Not sure that it will make sense for a cafe level, but here is my understanding of the current state. For the problem of "anonymous records", i.e. some datatype that may have different fields and their types we have following appoaches: 1. vinyl/hlist/etc. i.e. GADT base heterogeneous list. This one have O(N) random access and may not be a good fit for "anonymous record" use case 2. records: polymorphic data structures with type-level information about field names. Here we have a lot of datastructures one per number of fields multiply on 2 (lazy and strict variant). This one has O(1) random access. 3. fixed-vector-hetero/"wheel above". array based structures has O(1) access. I call code above a "wheel" because there were a number of similar implementations here and there. I'm not the author or contributor of any libraries above, so I may miss something, but here is my view: (let's not take a hlist into the scope, because it should be used in different scenarios). record provides a good approach + nice syntactic sugar (based on TH or preprocessor or possibly compiler). The downside here is that we have O(N) internal structures and exponential number of instances. You could take a look at records haddock page, use 10% zoom for the best feeling :). array-based approach have only one structure and 1 instance for any number of fields, and this is a benefit. But there is a cost as records could be used even without syntactic sugar (e.g. pattern matching), and with fixed vector it will be a bit tricker (if possible), for example my approach above as it stands will not be usable without syntactic sugar similar to the records one. There is one more problem, library heavily uses `unsafeCoerce` so it should be a part of the trusted base, of cause this is totally safe as typechecker checks that, nevertheless it may be a concern for the users. my wheel could reuse fixed-vector-hetero, as this is basically tagged structure over fixed vector, I have not reused that because of 2 reasons: 1. I have not found a nice way to update fields in immutable vector (maybe I just missed that part of API) 2. I don't see a nice way to give user a choice to make fields strict, and in a wheel it's possible to introduce another typelevel variable that will be used as a strictness marker, and lens that we build will appreciate that. Btw, here we may have a solution that is better that "all-or-nothing" one in the records package. this solution may well cope with solution for "overloaded records" that is worked on. 3. If fixed-vector will not be reused it's possible to convert structure in O(1). Anyway records have all required tooling around it, and seems like a best solution in this (anonymous records with O(1) field access) area at least for now. -- Alexander On 4 July 2015 at 17:19, Ben Franksen wrote:

Thanks, I didn't know of vector-fixed-hetero. Quite interesting.

Anyway, I am not so much interested in getting reasonable efficiency from any of the existing libraries right now, as I am in reaching consensus about what kind of compiler support would be needed to make any of them practical.

Cheers Ben

Alexander V Vershilov wrote:

...

(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 wrote:

...

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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-- Alexander

On 6 July 2015 at 10:51, Frank Staals wrote:

Alexander V Vershilov writes:

...

Not sure that it will make sense for a cafe level, but here is my understanding of the current state. For the problem of "anonymous records", i.e. some datatype that may have different fields and their types we have following appoaches:

1. vinyl/hlist/etc. i.e. GADT base heterogeneous list. This one have O(N) random access and may not be a good fit for "anonymous record" use case

2. records: polymorphic data structures with type-level information about field names. Here we have a lot of datastructures one per number of fields multiply on 2 (lazy and strict variant). This one has O(1) random access.

3. fixed-vector-hetero/"wheel above". array based structures has O(1) access. I call code above a "wheel" because there were a number of similar implementations here and there.

Somehow I feel the most promissing approach is missing from this list: The 'Just do it while compiling!' paper[1] describes how to get O(log n) query and update, by essentially using heterogeneous binomial trees. I think I also saw some haskell implementation in a github gist somewhere, but I could not find the link at the momement.

Yes, I tend to agree that this approach should be noted, I just was unaware of that article. As it seems it's better fit for functional paradigm that the "array" based and may have better properties when there are many updates and no fusion was implemented.

Having said that. I think the Big-O notation used here is misleading, since in 99.999% of all cases we have constant-sized records. Therefore, all approaches give O(1) access/update in these cases. If one really want to know what is fastest benchmarking is the only way to go here.

Benchmarking in the current case is always the answer, as for some use-cases memory overhead and locality may matter. And vector/data-structure approaches will give better results

Regards,

[1] http://www.fing.edu.uy/~mviera/papers/pepm13.pdf

--

- Frank

-- Alexander