Functions to mechanically derive TextShow, TextShow1, or TextShow2 instances, or to splice show-related expressions into Haskell source code. You need to enable the TemplateHaskell language extension in order to use this module.

Since: 2

deriveTextShow automatically generates a TextShow instance declaration for a data type, newtype, or data family instance. This emulates what would (hypothetically) happen if you could attach a deriving TextShow clause to the end of a data declaration.

Here are some examples of how to derive TextShow for simple data types:

{-# LANGUAGE TemplateHaskell #-}
import TextShow.TH

data Letter = A | B | C
$(deriveTextShow ''Letter) -- instance TextShow Letter where ...

newtype Box a = Box a
$(deriveTextShow ''Box) -- instance TextShow a => TextShow (Box a) where ...

If you are using template-haskell-2.7.0.0 or later (i.e., GHC 7.4 or later), deriveTextShow can also be used to derive TextShow instances for data family instances (which requires the -XTypeFamilies extension). To do so, pass the name of a data or newtype instance constructor (NOT a data family name!) to deriveTextShow. Note that the generated code may require the -XFlexibleInstances extension. Some examples:

{-# LANGUAGE FlexibleInstances, TemplateHaskell, TypeFamilies #-}
import TextShow.TH (deriveTextShow)

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

data family DataFam a b
newtype instance DataFam () b = DataFamB b
$(deriveTextShow 'DataFamB) -- instance TextShow b => TextShow (DataFam () b)

Note that at the moment, there are some limitations:

• The Name argument to deriveTextShow must not be a type synonym.
• deriveTextShow makes the assumption that all type variables of kind * require a TextShow constraint when creating the type context. For example, if you have data Phantom a = Phantom, then (deriveTextShow ''Phantom) will generate instance TextShow a => TextShow (Phantom a) where ..., even though TextShow a is not required. If you want a proper TextShow instance for Phantom, you will need to use makeShowbPrec (see the documentation of the make functions for more information).
• deriveTextShow lacks the ability to properly detect data types with higher-kinded type parameters (e.g., data HK f a = HK (f a)) or with kinds other than * (e.g., data List a (empty :: Bool)). If you wish to derive TextShow instances for these data types, you will need to use makeShowbPrec.
• Some data constructors have arguments whose TextShow instance depends on a typeclass besides TextShow. For example, consider newtype MyFixed a = MyFixed (Fixed a). Fixed a is a TextShow instance only if a is an instance of both HasResolution and TextShow. Unfortunately, deriveTextShow cannot infer that a must be an instance of HasResolution, so it cannot create a TextShow instance for MyFixed. However, you can use makeShowbPrec to get around this.

deriveTextShow1 automatically generates a Show1 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 TextShow1 clause to the end of a data declaration. Examples:

{-# LANGUAGE TemplateHaskell #-}
import TextShow.TH

data Stream a = Stream a (Stream a)
$(deriveTextShow1 ''Stream) -- instance TextShow1 TextStream where ...

newtype WrappedFunctor f a = WrapFunctor (f a)
$(deriveTextShow1 ''WrappedFunctor) -- instance TextShow1 f => TextShow1 (WrappedFunctor f) where ...

The same restrictions that apply to deriveTextShow also apply to deriveTextShow1, with some caveats:

• With deriveTextShow1, the last type variable must be of kind *. For other ones, type variables of kind * are assumed to require a TextShow context, and type variables of kind * -> * are assumed to require a TextShow1 context. For more complicated scenarios, use makeShowbPrecWith.
• If using -XDatatypeContexts, a datatype constraint cannot mention the last type variable. For example, data Ord a => Illegal a = Illegal a cannot have a derived TextShow1 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 TextShow1 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.

• In GHC 7.8, a bug exists that can cause problems when a data family declaration and one of its data instances use different type variables, e.g.,

 data family Foo a b c
 data instance Foo Int y z = Foo Int y z
 $(deriveTextShow1 'Foo)
 

To avoid this issue, it is recommened that you use the same type variables in the same positions in which they appeared in the data family declaration:

 data family Foo a b c
 data instance Foo Int b c = Foo Int b c
 $(deriveTextShow1 'Foo)
 

deriveTextShow2 automatically generates a TextShow2 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 TextShow2 clause to the end of a data declaration. Examples:

{-# LANGUAGE TemplateHaskell #-}
import TextShow.TH

data OneOrNone a b = OneL a | OneR b | None
$(deriveTextShow2 ''OneOrNone) -- instance TextShow2 OneOrNone where ...

newtype WrappedBifunctor f a b = WrapBifunctor (f a b)
$(deriveTextShow2 ''WrappedBifunctor) -- instance TextShow2 f => TextShow2 (WrappedBifunctor f) where ...

The same restrictions that apply to deriveTextShow and deriveTextShow1 also apply to deriveTextShow2, with some caveats:

• With deriveTextShow2, the last type variables must both be of kind *. For other ones, type variables of kind * are assumed to require a TextShow constraint, type variables of kind * -> * are assumed to require a TextShow1 constraint, and type variables of kind * -> * -> * are assumed to require a TextShow2 constraint. For more complicated scenarios, use makeShowbPrecWith2.
• If using -XDatatypeContexts, a datatype constraint cannot mention either of the last two type variables. For example, data Ord a => Illegal a b = Illegal a b cannot have a derived TextShow2 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 TextShow2 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 show an arbitrary data type or data family instance without having to make the type an instance of TextShow. For these cases, this modules 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, deriveTextShow cannot infer the correct type context for newtype HigherKinded f a = HigherKinded (f a), since f is of kind * -> *. However, it is still possible to derive a TextShow instance for HigherKinded without too much trouble using makeShowbPrec:

{-# LANGUAGE FlexibleContexts, TemplateHaskell #-}
import TextShow
import TextShow.TH

instance TextShow (f a) => TextShow (HigherKinded f a) where
    showbPrec = $(makeShowbPrec ''HigherKinded)