SmartConstructor

This module exposes a makeSmartCtor function which automatically creates smart constructors for custom newtype'd Haskell types. See http://wiki.haskell.org/Smart_constructors for a more in-depth discussion of smart constructors in Haskell.

Smart constructors are useful for imposing additional checks on values; given e.g.

import SmartConstructor

newtype Positive = Positive Int
newtype NonEmptyList a = NonEmptyList [a]
newtype Interval = Interval (Integer, Integer)

You can use makeSmartCtor to generate smart constructors as follows:

-- Defines 'makePositive :: Int -> Maybe Positive'
makeSmartCtor defaultOptions ''Positive [|(> 0)|]

-- Defines 'makeNonEmptyList :: [a] -> Maybe (NonEmptyList a)
makeSmartCtor defaultOptions ''NonEmptyList [|not . null|]

Notice how the third argument defines a predicate; the generated functions apply this predicate to the given value: if it yields true, the smart constructor call evaluates to a Just value. If the predicate yields false, the smart constructor evaluates to Nothing.

By default, the name for the smart constructor is derived from the type name. A custom name can be specified by modifying the ctorName field of the defaultOptions:

-- Defines 'createIV :: (Integer, Integer) -> Maybe Interval
makeSmartCtor defaultOptions{ ctorName = "createIV" } ''Interval [|uncurry (<=)|]

Documentation

data SmartCtorOptions

Values of the SmartCtorOptions type can be passed to makeSmartCtor in order to customize the generated constructor functions. At this point, only the name of the function can be changed.

Constructors

SmartCtorOptions

Fields

ctorName :: String

The desired name for the smart constructor function. An empty string will make makeSmartCtor derive the function name from the type by prepending make to the type name.

defaultOptions :: SmartCtorOptions

The default smart constructor generation options; the smart constructor function will be named after the type, e.g.

makeSmartCtor defaultOptions ''Foo [|const True|]

defines a function makeFoo.

makeSmartCtor :: SmartCtorOptions -> Name -> Q Exp -> Q [Dec]

The makeSmartCtor function creates a smart constructor for the given type, using the given predicate.