RowList Fun with Purescript

A small talk on RowList and some type-level programming in Purescript given at Small FP Conf 2017 in Tampere.

Reddit thread: https://www.reddit.com/r/purescript/comments/6xs5f2/rowlist_fun_with_purescript_slides_from_small_fp/

Transcript

1. RowList Fun with
   PureScript
   Justin Woo
   Small FP Conf 2017
   

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2. What is PureScript?
   ● Pure, Strict, Functional
   ● Higher Kinded Types
   ● Type classes with functional dependencies
   ● Tooling with purescript-ide server + client
   ● Docs for packages on Pursuit
   ● Easy-to-use build tool in Pulp
   ● Typed holes with type-directed search
   ● No runtime -- compiles to real JS
   ● Row types
   ● ...and more!
   

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3. What are Row types?
   ● An unordered collection of fields
   ○ Key is Symbol (a type-level string kind)
   ○ Value is of the kind provided to the row
   data Record :: # Type -> Type
   -- takes row of Type to define a Type
   type Person =
   { name :: String, age :: Number }
   type Person =
   Record (name :: String, age :: Number)
   ● Eff works this way!
   kind Effect
   data HTTP :: Effect
   data Eff :: # Effect -> Type -> Type
   main :: Eff ( http :: HTTP ) Unit
   

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4. Collection? How about as a List?
   ● What if we could use these rows as lists of fields? Wouldn’t that be fun?
   kind RowList
   data Nil :: RowList
   data Cons :: Symbol -> Type -> RowList -> RowList
   type PersonRow = (name :: String, age :: Number)
   type PersonRowList = (Cons "name" String (Cons "age" Number Nil))
   

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5. Type classes: “type-level case statements”
   ● How do you match and work with things on the type level? Type classes!
   ○ Using instances for each branch
   ● Simple example: getting keys from a record
   class Keys (xs :: RowList) where
   keysImpl :: RLProxy xs -> List String
   instance nilKeys :: Keys Nil where
   keysImpl _ = mempty
   -- Phantom types reminder
   data Maybe a = Nothing | Just a
   data Proxy a = Proxy
   data RLProxy (xs :: RowList) = RLProxy
   

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6. Cons instance of class Keys
   1. Pattern match on the RowList
   2. Reflect the Symbol to a string value
   3. Get the rest by calling keysImpl on the tail
   4. Cons this string onto the rest of the keys
   That’s it!
   instance consKeys ::
   ( IsSymbol name
   , Keys tail
   ) => Keys (Cons name ty tail) where
   keysImpl _ = first : rest
   where
   first = reflectSymbol (SProxy :: SProxy name)
   rest = keysImpl (RLProxy :: RLProxy tail)
   

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7. Exposing our method in a reasonable way
   ● We wrote a class for RowList, but this needs to work for row types from Records
   ● RowToList allows us to convert row to RowList for exactly this!
   keys :: forall row rl. RowToList row rl => Keys rl => Record row -> List String
   keys _ = keysImpl (RLProxy :: RLProxy rl)
   ● Since we have everything from the type level, the record ends up being a glorified Proxy
   

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8. Keys in action
   Usage:
   main =
   traverse_ print $ keys { a: 1, b: 2 }
   Output:
   "a"
   "b"
   

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9. ● Purescript-Simple-JSON: automatically parse record type aliases without any setup
   ○ Also writes JSON safely (e.g. no function stringifying)
   ● Purescript-Kancho: restrict Purescript types to Elm port types and generate Elm types
   ● Purescript-OhYes: restrict Purescript types to be Typescript-safe and generate type definitions, using
   polymorphic variants through Purescript-Variant to support discriminant-field union types in
   Typescript
   ● Purescript-ChocoPie: a Cycle.js-like library that exposes record-based APIs for creating cycles of Events
   from Purescript-Behaviors
   ● Purescript-IndexedDB-Safe by Antti Holvikari: type-safe wrappers for IndexedDB
   ● … and more!
   Other applications of RowList
   

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10. “Will I need to do this if I [use / start using]
    Purescript?”
    ● Writing type-level code using RowList?
    ○ Not really, but sometimes
    ○ And you will learn these from usage
    ● Defining your own type classes and working with them?
    ○ Not necessarily, but sometimes
    ○ And you will learn these from usage
    ● Understanding how to use constraints and throw them around?
    ○ Yes and no
    ○ You will learn these from usage
    ○ You can always start with the most concrete types and use more general ones over time
    ■ e.g. Monoid mempty vs []
    

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11. Thanks!
    Ask me Purescript questions on Twitter: @jusrin00
    Map/Zip/Keys examples on github.com/justinwoo/purescript-map-record
    IRC: #purescript on freenode
    Slack: #purescript on Functional Programming Slack
    Reddit: /r/purescript
    

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