Example:

This is particularly useful for creating instances for sophisticated data types. For example, deriveInvariant cannot infer the correct type context for newtype HigherKinded f a b c = HigherKinded (f a b c), since f is of kind * -> * -> * -> *. However, it is still possible to create an Invariant instance for HigherKinded without too much trouble using makeInvmap:

{-# LANGUAGE FlexibleContexts, TemplateHaskell #-}
import Data.Functor.Invariant
import Data.Functor.Invariant.TH

newtype HigherKinded f a b c = HigherKinded (f a b c)

instance Invariant (f a b) => Invariant (HigherKinded f a b) where
    invmap = $(makeInvmap ''HigherKinded)