polysemy-methodology - Domain modelling algebra in Haskell

One of Haskell’s primary and yet underused strengths is denotational design - to write code the way we wish it could be written, and then using the language to make that representation work in practice. Free Monads have been a huge step forward in providing general ways to produce domain specific languages that we can use to talk about the program using the semantics that are comfortable for us, and then repeatedly interpret that as slightly lower level DSLs until we reach the real code.

However, we have yet to fully bridge the gap between the architecture and what ultimately ends up running. The architecture process tends to work top-down, by taking very general problem statements to describe what the program should do, and then subdividing it into parts that meet those criteria. The development process typically works bottom-up, by collecting components together and composing them, and then hoping that what has been built does indeed match the architecture plan.

Typed holes and type applications can offer us a strategy to bridge this gap by exploring how we can describe decomposing problems into pieces in addition to composing solutions from parts. Here we look at a way to make a simple domain model and compile it directly.

We introduce an experimental library polysemy-methodology that works towards this purpose.

The Basic Approach

A simple program might look something like this:

prog :: Members '[ Input a
                 , Methodology a b
                 , Output b]
     => Sem r ()
prog = input @a >>= process @a @b >>= output @b

That is, this program transforms an Input a into an Output b by way of a Methodology a b that turns a into b. We can then type apply a and b and connect this to main.

If we have a solution readily available, we can consume a Methodology by running one of the interpreters runMethodologyPure or runMethodologySem.

Otherwise, we can use the other interpreters in this package to break the problem down into components or branches and solve each section separately. Each interpreter will produce a new set of Methodologys to be solved.

This allows us to work up a solution to a domain problem backwards, by running the program we intend to solve directly and using holes to guide the requirements.

Worked example

A worked example of this approach can be found in the flashblast repository. In this we want to take a configuration, and process it in some way an output of flashcards.

We might model this as such:

-- Domain.hs
import Polysemy
import Polysemy.Input
import Polysemy.Tagged
import Polysemy.Methodology
import Polysemy.Output

-- | A `DeckConfiguration` indicates how we create cards.
data DeckConfiguration

-- | A `CollectionsPackage` indicates the desired output format.
data CollectionsPackage

-- | The Construction Methodology for flashblast.
data ConstructionMethodology

-- | `flashblast` is a program that takes a `DeckConfiguration` and outputs a `CollectionsPackage`.
flashblast :: Members '[ Tagged DeckConfiguration (Input a)
                       , Tagged ConstructionMethodology (Methodology a b)
                       , Tagged CollectionsPackage (Output b)] r
           => Sem r ()
flashblast = do
  x <- tag @DeckConfiguration input
  k <- tag @ConstructionMethodology $ process x
  tag @CollectionsPackage $ output k

Notice that this is an abstract domain model. We have not committed to a particular representation of any of the three elements of this program. In fact, this file depends only on polysemy modules, which allows us to isolate the domain model from anything resembling real code.

However, we would also like to claim that what we say the program should do in abstraction is actually what we run for real. So it would be reassuring to be able to simply interpret this into real functions.

We commit to a concrete representation for the config and for the output only in the main application file, where we iterate over the decks.

-- Config.hs
data Spec =
    Pronunciation   [PronunciationSpec]
  | Excerpt         [ExcerptSpec]
  | BasicReversed   [BasicReversedCard]
  | MinimalReversed [MinimalReversedCard]
    deriving stock Generic

makePrisms ''Spec

data Deck = Deck {
  _resourceDirs :: ResourceDirs
, _exportDirs   :: ExportDirs
, _parts        :: [Spec]
} deriving stock Generic
-- Main.hs
data Deck = Deck {
  notes :: Map (Path Rel File) Text
, media :: [Path Rel File]
} deriving stock (Eq, Show, Generic)
  deriving Semigroup via GenericSemigroup Deck
  deriving Monoid via GenericMonoid Deck

main = do
  decks <- ...
  forM_ decks $ \x -> do
    flashblast @Config.Deck @Deck
      & runM

Here we will be told that we need to satisfy the Input, Output and Methodology effects.

