smartconstructor

This project builds an Haskell module SmartConstructor which exports a makeSmartCtor function which can be used to generate smart constructors for Haskell types. See the Haskell Wiki for a more in-depth discussion of smart constructors in Haskell.

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

{-# LANGUAGE TemplateHaskell #-}

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 (<=)|]