[Haskell-cafe] How to handle recoverable errors in multi-step computation?

[Lian Hung Hon]

Dear haskellers,

I have a multi-step computation. The steps are predefined, but users can
choose which steps to execute (something like a recipe). Here is an example
recipe:

Step 1 - Get data from network (multiple HTTP calls) and put into a list
Step 2 - Process the data (e.g. average, sum, median, etc.)
Step 3 - Persist result to database

Sometimes, Step 1 can fail for some of the HTTP calls. When this happens,
Step 2 should continue as much as possible using whatever data that has
been retrieved, but somehow indicate that an error has occurred and the
result is partial.

Q1: What is the idiomatic way of achieving this? Using throwError in
Control.Monad.Except aborts the computation, which isn't what I want.

Q2: (General software design) Furthermore, where should the error be
logged? Logging it in both Step 1 and 2 preserves modularity for each of
the steps, unfortunately it would result in duplicate error messages. What
is the best practice for this?

Regards,
Hon
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