The Config.Deck is divided into several different specs. We could simply write one giant function to solve the Methodology and annihilate the Methodology effect using runMethodologySem.

soln :: Members '[...] r => Config.Deck -> Sem r Deck
soln = ...

-- runMethodologySem @Config.Deck @Deck soln

But this would conflate our concerns - the different specs require different effects to execute, and having this single function require all effects wouuld be maintenance should we choose to remove any functionality. It would also increase our testing surface.

  • The MinimalReversedCards and BasicReversedCards are direct representations of what the output cards should look like, and so can be purely transformed.
  • ExcerptSpecs need to be transformed into cards by way of processing the specified video and subtitle track via ffmpeg.
  • PronunciationSpecs need to fetch the pronunciation data for the target words from a remote API.

What would be nice is if we could reach a point where we can make functions for each of with their respective effects isolated but without having to agglomerate all the effects into a single solution function.

It makes sense then to take our Methodology and break it down into sub Methodologys that can be reasoned about independently, rather than trying to satisfy the program with one function built up from parts. This way we can break the program down using only type applications and interpreters, and we only need to write any code once we are happy that the problem is sufficiently decomposed.

The interpreters in this library are operations that consume a Methodology and turn it into parts.

cutMethodology breaks the Methodology into two pieces, and will then require interpreters for each. So if we start with a Methodology b d, we can break it into Methodology b c and Methodology c d, each of which will require some solution. This is essentially reverse arrow composition.

b -----> d   ===>  (b ---> c), (c ---> d)

divideMethodology breaks the target into a pair, and connects the source to both of them, producing three Methodologys we need to solve. This is reverse fanout.

b ----> d ==> (b ---> c), (b ---> c'), ((c,c') ----> d)

decideMethodology breaks the source into an Either, allowing us to choose a Methodology to run as the result of another Methodology based on the source. This is reverse fanin.

b ----> d ===> (b---> Either c c'), (c ---> d), (c ---> d)

decomposeMethodology is cutMethodology specialised to HList as the center argument. This allows us to cut the Methodology into multiple parallel tracks.

b ----> d ===> b------d------f

Back to our example, we need to decompose our Config into the problems concerning each type of spec, then turn each of those into a Deck of its own, then collect the produced decks monoidally into the final output.

Dealing with HLists is a little awkward but the approach that will work is to deal with each strand individually, and use separateMethodologyInitial or separateMethodologyTerminal depending on whether the strand appears before or after the HList, which will separate the element of the HList into its own Methodology. Then, decompose this further or solve it.

type DeckSplit = '[[Config.MinimalReversedCard]
                 , [Config.BasicReversedCard]
                 , [Config.ExcerptSpec]
                 , [Config.PronunciationSpec]

main = do
  forM_ decks $ \x -> do
    flashblast @Config.Deck @Deck
      & untag @ConstructionMethodology
      & decomposeMethodology @Config.Deck @DeckSplit @Deck
        -- We pull out `Config.Deck -> [Config.MinimalReversedCard]` as its own `Methodology`.
        & separateMethodologyInitial @Config.Deck @[Config.MinimalReversedCard])
          -- And then immediately solve it purely.
          & runMethodologyPure _
        & separateMethodologyInitial @Config.Deck @[Config.BasicReversedCard]
          & runMethodologyPure _
        & separateMethodologyInitial @Config.Deck @[Config.ExcerptSpec]
          & runMethodologyPure _
        & separateMethodologyInitial @Config.Deck @[Config.PronunciationSpec]
          & runMethodologyPure _
        & endMethodologyInitial
          & separateMethodologyTerminal @[Config.MinimalReversedCard] @Deck
            & runMethodologyPure _
          & separateMethodologyTerminal @[Config.BasicReversedCard] @Deck
            & runMethodologyPure _
          & separateMethodologyTerminal @[Config.ExcerptSpec] @Deck
            & runMethodologySem _
          & separateMethodologyTerminal @[Config.PronunciationSpec] @Deck
            & runMethodologySem _

We have left holes that polysemy will now tell us need to be filled by nice clean a -> b or a -> Sem r b functions. Any effects we add here we can deal with after this block, or we can decompose this even further.

polysemy-methodology is available on hackage, github and gitlab.

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