Hello café, I've never written an Alternative instance for a newtype yet that doesn't look like this:
instance Alternative T where empty = T empty T x <|> T y = T (x <|> y)
Why does newtype deriving not work for Alternative? (It works fine for Monoid.) Thanks, Martijn.
Works for me on GHC6.10.4: {-# LANGUAGE GeneralizedNewtypeDeriving #-} module NewtypeDerive where import Control.Applicative newtype Foo f a = Foo (f a) deriving (Functor, Applicative, Alternative) newtype Bar a = Bar [a] deriving (Functor, Applicative, Alternative) -- ryan On Wed, Oct 14, 2009 at 2:16 PM, Martijn van Steenbergen < martijn@van.steenbergen.nl> wrote:
Hello café,
I've never written an Alternative instance for a newtype yet that doesn't look like this:
instance Alternative T where
empty = T empty T x <|> T y = T (x <|> y)
Why does newtype deriving not work for Alternative? (It works fine for Monoid.)
Thanks,
Martijn. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Martijn van Steenbergen wrote:
It doesn't work for this one:
newtype Split a = Split { runSplit :: [Either a (Char, Split a) ]}
But my handwritten instance remains identical.
The instance has the form [], not the form [Either _ (Char, Split _)]. Since they don't match exactly, it won't give you an instance automagically. It could have been the case that you intended some other instance besides []'s. All generalized newtype deriving does is derive instances for newtypes that wrap exactly what the instance is defined over. - Jake
participants (3)
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Jake McArthur -
Martijn van Steenbergen -
Ryan Ingram