ybogomolov / circuit-breaker-monad Goto Github PK

Using fast-check, add tests for circuit breaker, ensuring 100% coverage.

I am receiving this error when adding
const fetcher = circuitBreaker<Response>().run(defaultBreakerOptions);

any idea?

I would like to refactor this module a bit with the following points in mind:

1. Currently, circuit breaker was just a Reader which returned a so-called "enhanced fetch" – which was, according to signature, very similar to the State monad. If we use type isomorphism from product (pair) to a curried function, we get exactly a Reader of State monad:

   // This type:
   export type EnhancedFetch<T> =
     (request: Lazy<Promise<T>>, ref: IORef<BreakerState>) => 
       [IORef<BreakerState>, TaskEither<Error, T>];
   
   // ...is isomorphic to:
   export type EnhancedFetch<T> =
     (request: Lazy<Promise<T>>) => (ref: IORef<BreakerState>) => 
       [TaskEither<Error, T>, IORef<BreakerState>];
   
   // ...which gives us a State monad:
   export type EnhancedFetch<T> =
     (request: Lazy<Promise<T>>) => State<IORef<BreakerState>, TaskEither<Error, T>>;
   
   // ...and being further reduced to a Reader:
   export type EnhancedFetch<T> =
     Reader<Lazy<Promise<T>>, State<IORef<BreakerState>, TaskEither<Error, T>>>;

   I've decided to move the request and breaker state parts to a new BreakerEnvironment type, so the final type of circuit breaker would be:

   export interface BreakerEnvironment<T> {
     request: Lazy<Promise<T>>;
     breakerState: IORef<BreakerState>;
   }
   
   export type CircuitBreaker<T> = 
     Reader<BreakerOptions, State<BreakerEnvironment<T>, TaskEither<Error, T>>>;

   Of course, I can just put BreakerOptions part inside the BreakerEnvironment<> type (or compose them), but I don't like this idea. I want to segregate the concerns: Reader provides a read-only environment, while State deals with mutable parts. I want to keep request as a Lazy<Promise<T>> due to Promises in JS being eager. If this project migrates to fluture one day, I will change it to just a single Future.

2. As fp-ts is actively moving towards 2.0.0 release (see gcanti/fp-ts#823), this project as a part of its ecosystem needs to keep up. So I think it would be better to sync this project's major version to fp-ts's (at least, for now). I will not abandon v1 and will provide fixes for issues (if any are found).

3. According to usage in my projects, it's better to expose an explicit singleton with clojure-bound IORef<BreakerState> variable, than to build such instance ad-hoc in each project. I would like to keep balance between purity and convenience of practical usage.

4. As such refactoring effectively changes the types and behaviour of circuit breaker, I will remove note about this project being a port of Glue.CircuitBreaker from Haskell. This bothered me a bit, and now I can put my heart at ease.

Please vote or leave a comment about these changes!