[Haskell-cafe] Future Edison directions
Brian Hulley
brianh at metamilk.com
Tue Aug 1 16:54:14 EDT 2006
Robert Dockins wrote:
[snip]
> 7) Finally, I somehow feel like there should be a nice categorical
> formulation of these datastructure abstractions which would help to
> drive a refactoring of the API typeclasses in a principled way,
> rather than on an ad-hoc I-sort-of-think-these-go-together sort of
> way.
For the last few months (!!!) I've been thinking about the relationship
between measured sequences and plain sequences and also whether or not every
sequence should by indexable by Int. I'm wondering if something like the
following might be a possible factoring of the ops relating to
indexing/measurements:
-- from http://www.soi.city.ac.uk/~ross/papers/FingerTree.html
class Monoid v => Measured v a where
measure :: a -> v
instance Measured () a where measure _ = ()
-- then (also based mostly on FingerTree ideas)
class (Monoid v, Ord i) => IndexMeasure v i where -- no fundep
index :: v -> i
class BasicSeq c a | c -> a where
length :: c -> Int
empty :: c
isEmpty :: c -> Bool
atL :: c -> a
atR :: c -> a
pushL :: a -> c -> c
viewL :: Monad m => c -> m (a, c)
-- pushR, viewR
class (Measured v a, Measured v c, BasicSeq c a) => Measurable c v a |
c -> v where
-- precondition: pred is True for v `mappend` (measure c)
splitWithInternal :: (v -> Bool) -> v -> c -> (c, a, c)
splitWith :: (v -> Bool) -> c -> (c,c)
splitWith p t
| isEmpty t = (empty, empty)
| p (measure t) =
let
(l,x,r) = splitWithInternal p mempty t
in (l, pushL x r)
| otherwise = (empty, empty)
splitAt :: IndexMeasure v i => i -> c -> (c,c)
splitAt i = splitWith (\v -> i < index v)
size :: IndexMeasure v i => c -> i
size c = index (measure c)
-- take, drop, takeWith, dropWith, in terms of split and splitWith
atWith :: (v -> Bool) -> c -> a
atWith p t = (\(_,x,_)->x) (splitWithInternal p mempty t)
at :: IndexMeasure v i => i -> c -> a
at i = atWith (\v -> i < index v)
where splitWith p s returns (l,r) such that the measure of l `mappend` the
measure of the first element of r satisfies p (FingerTree paper has
explanation of this - I assume monotonic p for any useful use).
The idea of the above design would be to allow multiple indexes for the same
sequence (though the element type is the same in each case so possibly this
could be confusing though could be prevented by using a fundep in the
IndexMeasure class), as well as allowing sequences with an arbitrary measure
that isn't an index (just by having no instances of IndexMeasure) eg:
data TextBuffer = ...
newtype Line = Line Int
newtype CharPos = CharPos Int
data TextBufferMeasure = ...
instance IndexMeasure TextBufferMeasure Line where ...
instance IndexMeasure TextBufferMeasure CharPos where ...
instance Measureable TextBuffer TextBufferMeasure Char where ...
Line lineCount = size textbuf
CharPos charCount = size textbuf
(before, after) = splitAt (CharPos 56) textbuf
Of course this doesn't solve the problem of using nested sequences, but it
at least allows general measurement with predicate search to coexist with
simple indexing and size-with-respect-to-index where these are applicable to
the relevant concrete sequence.
Anyway just a very rough idea at the moment. I'm looking forward to seeing a
nice categorical factoring ;-)
Regards, Brian.
--
Logic empowers us and Love gives us purpose.
Yet still phantoms restless for eras long past,
congealed in the present in unthought forms,
strive mightily unseen to destroy us.
http://www.metamilk.com
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