Auke Booij b535257251 Avoid Arrows by interpreting monads ... TL;DR --- We go from this: ```haskell (| withRecordInconsistency ( (| modifyErrA ( do (info, dependencies) <- liftEitherA -< buildRelInfo relDef recordDependencies -< (metadataObject, schemaObject, dependencies) returnA -< info ) |) (addTableContext @b table . addRelationshipContext) ) |) metadataObject ``` to this: ```haskell withRecordInconsistencyM metadataObject $ do modifyErr (addTableContext @b table . addRelationshipContext) $ do (info, dependencies) <- liftEither $ buildRelInfo relDef recordDependenciesM metadataObject schemaObject dependencies return info ``` Background --- We use Haskell's `Arrows` language extension to gain some syntactic sugar when working with `Arrow`s. `Arrow`s are a programming abstraction comparable to `Monad`s. Unfortunately the syntactic sugar provided by this language extension is not very sweet. This PR shows how we can sometimes avoid using `Arrow`s altogether, without loss of functionality or correctness. It is a demo of a technique that can be used to cut down the amount of `Arrows`-based code in our codebase by about half. Approach --- Although _in general_ not every `Monad` is an `Arrow`, specific `Arrow` instantiations are exactly as powerful as their `Monad` equivalents. Otherwise they wouldn't be very equivalent, would they? Just like `liftEither` interprets the `Either e` monad into an arbitrary monad implementing `MonadError e`, we add `interpA` which interprets certain concrete monads such as `Writer w` into specific arrows, e.g. ones satisfying `ArrowWriter w`. This means that the part of the code that only uses such interpretable effects can be written _monadically_, and then used in _arrow_ constructions down the line. This approach cannot be used for arrow effects which do not have a monadic equivalent. In our codebase, the only instance of this is `ArrowCache m`, implemented by the `Rule m` arrow. So code written with `ArrowCache m` in the context cannot be rewritten monadically using this technique. See also --- - #1827 - #2210 PR-URL: https://github.com/hasura/graphql-engine-mono/pull/3543 Co-authored-by: jkachmar <8461423+jkachmar@users.noreply.github.com> GitOrigin-RevId: eb79619c95f7a571bce99bc144ce42ee65d08505		2022-02-22 18:09:50 +00:00
..
Extended.hs	server, pro: actually reformat the code-base using ormolu	2021-09-23 22:57:37 +00:00
Interpret.hs	Avoid Arrows by interpreting monads	2022-02-22 18:09:50 +00:00
Trans.hs	server, pro: actually reformat the code-base using ormolu	2021-09-23 22:57:37 +00:00