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26 lines
1.4 KiB
Markdown
26 lines
1.4 KiB
Markdown
This document explains what you are seeing in sections
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labelled "Only Before" and "Only After".
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In various places, `re-frame-trace` allows you to inspect values like `app-db`.
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Knowing the current value is useful, but
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you are sometimes more interested to know how a value has changed.
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The value might be `X` before the start of this epoch, and
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ended up as `X'`. So `re-frame-trace` will be showing you `X'` but you might woonder
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how `X` and `X'` are different. What got added or removed, and what was modified?
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To show such differences, `re-frame-trace` chooses to do a calculation best explained 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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Remember `X` is the value immediately `before` (this epoch). And `X'` is the value `after` (the epoch has completed).
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By [looking at the docs](https://clojuredocs.org/clojure.data/diff) for `clojure.data/diff`, you'll see
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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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respectively. The 3rd element is ignored because it's what hasn't changed, which isn't interesting.
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So, 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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it will be a worthwhile investment of your time.
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