On Tue, Jun 29, 2010 at 02:56:18PM -0500, Jeremy Shaw wrote:
On Jun 29, 2010, at 6:02 AM, Stephen Tetley wrote:
The "Applicative Programming with Effects Paper" has the "monodial accumulating" applicative instance on a sum type Conor McBride and Ross Paterson call Except:
data Except err a = OK a | Failed err
The applicatives-extra package defines a type:
type ErrorMsg = String data Failing a = Failure [ErrorMsg] | Success a
Which is a less general version of that type.
There's also a generalization, which I was going to put in transformers, but couldn't decide on the best name: data Sum p a = Return a | Others (p a) instance Functor p => Functor (Sum p) where fmap f (Return x) = Return (f x) fmap f (Others m) = Others (fmap f m) instance Applicative p => Applicative (Sum p) where pure = Return Return x <*> Return y = Return (x y) Return x <*> Others my = Others (x <$> my) Others mx <*> Return y = Others (mx <*> pure y) Others mx <*> Others my = Others (mx <*> my) instance Alternative p => Alternative (Sum p) where empty = Others empty Return x <|> _ = Return x Others _ <|> Return y = Return y Others mx <|> Others my = Others (mx <|> my)