[Haskell-cafe] Call for discussion: OverloadedLists extension
Michael Snoyman
michael at snoyman.com
Sun Sep 23 06:06:57 CEST 2012
(Prettier formatting available at: https://gist.github.com/3761252)
Many of us use the OverloadedStrings language extension on a regular
basis. It provides the ability to keep the ease-of-use of string
literal syntax, while getting the performance and correctness
advantages of specialized datatypes like ByteString and Text. I think
we can get the same kind of benefit by allowing another literal syntax
to be overloaded, namely lists.
## Overly simple approach
The simplest example I can think of is allowing easier usage of Vector:
[1, 2, 3] :: Vector Int
In order to allow this, we could use a typeclass approach similar to
how OverloadedStrings works:
class IsList a where
fromList :: [b] -> a b
instance IsList Vector where
fromList = V.fromList
foo :: Vector Int
foo = fromList [1, 2, 3]
## Flaws
However, such a proposal does not allow for constraints, e.g.:
instance IsList Set where
fromList = Set.fromList
No instance for (Ord b)
arising from a use of `Set.fromList'
In the expression: Set.fromList
In an equation for `fromList': fromList = Set.fromList
In the instance declaration for `IsList Set'
Additionally, it provides for no means of creating instances for
datatypes like Map, where the contained value is not identical to the
value contained in the original list. In other words, what I'd like to
see is:
[("foo", 1), ("bar", 2)] :: Map Text Int
## A little better: MPTC
A simplistic approach to solve this would be to just use MultiParamTypeClasses:
class IsList input output where
fromList :: [input] -> output
instance IsList a (Vector a) where
fromList = V.fromList
foo :: Vector Int
foo = fromList [1, 2, 3]
Unfortunately, this will fail due to too much polymorphism:
No instance for (IsList input0 (Vector Int))
arising from a use of `fromList'
Possible fix:
add an instance declaration for (IsList input0 (Vector Int))
In the expression: fromList [1, 2, 3]
In an equation for `foo': foo = fromList [1, 2, 3]
This can be worked around by giving an explicit type signature on the
numbers in the list, but that's not a robust solution. In order to
solve this properly, I think we need either functional dependencies or
type families:
## Functional dependencies
class IsList input output | output -> input where
fromList :: [input] -> output
instance IsList a (Vector a) where
fromList = V.fromList
instance Ord a => IsList a (Set a) where
fromList = Set.fromList
instance Ord k => IsList (k, v) (Map k v) where
fromList = Map.fromList
foo :: Vector Int
foo = fromList [1, 2, 3]
bar :: Set Int
bar = fromList [1, 2, 3]
baz :: Map String Int
baz = fromList [("foo", 1), ("bar", 2)]
## Type families
class IsList a where
type IsListInput a
fromList :: [IsListInput a] -> a
instance IsList (Vector a) where
type IsListInput (Vector a) = a
fromList = V.fromList
instance Ord a => IsList (Set a) where
type IsListInput (Set a) = a
fromList = Set.fromList
instance Ord k => IsList (Map k v) where
type IsListInput (Map k v) = (k, v)
fromList = Map.fromList
foo :: Vector Int
foo = fromList [1, 2, 3]
bar :: Set Int
bar = fromList [1, 2, 3]
baz :: Map String Int
baz = fromList [("foo", 1), ("bar", 2)]
## Conclusion
Consider most of this proposal to be a strawman: names and techniques
are completely up to debate. I'm fairly certain that our only two
choices to implement this extension is a useful way is fundeps and
type families, but perhaps there's another approach I'm missing. I
don't have any particular recommendation here, except to say that
fundeps is likely more well supported by other compilers.
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