Collecting successes and errors

traverse (“map with effects”)

Often we want to iterate through a collection of items, performing some effect for each item. This means we want some function that looks like

(a -> f b) -> t a -> result

where a -> f b is our effectful computation, t a is our collection (typically a list or an array of as) and result could take a few different shapes depending on the requirements of our program, especially in the common case when the effect f encapsulates some notion of failure (like TaskEither in fp-ts, or anything with ExceptT in its stack in Haskell).

A decision tree

• Is there a notion of “partial” success (succeed with a warning)?
  • Yes: Use traverse with These.
  • No: Continue.
• If everything succeeds, do you want to collect all successes?
  • Yes:
    • …and in case of failure I want to report just the first failure.

      Use traverse with Either/ExceptT.

    • …and in case of failure I want to report all failures.

      Use traverse with Validation.

  • No: Continue.
• Do you want just the first success (if anything succeeds)?
  • Yes:
    • …and in case of failure I want to report just the last failure.

      Use altAll/asum/ choice/oneOf with Either/ExceptT.

    • …and in case of failure I want to report all failures.

      Use altAll/asum/ choice/oneOf with Validation.

  • No: Continue.
• You want to just collect all successes and all failures?

  Use wilt (or partitionMap if the only “effect” is error handling).

wilt is from the Witherable type class in PureScript and fp-ts, and appears as mapEitherA in Haskell. Conceptually, “partitionMap with effects.” It’s a rarer bird than traverse.