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Handling optional record fields with undefined | a values and typesafe zero cost coercion.


The main idea behind this lib was taken from purescript-untagged-union library by @jvliwanag so all the credits should go to @jvliwanag. untagged-union provides a really interesting implementation of untagged unions for PureScript especially useful in the context of FFI bindings, so please check it out.

I've narrowed this idea down to handle only unions with undefined type. I really focus on optional record fields here.


You can encounter slow compilation time if you are going to process really large record types with the lib. I mean records with more than 80 or 100 fields can have an sigificant impact of module build time. Here you can find a related PS discourse thread for reference.


There are two coercing strategies provided by this lib. Don't worry they are both easy to use and the distinction between them is quite simple. I'm going to discuss this difference along the way.

Let me start with imports. This is a literate PureScript example (run as a part of the test suite) so we need them.

module Test.README where

import Prelude

import Data.Undefined.NoProblem (opt, Opt, undefined, (?), (!))
import Data.Undefined.NoProblem.Closed (coerce) as Closed
import Data.Undefined.NoProblem.Open (class Coerce, coerce) as Open
import Effect (Effect)
import Effect.Random (random)
import Test.Assert (assert)

An API author specifies a Record type with all the fields which are optional (wrapped in Opt) so the user can skip these record properties when calling a function.

type SimpleOptions =
  { a  String
  , b  Opt Number
  , c  Opt
    { d 
      { e  Opt
        { f  Opt String
        , g  Opt Number
        , h  String

To work with optional values we have some handy operators at our disposal:

  • a value accessor ! ∷ Opt a → a → a which expects a default value

  • a "pseudo bind": ? ∷ Opt a → (a → Opt b) → Opt b opertor which allows us to dive for example into optional record values.


Let me start with Open.coerce function. We are going to build a function which internally works with the SimpleOptions record value defined above. Both coerce functions (Open.coerce and Closed.coerce) are able to "fill" missing fields in a given record (recursively) with Opt a if that is a part of the initial type and transform proper values to Opt ones if it is needed. This is a purely typelevel transformation.

-- | This signature is optional
consumer   r. Open.Coerce r SimpleOptions  r  Number
consumer r =
    -- | We should provide an info to which type we try to coerce
    opts = Open.coerce r ∷ SimpleOptions

    -- | We can access and traverse optional values using "pseudoBind" function.
    -- | Side note: we can also close such a chain with `# toMaybe` easily.
    g = opts.c ? _.d.e ? _.g ! 0.0
    opts.b ! 0.0 + g

The Coerce constraint checks if we can use coerce safely.

Calling our consumer

Now we are ready to use our function. As you can see our argument value lacks multiple fields and uses values directly in the places where Opt are really expected in the SimpleOptions type (like c should be Opt {... } and g should have type Opt Number):

recordCoerce  Effect Unit
recordCoerce = do
    argument =
       { a: "test"
       , c:
         { d:
           { e: { g: 8.0, h: "test" }}

    result = consumer argument
  assert (result == 8.0)

Optionality is just a value

It is worth nothing that optional field value is just a value. Its type is extended with additional resident - undefined. There are two constructor provided for Opt: opt ∷ ∀ a. a → Opt a and undefined ∷ ∀ a. Opt a.

You can accept or build and assemble these values on the way and pass them down to the consumers below.

optValues :: Effect Unit
optValues = do
  -- | Under some circumstances we want
  -- | to setup part of the record
  setup ← (_ < 0.5) <$> random

    { b, g } = if setup
      -- | Could be also done with `coerce`.
      then { b: opt 20.0, g: opt 25.0 }
      -- | Could be also `coerce { }`.
      else { b: undefined, g: undefined }

    $ (consumer { a: "test", b, c: { d: { e: { g, h: "test" }}}})
    == (if setup then 45.0 else 0.0)

NoProblem.Open.* approach


There is an inherent problem with coercing polymorphic types in this case. Internally I'm just not able to match a polymorphic type like a with expected type like Int because I don't want to close the instance chains and commit to a given type (using something like TypeEquals) in this case.

In other words when you use Open.coerce and Open.Coerce then whenever the user provides values like Nothing or [] as a part of the argument value these pieces should be annotated.

type OptionsWithPolymorphicValue = { x :: Opt (Array Int) }

openCoerceNonPolymorphicArray  Effect Unit
openCoerceNonPolymorphicArray = do
    -- | This `Array Int` signature is required
    argument = { x: [] :: Array Int }

    v = Open.coerce argument ∷ OptionsWithPolymorphicValue

  assert $ (v.x ! [1] == [])


You can always provide an Open.Coerce instance for your types and allow coercing of its "internals". Please check examples in the NoProble.Open module where you can find instances for Array, Maybe etc.

NoProblem.Closed.* approach

There is realy no difference in the API provided by this module so we have Coerce class and coerce function here. The only difference is that I'm closing the instance chain and trying to force unification in the last instance.


When you reach for this type of coercing you can expect a better behavior in the case of polymorphic values. The previous example works now without annotation for the array in x prop:

closedCoercePolymorphicArray  Effect Unit
closedCoercePolymorphicArray = do
    argument = { x: [] }

    r = (Closed.coerce argument :: OptionsWithPolymorphicValue)

  -- | We can retrive the empty array value which has now type `Array Int`
  assert (r.x ! [8] == [])


The downside of the Closed.Coerce class is that you are not able to provide more instances for it. Because we are closing here an instance chain with this unification case instance coerceUnify :: (TypeEquals a b) => Coerce a b there is no way for you to provide additional instances.



I try to provide some debug info which should help when there is a type mismatch. For example this kind of polymorphic array value in the z field causes problem:

type NestedError =
  { l :: Array { x :: Opt Int, y :: Int, z :: Opt (Array  Int) }}

x = coerce { l: [{ y: 9, z: [] }]} :: NestedError

and we can get quite informative compile time error message with property path like:

Type mismatch on the path: { l."Array".z."Array" }. Expecting


but got


If one of the types above is a type variable like `t2` or `t37`
it probably means that you should provide type annotation to some
parts of your value. Something like `[] ∷ Array Int` or `Nothing ∷ Maybe String`.

I'm trying to cover as many cases as I can but it is of course possible that you are going to get just generic complier error.


In the case of Closed constraint errors I think that I'm not able to properly format and render errors like I've done in the previous case. So I have included the type path of the properties in the typeclass parameters and it can be somewhat extracted from the generic error. The path is currently provided in the reverse order. In the below case we see that the unification problem is related to the property type on the path (SCons "Array" (SCons "x" SNil)) which translates into something like "x.Array.__".

  Could not match type


  with type


while solving type class constraint

  Data.Undefined.NoProblem.Closed.TypeEqualsOnPath String
                                                   (SCons "Array" (SCons "x" SNil))

while applying a function coerce