Thursday, April 16, 2015

Elixir functions are not Functions

I was banging my head against a simple elixir test this morning. It's a test that's been in my code for months and that never worked for some reason. This is a boiled down example:


defmodule RaiseTest do
        def testraise do
                raise "This is an error"
        end
end

defmodule RaiseTestTest do
  use ExUnit.Case
  test "assert_raise works" do
    assert_raise(RuntimeError, RaiseTest.testraise )
  end
end

mix test

  1) test assert_raise works (RaiseTestTest)
     test/raise_test_test.exs:4
     ** (RuntimeError) This is an error
     stacktrace:
       (raise_test) lib/raise_test.ex:4: RaiseTest.testraise/0
       test/raise_test_test.exs:5


Finished in 0.03 seconds (0.03s on load, 0.00s on tests)
1 tests, 1 failures
My error was finally pointed out to me this morning on the elixir list and opened my eyes to a consistent flaw in my thinking about elixir. The fix of course is to actually provide a function to assert_raise, what I was actually providing was an expression that could return anything. 

 test "assert_raise works" do
    assert_raise(RuntimeError, fn -> RaiseTest.testraise end ) 
end

The key misunderstanding in my head was that function when used in the elixir context actually maps to what I think of as a function reference from my years of C programming. Module functions return expressions which can be anything. The test failed because it was calling TestRaisetest to see if it returned a function it could use in the test. 
When an elixir function requires a "function" as an argument, it really means a closure that can be executed.
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