Elixir New Added 10 September 2026

Elixir: (FunctionClauseError) no function clause matching

Every clause of the function was tried and none matched the arguments, either on the pattern or on a guard. Elixir prints the attempted clauses and the arguments it was given, so the useful comparison is between the two: a nil where a map was expected, a string key where an atom was expected, or a guard that quietly excluded the value.

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

# The report lists the clauses that were tried and the arguments given
mix run -e 'MyApp.Orders.total(nil)'

# Look at the value before it reaches the function
value |> IO.inspect(label: "into total/1") |> MyApp.Orders.total()

# String keys and atom keys do not match each other
def total(%{"items" => items}), do: sum(items)   # from JSON
def total(%{items: items}), do: sum(items)       # from a struct

# Guards silently exclude: is_integer/1 rejects a float, and a binary is not a charlist
def retries(n) when is_integer(n) and n >= 0, do: n

# Add a last clause while debugging, then remove it
def total(other), do: raise(ArgumentError, "unexpected: " <> inspect(other))

# Dialyzer finds many of these before runtime
mix dialyzer

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

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