Soundcheck.Check.Warning.PossiblyUnsetAssign (soundcheck v0.1.0)

Basics

This check has a base priority of high and works with any version of Elixir.

Explanation

Flags template reads of assigns that mount/3 does not set on every path before the first render. LiveView raises KeyError on the first render that reaches such a read.

Before:

def mount(_params, _session, socket), do: {:ok, socket}

def handle_event("refresh", _params, socket),
  do: {:noreply, assign(socket, :stats, Stats.load())}

def render(assigns), do: ~H"<p>{@stats.total}</p>"

After:

def mount(_params, _session, socket), do: {:ok, assign(socket, :stats, nil)}

def render(assigns), do: ~H"<p :if={@stats}>{@stats.total}</p>"

The check follows mount/3 in both the disconnected and the connected context. A key counts as set before the first render when mount/3 sets it on every path, when handle_params/3 or an on_mount hook may set it, or when it is listed in assume_set. Only reads that run on every render count: reads inside :if elements, EEx if/case/for blocks, and slots are skipped. Keys set only when connected are left to AssignSetOnlyWhenConnected (SC202).

When mount/3 passes the socket to a function soundcheck does not model, the check makes no claim. When handle_params/3 does, or an on_mount hook cannot be read, findings are unverifiable.

Confidence: medium. Tier: semantic (run mix compile before mix credo).

Check-Specific Parameters

Use the following parameters to configure this check:

:excluded_paths

Paths to skip (atoms, binaries as path segments, or regexes).

This parameter defaults to [~r/\/test\//, "test"].

:assume_set

Keys the team's conventions guarantee before the first render.

This parameter defaults to [:current_user, :current_scope, :flash, :live_action, :page_title].

:strict

Report unverifiable findings too.

This parameter defaults to false.

General Parameters

Like with all checks, general params can be applied.

Parameters can be configured via the .credo.exs config file.