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ERRORS.md

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Errors in flyd

Ethos

  • It should not be the concern of flyd to handle exceptions for the user -- any throw should result in a hard failure.
  • Silent failures are bad (current way flyd handles Promise.reject)
  • Be as unopinionated in implementation as possible
  • Be functional in design
  • Be as backward compatible as possible with the current api

Concepts

  • The stream is of events
  • Each stream has a left and a right side (like an Either)
  • The right side is the domain objects
  • The left side is meta (in most cases errors)
  • By default the api operates on the right side

The Api

s is a stream

Setting data s(...) is overloaded

  • s(value) is the default case takes a value makes it a right and pushes it down the stream
  • s(promise) if the promise resolves pushes a right, otherwise pushes a left
  • s(either) pushes down right or left based on either.left either.right
  • s.left(value) sets the stream to a left of value

Getting data

  • s() get the last right value or throws an exception if there is a left value
  • s.left() get the last left value or throws an exception if there is a right value
  • s.either() get the last value out as an Either

Checking stream state

  • s.isLeft() return true if the stream contains a left value
  • s.isRight() return true if the stream contains a right value

Core functions

  • .map() works only on rights and ignores lefts
  • .mapEither() gets all events as an Either
  • .combine() and .merge() stay the same they work on streams
  • ap() works on rights only
  • .scan() works on rights only
  • .on() works on rights only

The Either implementation

There are no additional dependencies and we have provided a minimal implementation for basic use. If you plan on using .mapAll we recommend overriding the methods in flyd.Either. You can use folktale/data.either for example as shown below.

var DE = require('data.either');
flyd.Either.Right = DE.Right;
flyd.Either.Left = DE.Left;
flyd.Either.isEither = function(obj) { return obj instanceof DE; };
flyd.Either.isRight = function(e) { return e.isRight; };
flyd.Either.getRight = function(e) { return e.value; };
flyd.Either.getLeft = function(e) { return e.value; };

Other functionality

Keeping with the ethos of flyd any further functions like .swap or .onAll should be implemented as modules.