[Haskell-cafe] Turning all the Nothings into Just defaultValue using Data.Generics

Alexey Rodriguez mrchebas at gmail.com
Thu Nov 13 05:37:17 EST 2008

Hi David!

2008/11/12 David Fox <ddssff at gmail.com>:
> I want to use Data.Generics to write a function to turn all the Nothings in
> a data structure into Just defaultValue, as shown below.  I get the
> following error because the compiler doesn't know enough about Maybe a for
> mkT to create the generic function that everywhere requires, I guess.
> Test.hs:26:16:
>     Ambiguous type variable `a' in the constraints:
>       `Typeable a'
>         arising from a use of `mkT' at Senior/Test2.hs:26:16-30
>       `Default a'
>         arising from a use of `justDefault' at Senior/Test2.hs:26:20-30
>     Probable fix: add a type signature that fixes these type variable(s)
> Here is the example.  It all works except for "test".  Any suggestions how
> to do this?

The mkT function is used to turn a *monomorphic* transformation into a
polymorphic one. Because justDefault is polymorphic and because mkT
does not fix the type of justDefault, the constraints of justDefault
won't be resolved. So you may need to fix the type of justDefault.

How to solve this? You could give up your type class design and write
the type specific cases of justDefault using "extT". If you really
want to use your type class design and hence keep "test" open to new
Default instances, then SYB is most likely the wrong library for you.

You could on the other hand use Scrap Your Boilerplate with class,
which you can download from hackage[1]. This library was designed so
that generic functions can be extended with additional type cases even
after they have been defined. This fits your situation since you can
give new default values using a type class. Using this library, your
example looks like this:

> import Data.Generics.SYB.WithClass.Basics
> import Data.Generics.SYB.WithClass.Derive
> import Data.Generics.SYB.WithClass.Instances
> import Language.Haskell.TH()
> class Default a where
>     defaultValue :: a
> instance Default Int where
>     defaultValue = 0
> instance Default String where
>     defaultValue = ""
> instance Default (Maybe a) where
>     defaultValue = Nothing
> data AB = AB {b :: Int, c :: Maybe String} deriving (Show)
> -- Derive data instances
> $(derive [''AB])
> instance Default AB where
>     defaultValue = AB {b = defaultValue, c = defaultValue}
> -- Dictionary and proxy for the justDefault function
> data JustDefaultD a = JustDefaultD { justDefaultD :: a -> a }
> justDefaultCtx :: Proxy JustDefaultD
> justDefaultCtx = undefined
> -- Default case for justDefault (non Maybe type)
> instance Sat (JustDefaultD a) where
>   dict = JustDefaultD id
> -- Maybe case for justDefault
> instance Default a => Sat (JustDefaultD (Maybe a)) where
>   dict = JustDefaultD justDefault
>     where
>       justDefault Nothing = Just defaultValue
>       justDefault (Just x) = Just x
> test = everywhere justDefaultCtx (justDefaultD dict) (defaultValue :: AB)

You will need the everywhere traversal which is not shipped in the
package. You can download it from gp-bench[2].



[1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/syb-with-class
[2] http://darcs.haskell.org/generics/comparison/syb3/Traversals.lhs

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