(forgot to reply to cafe list
Hi.
You can take a look at vector-fixed-hetero [1],
that can act as anonymous structure with arbitrary number of fields,
convertion to and from
datatypes with the same structure and many more features. It's missing
field names though
and syntactic sugar that 'records' package have. If you like to
reinvent a wheel, then
you can use following appoach:
introduce some typelevel info
data a :? (b :: Symbol)
data FieldName (t :: Symbol) = FieldName
introduce generic structure for records with any number of fiels
newtype ARec (m :: [*]) = ARec { unRec :: Array Any }
type instance TProxy (ARec m) = m
type instance VProxy (ARec m) = ARec
rerec :: ARec a -> ARec b
rerec = ARec . unRec
indexRec :: (KnownNat (FieldIndex name m), FieldType name m ~ a)
=> proxy name -> ARec m -> a
indexRec name v = unsafeCoerce $
indexArray (unRec v) (fromIntegral (natVal (indexProxy name v)))
updateRec :: (KnownNat (Length m), KnownNat (FieldIndex name m),
FieldType name m ~ a)
=> proxy name -> a -> ARec m -> ARec m
updateRec name b v = runST $ do
out <- newArray len undefined
copyArray out 0 (unRec v) 0 len
writeArray out idx (unsafeCoerce b)
ARec <$> unsafeFreezeArray out
where
idx = fromIntegral (natVal (indexProxy name v))
len = fromIntegral (natVal (lengthProxy v))
you'll need some typelevel magic for that:
type family FieldType (n :: Symbol) m where
FieldType n ((a :? n) ': z) = a
FieldType n (b ': z) = FieldType n z
type family FieldIndex (n::Symbol) m :: Nat where
FieldIndex n ((a :? n) ': z) = 0
FieldIndex n ( b ': z) = 1 + FieldIndex n z
indexProxy :: (KnownNat c, FieldIndex n m ~ c) => proxy1 n -> proxy2 m
-> Proxy c
indexProxy _ _ = Proxy
type family Length m where
Length '[] = 0
Length (x ': xs) = 1 + Length xs
lengthProxy :: (KnownNat c, Length n ~ c) => proxy n -> Proxy c
lengthProxy _ = Proxy
then you can implement lenses:
instance (KnownNat (FieldIndex n m), KnownNat (Length m), FieldType n m ~ a)
=> HasField (n :: Symbol) (ARec m) a where
getField = indexRec
updateField p = flip (updateRec p)
fieldLens' :: (HasField name z a, FieldType name (TProxy z) ~ a)
=> FieldName name -> Lens z z a a
fieldLens' name = \f m -> fmap (updateField name m)
(f $ getField name m)
type family UpdateType (n :: Symbol) z a b where
UpdateType n ((a :? n) ': z ) a b = (b :? n) ': z
UpdateType n ( z ': zs) a b = z ': UpdateType n zs a b
fieldLens :: ( ARec m ~ z, ARec m' ~ z', m' ~ UpdateType name m a b
, FieldType name m ~ a, FieldType name m' ~ b
, KnownNat (FieldIndex name m), KnownNat (FieldIndex name m')
, KnownNat (Length m), KnownNat (Length m')
) => FieldName name -> Lens z z' a b
fieldLens name = \f m -> fmap (updateField name (rerec m))
(f $ getField name m)
this approach is more or less the same as records package with only one
datastructure and almost the same syntactic sugar can be applied, the
only missing thing is that pattern matching will be more difficult that with
records.
At this point it's not possible to write strict fields, but it can be
easily extended.
If someone is interested in this sort of wheel, I can prepare a package and some
docs about and with coercion with other solutions like
fixed-vector-hetero and records.
[1] https://hackage.haskell.org/package/fixed-vector-hetero
On Sat, Jul 4, 2015, 16:08 Ben Franksen
Marcin Mrotek wrote:
Okay, perhaps I'm too newbie to understand the big picture, but it seems to me you can get either:
a) O(1) access to any, arbitrarily selected (at runtime) field b) O(1) append
I guess option a) is better performance-wise, as appending is usually done less often than selecting (an O(1) slice is already possible with independently typed regular Haskell records) but dependently-typed-list-based implementation, or at the very least Vinyl (I haven't ever used HList) has the advantage of being dead simple in both implementation and usage. I mean, with Vinyl, you can write manual recursion over Rec's like:
foo :: Rec ... -> Rec ... foo RNil = ... foo (r :& rs) = ...
whenever GHC's typechecker gives up and goes on a strike; and I dare to say, with commonly used record sizes (have you ever used a record with more than, let's say, 10 fields?) the speed tradeoff is not noticeable.
While more than 10 fields in a record is uncommon for typical library APIs and simple programs, real world projects can grow much larger records. One example is configuration data for complex programs (like Darcs or even GHC) with many options. It would be so nice if we could use record types for the configuration! Another application could in control system toolkits like EPICS [1], which currently has (actually: generates) C records with potentially hundreds of fields.
If lookup is / remains linear we can never efficiently support these kinds of applications and that would be very sad.
I think the most reasonable default is O(1) for lookup and O(n) for extension, like in Nikita Volkov's record package. It is quite unfortunate that this package limits the number of fields! If GHC would offer generic support for tuples of arbitrary size (with the same efficiency as today) this limitation could be avoided and all would be well.
Cheers Ben
[1] http://www.aps.anl.gov/epics/ -- "Make it so they have to reboot after every typo." ― Scott Adams
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