A closure is a function value that retains access to variables from the lexical scope where it was created. Holding the function reference also keeps those captured variables reachable. In Elixir, closures appear naturally in lambdas passed to collection functions and concurrent processes. section 2.4.9 section 5.2.1
Capturing a value for concurrent work
Define a function that accepts a query description, then spawn an inner function that uses it: `async_query = fn query_def -> spawn(fn -> query_result = run_query.(query_def); IO.puts(query_result) end) end`. Calling `async_query.("query 1")` passes the query data into the new process through the closure. Running it over `1..5` starts five independent computations; results can appear in any order. section 5.2.1 section 5.2.1
Closures have two practical properties:
- They capture a specific memory location, so rebinding a variable later does not change what an existing closure observes. section 2.4.9
- They can shorten callbacks: `Enum.each([1, 2, 3], &IO.puts/1)` replaces an explicit lambda while preserving the function argument. section 2.4.9
- They do not provide shared mutable state between BEAM processes; captured data is deep copied when passed to another process. section 5.2.1
