Elixir New Added 19 September 2026

Elixir: protocol String.Chars not implemented for %{} of type Map

String interpolation calls to_string/1, and only a few types implement it: maps, lists, tuples and structs do not. The failure is at runtime rather than compile time, so it usually appears in an error path or a log line that only runs when something else has already gone wrong.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

# Interpolation of a map raises, inspect renders anything
user = %{id: 1, name: "ada"}
"user: #{user}"          # ** (Protocol.UndefinedError)
"user: #{inspect(user)}" # user: %{id: 1, name: "ada"}

# Logger metadata and messages have the same rule
require Logger
Logger.info("payload: #{inspect(payload)}")

# nil is the other common one: it is an Atom and has no String.Chars
"hello #{user[:nickname] || "friend"}"

# For a struct you print often, implement the protocol once
defimpl String.Chars, for: Money do
  def to_string(%Money{cents: c, currency: cur}), do: "#{c / 100} #{cur}"
end

# Inspect is for humans and may truncate: pass limit: :infinity when it matters
inspect(payload, limit: :infinity, pretty: true)

How to diagnose Elixir errors

Elixir errors are shaped by the actor model: a GenServer.call timeout does not mean the server crashed, it means the server was busy for longer than the caller was willing to wait. The right question is usually "what is that process doing?" rather than "why did the call fail?". Because supervisors restart failed processes automatically, transient errors can also hide in the logs while the system appears healthy.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

Attach to a running node with iex --remsh and inspect the process: Process.info(pid, :message_queue_len). A growing mailbox means the process is the bottleneck.
Use :observer.start() to see the supervision tree, process memory and message queues visually.
Move long-running work out of handle_call. Use handle_cast, a Task, or a dedicated pool so the GenServer stays responsive.
Check restart intensity. A supervisor that exceeds max_restarts takes down its own supervisor, producing a cascade that looks like an unrelated failure at the top.
For compile errors in dependencies, run mix deps.compile --force and check that any required native toolchain (make, gcc, erlang headers) is installed.

Tools worth reaching for

:observer.start()
iex --remsh
Process.info/2
mix deps.tree
:recon

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

hexdocs.pm hexdocs.pm

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