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The Descriptor


One of my eventual goals with this blog is a post, or series of posts self-importantly titled Effective Haskell1. It’s a response to the recent Simple/Boring/Junior Haskell movement. It’s true that when writing Haskell in production, you need to be cautious not to accrue accidental complexity. My issue with the argument is the implication that we’re giving something up. Instead, I’d argue that using “advanced” features like GADTs, Type Families and even type classes is often unnecessary in the first place2, and a failure to find a simpler solution.

I’m not sure if I’ll ever actually finish a single post in Effective Haskell, but what I’m going to do in the meantime is use this blog to collect examples of what I think would classify as Effective Haskell.

In this inaugural post, we will be studying a technique (ostensibly) for iterating over record fields and adding metadata to them, without using Template Haskell or Generics. I’m calling it a descriptor because that’s what it reminds me of, but if somebody else has named it before, or just knows a more fitting name, please let me know.

Motivating example

Our initial motivating example is from where I initially stumbled into this technique.

The goal of the library in this example is to give a way to define graphics pipelines in a safe and efficient way. The user of the library formulates the global arguments (uniforms in GL parlance) to a pipeline on the Haskell side as a data type:

data Blinn f = Blinn
  { viewPos       :: f (V3 Float)
  , lightAmbient  :: f (V3 Float)
  , lightDiffuse  :: f (V3 Float)
  , lightSpecular :: f (V3 Float)
  , shininess     :: f Float
  , mvpMatrices   :: MVP f

The shaders (GPU code) are written in a different language, and we an only compile them manually at runtime. From this Haskell record, we then need to:

  1. Reject types that the GPU does not support
  2. Match up these arguments to the shader code
  3. Make sure the types of the variables match between our code and the GPU
  4. Check for missing/unbound/duplicate variables
  5. Get the locations of those variables on the GPU
  6. Assign initial values
  7. Provide a safe way to update the variables at runtime

All of that is handled by createProgram, which creates our pipeline and only requires that the user pass a descriptor of the Blinn record:

(program, log) <- createProgram "glsl/common.vert" "glsl/blinn.frag" $
  \f -> Blinn <$> f viewPos       0   "viewPos"
              <*> f lightAmbient  0.2 "light.ambient"
              <*> f lightDiffuse  1   "light.diffuse"
              <*> f lightSpecular 1   "light.specular"
              -- (lines omitted)
              <*> f shininess     32  "material.shininess"
              <*> descMvp (\f' -> f (f' . mvpMatrices))

The descriptor is the function from the second line downward. This descriptor contains, for every field, the respective field accessor, an initial value, and the name of the variable in the shader code. If createProgram succeeds it returns a Program Blinn value, and now the field accessors of the original struct can e.g. be used as type-safe update functions:

withProgram program $ do
  objViewPos    $= x
  (model . mvp) $= modelMat
  (view . mvp)  $= viewMat
  drawMesh mesh

The point is this; a descriptor is an ordinary value, that you can have users provide about their data type. You can then use this value to do complicated things, in a type-safe manner, without the library ever having seen the original type, and without the user having to be aware of the machinery.

What the descriptor ultimately looks like will depend on the application3, but the principles stay the same. You might already be able to discern a lot by looking it how they’re used above, but let’s see how they work.

Simple Descriptors

Let’s say we’re writing a library that provides a way to ask for data on the command line. We expose a function that, given a label, parsing function, and verification function, asks for and yields a single value:

ask :: String -> (String -> Maybe a) -> (a -> Bool) -> IO a
ask label parse check = go where
  go = do
    putStrLn $ "What's your " <> label <> "?"
    parse <$> getLine >>= \case
      Just r | check r -> return r
      _ -> putStrLn "Invalid response" >> go

And the user then uses Applicative to assemble into a function asking for an entire record:

data Person = Person
  { pName :: String
  , pAge  :: Int

askPerson :: IO Person
askPerson = Person
  <$> ask "name" Just      ((>1) . length . words)
  <*> ask "age"  readMaybe (\a -> a >= 18 && a <= 99)

