Log-based expectation matcher for DoubleDown dispatch logs.

Declares expectations against the dispatch log after execution. Matches on the full {contract, operation, args, result} tuple — including results, which is meaningful because DoubleDown handlers (especially Repo.Stub) do real computation (changeset validation, PK autogeneration, timestamps).

Basic usage

DoubleDown.Log.match(:insert, fn
  {_, _, [%Changeset{data: %Thing{}}], {:ok, %Thing{}}} -> true
end)
|> DoubleDown.Log.reject(:delete)
|> DoubleDown.Log.verify!(MyContract)

Matcher functions only need the positive matching clauses — FunctionClauseError is caught and interpreted as "didn't match". No need for a _ -> false catch-all, though returning false explicitly can be useful for excluding specific values that are hard to exclude with pattern matching alone.

Matching on results

Unlike Mox/Mimic where asserting on return values would be circular (you wrote the stub), DoubleDown handlers do real computation. Matching on results is a meaningful assertion:

DoubleDown.Log.match(:insert, fn
  {_, _, [%Changeset{data: %Thing{}}],
   {:ok, %Thing{id: id}}} when is_binary(id) -> true
end)
|> DoubleDown.Log.verify!(RepoContract)

Counting occurrences

DoubleDown.Log.match(:insert, fn
  {_, _, [%Changeset{data: %Discrepancy{}}], {:ok, _}} -> true
end, times: 3)
|> DoubleDown.Log.verify!(DoubleDown.Repo)

Multi-contract

Build separate matcher chains and verify each against its contract:

todos_log =
  DoubleDown.Log.match(:create_todo, fn {_, _, _, {:ok, _}} -> true end)

repo_log =
  DoubleDown.Log.match(:insert, fn {_, _, _, {:ok, _}} -> true end)

DoubleDown.Log.verify!(todos_log, MyApp.Todos)
DoubleDown.Log.verify!(repo_log, DoubleDown.Repo)

Matching modes

Loose (default)

Matchers must be satisfied in order within each operation, but other log entries are allowed between them. Different operations are matched independently (no cross-operation ordering):

DoubleDown.Log.match(:insert, matcher)
|> DoubleDown.Log.match(:update, matcher)
|> DoubleDown.Log.verify!(MyContract)
# Passes if log contains an insert and an update,
# regardless of other entries or relative order.

Strict

Every log entry for the contract must be matched. No unmatched entries allowed:

DoubleDown.Log.match(:insert, matcher)
|> DoubleDown.Log.match(:update, matcher)
|> DoubleDown.Log.verify!(MyContract, strict: true)

Relationship to existing APIs

Built on DoubleDown.Testing.get_log/1. Completely decoupled from handler choice — works with Repo.Stub, Repo.OpenInMemory, set_fn_handler, set_stateful_handler, or DoubleDown.Double.

Can be used alongside DoubleDown.Double — Handler for fail-fast validation and producing return values, Log for after-the-fact result inspection.