Hello, I can *almost* do it like this: test = (id `ext1T` justDefault) (defaultValue :: A) justDefault :: forall f. (Default f, Data f) => Maybe f -> Maybe f justDefault Nothing = defaultValue justDefault (Just x) = Just x Except it fails with: Could not deduce (Default d1) from the context (Data d1) arising from a use of `justDefault' at /tmp/type.hs:31:19-29 Possible fix: add (Default d1) to the context of the polymorphic type `forall d1. (Data d1) => t d1 -> t d1' In the second argument of `ext1T', namely `justDefault' In the expression: (id `ext1T` justDefault) (defaultValue :: A) In the definition of `test': test = (id `ext1T` justDefault) (defaultValue :: A) If we could figure out a way to write justDefault so that it did not require the Default class, then things would work. It would be nice if there was a way to do one thing if a value is an instance of Default and something else if it is not. Here is some psuedo-Haskell code showing what I mean: justDefault :: forall f. (Data f) => Maybe f -> Maybe f justDefault Nothing | (Default f) => defaultValue | _ => Nothing justDefault (Just x) = Just x Any ideas? j. At Wed, 12 Nov 2008 09:46:05 -0800, David Fox wrote:
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] [1.1 ] 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?
{-# LANGUAGE DeriveDataTypeable, FlexibleContexts, FlexibleInstances, MultiParamTypeClasses, RankNTypes, TemplateHaskell, TypeSynonymInstances #-} {-# OPTIONS_GHC -fallow-undecidable-instances #-} module Test where
import Data.Generics
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 A = A {b :: Int, c :: Maybe String} deriving (Show, Data, Typeable)
instance Default A where defaultValue = A {b = defaultValue, c = defaultValue}
test = everywhere (mkT justDefault) (defaultValue :: A) where justDefault Nothing = Just defaultValue justDefault (Just x) = Just x [1.2
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