This is the basis for our descriptor. The arguments to ask in askPerson describe general properties of the fields of Person that might be useful in other contexts. We turn askPerson into a descriptor as follows:

  1. we factor out the ask so it takes a general field function as an argument
  2. we add one extra argument to field, the respective record field accessor
  3. we generalize the IO into any Applicative
descPerson :: Descriptor Person
descPerson field = Person
  <$> field pName "name" Just      ((>1) . length . words)
  <*> field pAge  "age"  readMaybe (\a -> a >= 18 && a <= 99)

The here Descriptor is a type synonym that forces us to be polymorphic:

type Descriptor s = forall m. Applicative m
  => (forall a. (s -> a)
             -> String
             -> (String -> Maybe a)
             -> (a -> Bool)
             -> m a
  -> m s

And that’s it. As a first exercies, we can use Descriptor to construct something equivalent to the askPerson we defined above:

askDesc :: Descriptor p -> IO p
askDesc desc = desc (const ask)

askPerson :: IO Person
askPerson = askDesc descPerson

But what have we gained? We could swap out ask for a similar function, of course. But there is a point to passing the record field accessor to field; it allows us to work with existing data. For example, we can perform just the validation:

validate :: Descriptor p -> p -> [String]
validate desc pers = execWriter $ desc $ \field lbl _ p ->
  let a = field pers
   in unless (p a) (tell ["Invalid " <> lbl]) $> a
λ> validate descPerson (Person "aa" 45)
["Invalid name"]

Or, we can enumerate all the fields in a descriptor:

fields :: Descriptor p -> [String]
fields desc = execWriter $ desc $ \_ lbl _ _ ->
  tell [lbl] $> undefined
λ> fields descPerson
["name", "age"]

That’s the gist of a descriptor; a function applied, to each field of a record, with some arguments, polymorphic over any applicative. How you structure the field function depends on what you use the descriptor for, but this outlines the general idea.

It’s a pretty neat trick, but unfortunately, there are some issues here:

  1. We can give nonsensical Descriptors that still type check:
descNonsense :: Descriptor Person
descNonsense _ = pure $ Person "太郎" 3
  1. We need an unfortunate undefined to make the fields definition above type check.
  2. validate is a bit contrived; only outputting a list of invalid fields is hard to deal with safely.

All of that is solved when we use Higher-Kinded Data (HKD), which is where this technique really comes into its own.

Descriptors with Higher-Kinded Data

Higher-Kinded Data is a pattern where you parameterize record fields over some functor, like this:

data HPerson f = HPerson
  { hName :: f String
  , hAge  :: f Int

With HKD, HPerson Identity is equivalent to the original Person record, but we also get HPerson Maybe that might have missing fields, HPerson (Const a) that has a value of type a for every field, etc.

We can apply the idea of the descriptor to HKD almost verbatim. Our new descPerson and askDesc look pretty much the same at the term level:

descHPerson :: HDescriptor HPerson
descHPerson field = HPerson
  <$> field hName "name" Just ((> 1) . length . words)
  <*> field hAge "age" readMaybe (\a -> a > 18 && a < 99)

askHDesc :: HDescriptor s -> IO (s Identity)
askHDesc desc = desc $ \_ lbl parse check -> Identity <$> ask lbl parse check

In HDescriptor s our s is now also polymorphic over the base functor. This is its type:

type HDescriptor s = forall m f. Applicative m
  => (forall a. (forall g. s g -> g a) -- or, equivalently, Field s a, see below
             -> String
             -> (String -> Maybe a)
             -> (a -> Bool)
             -> m (f a)
  -> m (s f)

Let’s revisit the issues with the non-HKD approach.

