# `gno.land/p/moul/reactions/v0`
The engine behind an embeddable reaction block: pages, tallies, a closed emoji
palette, and the markdown that shows them. No realm state, no chain reads, no
authority decisions. The realm on top owns all three.
Live realm: [`gno.land/r/moul/reactions/v0`](https://github.com/moul/gno-contracts/tree/main/r/moul/reactions).
## The model
A `Board` maps a **page key** to a `Page`. A page holds exactly three things:
| | |
|---|---|
| a count per palette key | what the strip shows |
| the **current** reaction of each address | so a tally counts addresses and not clicks, and a reader can change their mind or take it back |
| the last reaction | which key, from whom, at what height |
There is no text field anywhere. That is what makes the whole thing runnable
with no moderation: reactions come from a closed set of six, so there is nothing
to take down.
```go
b := reactions.NewBoard()
b.React("gno.land/r/moul/home", caller, reactions.KeyFire, height) // false if rejected
b.Unreact("gno.land/r/moul/home", caller) // false if there was none
p := b.Page("gno.land/r/moul/home") // nil when untouched, and nil is safe to use
p.Count(reactions.KeyFire) // 1
p.Total() // addresses reacting, not clicks
p.Of(caller) // "fire", or "" for none
key, who, height := p.Last()
md := reactions.Block(realmPath, page, p) // the whole widget
```
Every `Page` read is nil-safe and answers as if the page were empty, so a host
realm renders the block without ever checking whether the page exists.
## Three decisions worth knowing before you use it
**A reaction key is an ASCII name, not a glyph.** `"up"`, not `👍`. A name
survives a URL query parameter, an avl key and a markdown escaper with no
encoding question anywhere, and the palette can change a glyph without
rewriting stored state. `Emoji(key)` is the presentation half.
**No glyph in the palette uses U+FE0F or a zero-width joiner.** The link title
goes through `ui.Inline`, which strips exactly those, so a glyph needing a
variation selector would come out the other side as its monochrome form. ❤️ is
the casualty; `heart` is 💜, a codepoint that is colour on its own. Pinned by
`TestEmojiSurviveTheEscaper`.
**`Unreact` does not rewind the last-reaction line.** It records an event that
happened, and recomputing it would mean walking every address on the page, so
`Page.Last` keeps naming a reaction nobody holds any more.
What is *shown* is a separate decision, and `Summary` makes it: a page whose
reactors have all left renders as empty, not as a withdrawn reaction. Gating on
the last key alone would print "last 👍 by `g1…` · 0 reactions" above an empty
strip, which contradicts itself. `TestSummaryGoesEmptyWhenEveryoneLeaves` pins
the pair.
## Page keys
A page key is a package path, and `ValidPage` is what bounds this package's
exposure: at least one `/`, no empty element, and only lowercase ASCII letters,
digits, `.`, `-`, `_`, `:` and `/`, up to `MaxPageLen` bytes. That excludes every
character a markdown escaper exists for, plus every byte above ASCII, so a
stored key can never be a bidi run, a zero-width joiner or an image.
`:` is in the set so a host realm can key one block per article on the path it
already serves: `gno.land/r/you/blog:hello-world`.
This package only ever puts a key in a URL. A realm that shows the key **as
text** escapes it there; `ValidPage` and `ui.Inline` protect against different
mistakes.
## Not to be confused with
[`r/moul/x/daily/reactions`](https://github.com/moul/gno-contracts/tree/main/r/moul/x/daily/reactions)
is an older, unrelated experiment: a single self-contained board with its own
"topics", no embedding, and no way for a reader to change or remove a reaction.
Nothing is shared between the two.
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---
Part of **[moul/gno-contracts](https://github.com/moul/gno-contracts)** — moul's versioned gno.land contracts. See the repository for the full catalog, build/test tooling, and usage.
**Dependency graph:**
> ⚠️ **Disclaimer:** provided as-is, without warranty; not security-audited. Full disclaimer: [DISCLAIMER](https://github.com/moul/gno-contracts/blob/main/DISCLAIMER.md).
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Signatures reconstructed verbatim from vm/qfuncs — interface params keep their inline definitions.