2018-01-17 12:25:41 +00:00
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This document briefly explains what you are seeing in those mysterious sections
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labelled "Only Before" and "Only After". Enlightenment is nigh.
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2018-01-18 01:23:12 +00:00
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`re-frame-trace` allows you to inspect important values like `app-db`.
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While knowing current values is useful,
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you are often more interested to know how a value has changed.
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The value might start off as, say, `X` (before the event happened) but
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2018-01-18 00:50:18 +00:00
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ended up as `X'`. How are `X` and `X'` different, you wonder?
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2018-01-17 12:25:41 +00:00
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What got added or removed? What was modified?
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2018-01-18 01:23:12 +00:00
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So, how then to display changes in a way that's easy to understand? I'm glad you asked.
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2018-01-18 00:50:18 +00:00
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`re-frame-trace` chooses to do a calculation best shown by this pseudo code:
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```clj
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(let [[only-before only-after _] (clojure.data/diff X X')]
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...)
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```
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2018-01-18 01:23:12 +00:00
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Remember X is the value immediately `before` (this epoch). And `X'` is the value `after` (the epoch has completed).
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2018-01-17 12:25:41 +00:00
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By [looking at the docs](https://clojuredocs.org/clojure.data/diff) on `diff`, you'll see
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2018-01-18 00:50:18 +00:00
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that it calculates how two values differ, and returns a triple of values. `re-frame-trace`
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captures and displays the first two elements of this triple as "only before" and "only after"
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2018-01-18 01:23:12 +00:00
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respectively. The 3rd element is ignored because it is not very interesting - it says
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what hasn't changed, so it isn't shown.
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2018-01-17 12:25:41 +00:00
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To correctly interpret "Only Before" and "Only after", you'll need to spend a bit
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of time properly familiarising yourself with how `clojure.data/diff` works, but
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2018-01-18 01:23:12 +00:00
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it will be a worthwhile time investment.
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