From Either to Variant

Adding More Conditions

Let's forget Coproduct for a moment and return back to the Either level. When we are writing code, we often need to refactor things. If we are using nested Either types everywhere, there are three ways we could change our code:

  1. Change the order of the types position: Either first second <=> Either second first
  2. Add/Remove another type: Either first (Either second third) <=> Either first second
  3. Change one type to another: Either Int Int <=> Either Int String

If we made one of the changes above, we would need to change our function's type signatures slightly just to get the code to compile again. If we must do this after writing 20 functions, that's a lot of wasted development time!

Moreover, purescript-either only grants us the ability to define 10 total types in a nested manner. Once we go above that number, we need to write our own convenience functions. If we do this in multiple projects, that's more wasted developer time.

So, what are our options here? We saw previously that we could not define our own special nested Either type without knowing at the type-declaration-time what the next few types are. However, we do know at compile-time what those types will be. We also know at compile-time how many types will exist in a nested Either-like type. This implies that we might want to stop looking at value-level programming for our solutions and instead look at type-level programming. More specifically, we should look at row kinds.

We can use row kinds specified to type (i.e. # Type) to specify n-different types that are known at compile-time. As an example, we can look at Record, which is a nested Tuple that uses row kinds to work:

-- first AND second AND third AND fourth AND last
Tuple first (Tuple second (Tuple third (Tuple fourth last)))
Record (a :: first, b :: second, c :: third, d :: fourth, e :: last)
-- which is more commonly seen using "{}" syntax
       {a :: first, b :: second, c :: third, d :: fourth, e :: last}

Is it possible to take this same idea and apply it to Either? Yes, which is what purescript-variant does:

-- first OR second OR third OR fourth OR last
Either first (Either second (Either third (Either fourth last)))
Variant (a :: first, b :: second, c :: third, d :: fourth, e :: last)
-- Unfortunately, there is no equivalent "{}"-like syntax for variants

The advantage of using the row kinds via # Type is that it makes our nested Either type "open" via row polymorphism. Using Record as an example, we'll see that each of the three "prototype code" requirements from above can be achieved without refactoring!

Looking at our requirements from before...

  1. Change the order of the types position: Either first second <=> Either second first
  2. Add/Remove another type: Either first (Either second third) <=> Either first second
  3. Change one type to another: Either String Boolean <=> Either Int Boolean

... we can see that Record meets our requirements because of row polymorphism:

-- the function
getName :: forall allOtherFields. { name :: String | allOtherFields } -> String
getName :: { name: theName } = theName

-- normal version
getName { name: "John", dogName: "Spot" }
-- 1) changed type's order
getName { dogName: "Spot", name: "John" }
-- 2a) added a type
getName { name: "John", dogName: "Spot", age: 20 }
-- 2b) removed a type
getName { name: "John" }
-- 3) changed "dogName" from type `String` to `DogNames`
getName { name: "John", dogName: Spot }

All five exampls from above compile. Since Variant uses row kinds via # Type, too, it also benefits from these advantages.

Using Variant

Here's the link to the library: purescript-variant.

Record has support at the syntax-level in Purescript. So, instead of writing Record (key :: ValueType), we can write {key :: ValueType}. Unfortunately, Variant is not supported at the syntax-level. Thus, we must be explicit and use type-level programming. The following code shows how to write inject and project via Variant and Symbols, type-level Strings.

If you haven't already done so, read through the Syntax folder on Type-Level Programming Syntax


injectFruit :: forall v. Fruit -> Variant (fieldName :: Fruit | v)
injectFruit fruit = inj (Proxy :: Proxy "fieldName") fruit


projectFruit :: forall v. Variant (fieldName :: Fruit | v) -> Maybe Fruit
projectFruit variant = prj (Proxy :: Proxy "fieldName") variant

Pattern Matching in Variant

The other functions that Variant provides can be see via its docs. I created the following table after looking at the project's test's source code. Some functions seem to exist to fit different people's syntax preferences:

Exhaustively pattern matches types by...Allows "open" Variant values?Corresponding function's syntax
Providing default value for missing casesYesdefault defaultValue <combinator chain> variantArg
Matching all casesNomatch { eachField: (\a -> {- body for each field -}) } variantArg
Matching all casesNocase_ <combinator chain> variantArg

where a <combinator chain> is:

  • single element chain: # combinator
  • multi-element chain: # combinator1 # combinator2 ... # combinatorN
CombinatorExpected arguments
on(Proxy :: Proxy "fieldName") (\value -> {- body -})
onMatch{ fieldNameN :: (FieldType -> a) }

Besides those above, Variant also has expand and contract. One takes a Variant that has more fields than just those specified in some function and "expands" it into its full number of nested types. The other takes a fully-expanded Variant and "contracts" it down to a smaller subset of its nested types.

Updating Our Solution

If we return to our FruitGrouper solution from before and use Variant instead of FruitGrouper, here's what we get.

Here's the list of run commands:

  • showFruitVariant (injFruit Apple)
    • At step 3 below, only
  • showFruitVariant (injFruit2 Orange)
    • This function still works despite showFruitVariant never knowing anything about a Fruit2 type!
  • showFruit2Variant (injFruit3 Cherry)
    • This function still works despite showFruit2Variant never knowing anything about a Fruit3 type!
  • showFruit3Variant (injFruit2 Orange)
    • This function still works despite showFruit3Variant never knowing anything about a Fruit2 type!

For each of the above run commands, do the following to "simulate" one file existing before another one:

  1. Start a REPL
  2. Copy and paste each "file" below into the REPL.
  3. Run the commands at the end of this snippet
-- File1's original code: once compiled, it cannot change
import Data.Maybe
import Data.Variant
import Data.Function
import Data.Symbol

data Fruit
  = Apple
  | Banana

showFruit :: Fruit -> String
showFruit Apple = "apple"
showFruit Banana = "banana"

showFruitVariant :: forall v. Variant (fruit :: Fruit | v) -> Maybe String
showFruitVariant = default Nothing
  # onMatch
    { fruit: \appleBanana -> Just (showFruit appleBanana)

injFruit :: forall v. Fruit -> Variant (fruit :: Fruit | v)
injFruit fruit = inj (Proxy :: Proxy "fruit") fruit

-- File 2. This should work without the previous file being recompiled
data Fruit2 = Orange

showFruit2 :: Fruit2 -> String
showFruit2 Orange = "orange"

showFruit2Variant :: forall v
                     . Variant (fruit2 :: Fruit2 | v)
                    -> Maybe String
showFruit2Variant = default Nothing
  # onMatch
    { fruit2: \orange -> Just (showFruit2 orange)

injFruit2 :: forall v. Fruit2 -> Variant (fruit2 :: Fruit2 | v)
injFruit2 fruit2 = inj (Proxy :: Proxy "fruit2") fruit2

-- File 3. This should work without the previous 2 files being recompiled
data Fruit3 = Cherry

showFruit3 :: Fruit3 -> String
showFruit3 Cherry = "cherry"

showFruit3Variant :: forall v
                     . Variant (fruit3 :: Fruit3 | v)
                    -> Maybe String
showFruit3Variant = default Nothing
  # onMatch
    { fruit3: \cherry -> Just (showFruit3 cherry)

injFruit3 :: forall v
          -> Fruit3
           . Variant (fruit3 :: Fruit3 | v)
injFruit3 fruit3 = inj (Proxy :: Proxy "fruit3") fruit3

Now run the following commands in the REPL: