Documentation

Derives a Listable instance for a given type Name.

Consider the following Stack datatype:

data Stack a = Stack a (Stack a) | Empty

Writing

deriveListable ''Stack

will automatically derive the following Listable instance:

instance Listable a => Listable (Stack a) where
  tiers = cons2 Stack \/ cons0 Empty

Warning: if the values in your type need to follow a data invariant, the derived instance won't respect it. Use this only on "free" datatypes.

Needs the TemplateHaskell extension.

Same as deriveListable but does not warn when the requested instance already exists. The function deriveListable is preferable in most situations.

Derives a Listable instance for a given type Name cascading derivation of type arguments as well.

Consider the following series of datatypes:

data Position = CEO | Manager | Programmer

data Person = Person
            { name :: String
            , age :: Int
            , position :: Position
            }

data Company = Company
             { name :: String
             , employees :: [Person]
             }

Writing

deriveListableCascading ''Company

will automatically derive the following three Listable instances:

instance Listable Position where
  tiers = cons0 CEO \/ cons0 Manager \/ cons0 Programmer

instance Listable Person where
  tiers = cons3 Person

instance Listable Company where
  tiers = cons2 Company

Given a type Name, derives an expression to be placed as the result of tiers:

consN C1 \/ consN C2 \/ ... \/ consN CN

This function can be used in the definition of Listable instances:

instance Listable MyType where
  tiers = $(deriveTiers)

Given a type Name, derives an expression to be placed as the result of list:

concat $ consN C1 \/ consN C2 \/ ... \/ consN CN