Dear haskell-cafe, I've read in Reddit that pattern synonyms have been merged in GHC HEAD [ http://www.reddit.com/r/haskell/comments/1vpaey/pattern_synonyms_merged_into... ]. I would like to know whether associated patterns, that is, patterns which come under the umbrella of a type class, have also been implemented. I think that associated patterns would fill the gap in difference of features between type classes and common data types. For plain data types, you can declare both functions and patterns (either via constructors or now via pattern synonyms). However, you can only declare functions (either term-level or type-level) in type classes. This means that the pattern match mechanism, very useful to get clear code, is not useful if you want to use type classes. Alejandro.
Hi Alejandro,
If your inspiration for "associated __" comes from the -XTypeFamilies,
consider that you can take a working program, move all associated
type/data families to top-level, and the program will still work. The
only point of associating them with a class is to help prevent people
from forgetting to write the type instance, and possibly to improve
documentation.
So maybe using a class method in the definition of a pattern synonym
is close enough:
{-# LANGUAGE ViewPatterns, PatternSynonyms #-}
import qualified Data.Sequence as Seq
class Listy t where
listySplit :: t a -> Maybe (a, t a)
listyUnsplit :: a -> t a -> t a
instance Listy [] where
listySplit (a:b) = Just (a,b)
listySplit _ = Nothing
listyUnsplit = (:)
instance Listy Seq.Seq where
listySplit (Seq.viewl -> a Seq.:< b) = Just (a,b)
listySplit _ = Nothing
listyUnsplit = (Seq.<|)
pattern L x xs <- (listySplit -> Just (x,xs))
-- example
sum1 :: (Listy t, Num a) => t a -> a
sum1 (L x xs) = x + sum1 xs
sum1 _ = 0
I don't see a way to make the expression `L x xs' stand for
listyUnsplit. Maybe someone else can figure this out, or this is a
forthcoming feature?
--
Adam
On Tue, Jan 21, 2014 at 5:31 AM, Alejandro Serrano Mena
Dear haskell-cafe, I've read in Reddit that pattern synonyms have been merged in GHC HEAD [http://www.reddit.com/r/haskell/comments/1vpaey/pattern_synonyms_merged_into...].
I would like to know whether associated patterns, that is, patterns which come under the umbrella of a type class, have also been implemented.
I think that associated patterns would fill the gap in difference of features between type classes and common data types. For plain data types, you can declare both functions and patterns (either via constructors or now via pattern synonyms). However, you can only declare functions (either term-level or type-level) in type classes. This means that the pattern match mechanism, very useful to get clear code, is not useful if you want to use type classes.
Alejandro.
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
It's been merged in, so you should build head and find out! (Seriously,
we're very close to an official RC, if you use a unixy platform current
head should look very similar to the official RC)
On Tuesday, January 21, 2014, Alejandro Serrano Mena
Dear haskell-cafe, I've read in Reddit that pattern synonyms have been merged in GHC HEAD [ http://www.reddit.com/r/haskell/comments/1vpaey/pattern_synonyms_merged_into... ].
I would like to know whether associated patterns, that is, patterns which come under the umbrella of a type class, have also been implemented.
I think that associated patterns would fill the gap in difference of features between type classes and common data types. For plain data types, you can declare both functions and patterns (either via constructors or now via pattern synonyms). However, you can only declare functions (either term-level or type-level) in type classes. This means that the pattern match mechanism, very useful to get clear code, is not useful if you want to use type classes.
Alejandro.
participants (3)
-
adam vogt -
Alejandro Serrano Mena -
Carter Schonwald