  1. HDescriptor cannot choose the underlying functor, it has to use field to construct it. We can no longer construct a nonsensical HDescriptor without explicitly using undefined.
  2. We can write fields using Proxy instead of undefined:
hfields :: HDescriptor p -> [String]
hfields desc = execWriter $ desc $ \_ lbl _ _ -> tell [lbl] $> Proxy
  1. We can now use our validation function to check an existing record “in-place”, rather than only outputting a list of wrong fields. Compare this type to that of validate:
hvalidate :: HDescriptor s -> s Identity -> s Maybe
hvalidate desc s = runIdentity $ desc $ \f _ _ check ->
  f s <&> (\a -> if check a then Just a else Nothing)

Here’s something that we couldn’t do at all before. Imagine that we get a HPerson (Const String) from, say, a web form. We can then use the HDescriptor to parse and check each field individually.

hParseCheck :: HDescriptor s -> s (Const String) -> s Maybe
hParseCheck desc s = runIdentity $ desc $ \f _ parse check -> pure $
  case parse $ getConst (f s) of
    Just r | check r -> Just r
    _ -> Nothing

HKD type classes

When you use HKD, you typically want to be able to map/traverse/<*> the fields of your record. There are libraries like higgledy, barbies, barbies-th, or hkd that help you derive the required instances (and other nice things). We can show that a descriptor gives you the same power:

dmap :: HDescriptor s ->
  (forall a. f a -> g a) -> s f -> s g
dmap desc fn s = runIdentity $
  desc $ \f _ _ _ -> pure $ fn (f s)

dtraverse :: Applicative m => HDescriptor s ->
  (forall a. f a -> m (g a)) -> s f -> m (s g)
dtraverse desc fn s =
  desc $ \f _ _ _ -> fn (f s)

dpure :: HDescriptor s ->
  (forall a. f a) -> s f
dpure desc a = runIdentity $
  desc $ \_ _ _ _ -> pure a

dliftA2 :: HDescriptor s ->
  (forall x. f x -> g x -> h x) -> s f -> s g -> s h
dliftA2 desc fn sf sg = runIdentity $
  desc $ \f _ _ _ -> pure $ fn (f sf) (f sg)

This doesn’t necessarily mean that descriptors compete with the libraries above. The actual use cases are different, descriptors work best when you have to provide an interface to library users and don’t want to force them to use Template Haskell, Generics, or dependencies.

Structs and FFI

Briefly, before we continue: every record field accessor of an HKD has type forall f. s f -> f a. To avoid having to quantify the f every time, we’re going to assign it a type signature:

type Field s a = forall f. s f -> f a

For example, hName :: Field HPerson String and hAge :: Field HPerson Int.

Updating a single field

One of the issues with normal Storable-based FFI is that, even if you define a Storable instance for a user-defined struct, you cannot perform any field-wise updates on it. With HKD we can, as follows:

data MyStruct f = MyStruct
  { versionMajor        :: f Int
  , versionMinor        :: f Int
  , frictionCoefficient :: f Double
  , baconNumber         :: f Word8

data SPtr struct = SPtr
  { sBase    :: Ptr ()
  , sOffsets :: struct (Const Int)

setField :: Storable a => SPtr struct -> Field struct a -> a -> IO ()
setField (SPtr base offsets) field = poke ptr
    ptr = plusPtr base . getConst . field $ offsets

As you can see, the trick is to use MyStruct (Const Int) to store the offset of every field. We can then update a single field using

setField ptr baconNumber 1

-- Or, if you want to get fancy,
let ($=) :: Storable a => Field struct a -> a -> ReaderT (SPtr struct) IO ()
    ($=) = ...
flip runReaderT ptr $ do
  versionMajor $= 2
  versionMinor $= 1

The field accessors of MyStruct now double as field accessors for our foreign struct. I’m leaving getField as an exercise, but it works the same way.

