Auke Booij 1007ea27ae server: refactor MonadSchema into MonadMemoize ... Followup to hasura/graphql-engine-mono#4713. The `memoizeOn` method, part of `MonadSchema`, originally had the following type: ```haskell memoizeOn :: (HasCallStack, Ord a, Typeable a, Typeable b, Typeable k) => TH.Name -> a -> m (Parser k n b) -> m (Parser k n b) ``` The reason for operating on `Parser`s specifically was that the `MonadSchema` effect would additionally initialize certain `Unique` values, which appear (nested in) the type of `Parser`. hasura/graphql-engine-mono#518 changed the type of `memoizeOn`, to additionally allow memoizing `FieldParser`s. These also contained a `Unique` value, which was similarly initialized by the `MonadSchema` effect. The new type of `memoizeOn` was as follows: ```haskell memoizeOn :: forall p d a b . (HasCallStack, HasDefinition (p n b) d, Ord a, Typeable p, Typeable a, Typeable b) => TH.Name -> a -> m (p n b) -> m (p n b) ``` Note the type `p n b` of the value being memoized: by choosing `p` to be either `Parser k` or `FieldParser`, both can be memoized. Also note the new `HasDefinition (p n b) d` constraint, which provided a `Lens` for accessing the `Unique` value to be initialized. A quick simplification is that the `HasCallStack` constraint has never been used by any code. This was realized in hasura/graphql-engine-mono#4713, by removing that constraint. hasura/graphql-engine-mono#2980 removed the `Unique` value from our GraphQL-related types entirely, as their original purpose was never truly realized. One part of removing `Unique` consisted of dropping the `HasDefinition (p n b) d` constraint from `memoizeOn`. What I didn't realize at the time was that this meant that the type of `memoizeOn` could be generalized and simplified much further. This PR finally implements that generalization. The new type is as follows: ```haskell memoizeOn :: forall a p. (Ord a, Typeable a, Typeable p) => TH.Name -> a -> m p -> m p ``` This change has a couple of consequences. 1. While constructing the schema, we often output `Maybe (Parser ...)`, to model that the existence of certain pieces of GraphQL schema sometimes depends on the permissions that a certain role has. The previous versions of `memoizeOn` were not able to handle this, as the only thing they could memoize was fully-defined (if not yet fully-evaluated) `(Field)Parser`s. This much more general API _would_ allow memoizing `Maybe (Parser ...)`s. However, we probably have to be continue being cautious with this: if we blindly memoize all `Maybe (Parser ...)`s, the resulting code may never be able to decide whether the value is `Just` or `Nothing` - i.e. it never commits to the existence-or-not of a GraphQL schema fragment. This would manifest as a non-well-founded knot tying, and this would get reported as an error by the implementation of `memoizeOn`. tl;dr: This generalization _technically_ allows for memoizing `Maybe` values, but we probably still want to avoid doing so. For this reason, the PR adds a specialized version of `memoizeOn` to `Hasura.GraphQL.Schema.Parser`. 2. There is no longer any need to connect the `MonadSchema` knot-tying effect with the `MonadParse` effect. In fact, after this PR, the `memoizeOn` method is completely GraphQL-agnostic, and so we implement hasura/graphql-engine-mono#4726, separating `memoizeOn` from `MonadParse` entirely - `memoizeOn` can be defined and implemented as a general Haskell typeclass method. Since `MonadSchema` has been made into a single-type-parameter type class, it has been renamed to something more general, namely `MonadMemoize`. Its only task is to memoize arbitrary `Typeable p` objects under a combined key consisting of a `TH.Name` and a `Typeable a`. Also for this reason, the new `MonadMemoize` has been moved to the more general `Control.Monad.Memoize`. 3. After this change, it's somewhat clearer what `memoizeOn` does: it memoizes an arbitrary value of a `Typeable` type. The only thing that needs to be understood in its implementation is how the manual blackholing works. There is no more semantic interaction with _any_ GraphQL code. PR-URL: https://github.com/hasura/graphql-engine-mono/pull/4725 Co-authored-by: Daniel Harvey <4729125+danieljharvey@users.noreply.github.com> GitOrigin-RevId: 089fa2e82c2ce29da76850e994eabb1e261f9c92		2022-08-04 13:45:53 +00:00
..
Aeson	server: Data Connectors aggregation agent API contract [GDW-95]	2022-07-20 05:21:54 +00:00
Parser	server: refactor MonadSchema into MonadMemoize	2022-08-04 13:45:53 +00:00
QuickCheck	server: Splits QuickCheck extension and orphan instance modules	2022-02-22 15:33:37 +00:00