[Haskell-cafe] Factory methods in Haskell

[Derek Elkins]

On Sat, 2009-01-24 at 15:43 -0600, Jeremy Shaw wrote:
> Hello,
> 
> I was reading about the "Factory Method Pattern" on wikipedia, and
> noticed that the very first example was written in Haskell. Sweet!
> 
> http://en.wikipedia.org/wiki/Factory_method_pattern#Haskell
> 
> Unfortunately, it looks to me like it is missing the 'factory' part.

It is.

> 
> I have attempted to implement something more factory like (see
> attached). I am wonder what other people think. Is the code on
> wikipedia really demoing a factory method? Is the code attached any
> better? Is there an even better what to write this in Haskell?

Answering your questions in order: no, yes, yes but the essence is right
(there are other ways as well)  However since dynamic dispatch is rare
in Haskell, the factory method does not really come up.  It does occur
but not in a way that most people think of as "the factory method."

> plain text document attachment (Pizza.hs)
> {-# LANGUAGE ExistentialQuantification #-}
> import Numeric (showFFloat)
> 
> -- * A type which can hold different types of pizzas
> 
> data Pizza = forall a. (PizzaMethods a) => Pizza a
> 
> -- * A type class with functions common to different types of pizza
> 
> class (Show a) => PizzaMethods a where
>     price' :: a -> Double
> 
> -- * Getter functions for the pizza type
> 
> price :: Pizza -> Double
> price (Pizza p) = price' p
> 
> pizzaType :: Pizza -> String
> pizzaType (Pizza p) = show p

Get rid of these functions and get rid of the ' in price' and simply
make Pizza an instance of PizzaMethods.

> 
> -- * Some types of pizza
> 
> data HamAndMushroom = HamAndMushroom deriving (Read, Show)
> data Deluxe         = Deluxe         deriving (Read, Show)
> data Hawaiin        = Hawaiin        deriving (Read, Show)
> 
> -- * Prices of various pizzas
> 
> instance PizzaMethods HamAndMushroom where
>     price' _ = 8.50
> 
> instance PizzaMethods Deluxe where
>     price' _ = 10.50
> 
> instance PizzaMethods Hawaiin where
>     price' _ = 11.50
> 
> -- * A pizza factory
> 
> pizzaFactory :: String -> Pizza
> pizzaFactory pizzaType
>     | pizzaType == "HamAndMushroom" = Pizza HamAndMushroom
>     | pizzaType == "Deluxe" = Pizza Deluxe
>     | pizzaType == "Hawaiin" = Pizza Hawaiin
>     | otherwise = error "We don't serve your kind here." 
> 
> -- * An order at the pizza factory
> 
> main = 
>     let pizza = pizzaFactory "HamAndMushroom"
>     in putStrLn $ "You can get a " ++ pizzaType pizza ++ " for $" ++ showFFloat (Just 2) (price pizza) "."
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