Constructing the SPtr

Where does the SPtr actually come from? As you might have guessed, we can make one with a descriptor.

ptr <- newSPtr $ \field -> MyStruct
  <$> field versionMajor        1
  <*> field versionMinor        9
  <*> field frictionCoefficient 0.9
  <*> field baconNumber         0

The second argument to field is the initial value of each field.

newSPtr traverses the constructor, creating the record of the offsets for each field. It then mallocs the total size, and assigns each field its initial value:

newSPtr :: SDescriptor s -> IO (SPtr s)
newSPtr desc = do
  base <- mallocBytes size
  desc $ \f a -> poke (plusPtr base . getConst . f $ offsets) a $> Proxy
  pure (SPtr base offsets)
    (offsets, size) = flip runState 0 $
      desc $ \_ a -> state (\s -> (Const s, s + sizeOf a))

What makes SDescriptor different from our previous descriptors is that it has a type class constraint on the field function:

type SDescriptor struct = forall m f. Applicative m
  => ( forall a. Storable a
              => Field struct a
              -> a
              -> m (f a)
  -> m (struct f)

This means that, as soon as one of the fields of MyStruct is not Storable, you cannot write a SDescriptor for it. Conversely, the existence of the SDescriptor MyStruct proves that every field of MyStruct is Storable. For example, you could not add a String field to MyStruct, since String aren’t Storable. We’ll look into how you might deal with strings in the section on arrays below.

Nested structs

The initial example already hinted at the fact that structs/descriptors can be nested. The data definition is fairly straightforward, no different from how you would normally do it with HKD:

data MySuperStruct f = MySuperStruct
  { someInt :: f Int
  , nestedData :: MySubStruct f

As for the descriptor itself, you simply call the descriptor for the nested struct in the place it occurs, but you’ll have to prepend the record field accessor as follows:

descMySuperStruct :: SDescriptor MySuperStruct
descMySuperStruct field = MySuperStruct
    <$> field someInt 1
    <*> descMySubStruct (\subField -> field (subField . nestedData))


As a final thought, let’s think about how to approach structs that contain arrays. This will be just one of the ways to tackle it, but there are ways to go e.g. statically known sizes.

The trick here is to give our records two4 functor parameters:

data Image fArr fPrim = Image
  { imgW    :: fPrim Int
  , imgH    :: fPrim Int
  , imgData :: fArr Word8

Correspondingly, our descriptor now takes two function arguments, with the one for arrays taking an extra one indicating the size:

myArrStructDescriptor :: ArrDescriptor MyStructWithArrays
myArrStructDescriptor array field = MyStructWithArrays
  <$> field imgW 99
  <*> field imgH 99
  <*> array imgData (99 * 99 * 3) 0

I’ll give the type of ArrDescriptor below for completeness’ sake, but even more than before it’s not about the specifics of this approach, but the general idea; you can have multiple field-style functions. In this case the difference is between primitive updates in fPrim and indexed updates in fArr, but you could, for example, also have read/write-only fields.

type ArrDescriptor struct = forall m fArr fField. Applicative m
  => ( forall a. Storable a
              => (forall gArr gField. struct gArr gField -> gArr a)
              -> Int
              -> a
              -> m (fArr a)
  -> ( forall a. Storable a
              => (forall gArr gField. struct gArr gField -> gField a)
              -> a
              -> m (fField a)
  -> m (struct fArr fField)


When you’re in the trenches of a tutorial like this, it can be hard to see the forest for the trees. Especially when working with nested structs and arrays, our types got pretty involved. However, I hope I have also been able to convince you that when this approach works, it can work really well. The library author (person who defines the descriptor) gets a lot of power, and the user (person who implements the descriptor) only has to define a single generic traversal. Furthermore, since we aren’t using any existing abstractions, we get to completely tailor it to our own needs, as you saw in the array example.

Ultimately, I’m not sure if the ideas here are going to be useful for many people. I have worked with libraries that horribly over-complicated their FFI so I know there are at least some people who might find this useful, but that’s not really the point of this post. Most importantly, I think it’s a neat example of how we can write wonderful abstract interfaces with just RankNTypes and some polymorphism in the right places.

If you have any questions or criticism, feel free to contact me.

  1. Alternate title: Gospel of RankNTypes, the One True Extension↩︎

  2. We can actually write incomprehensible code without them!↩︎

  3. Which is why this works better as a blog post than a library↩︎

  4. Or more. You might want to make a special case for String types, or dynamically sized arrays…↩︎

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