"def" without "do"

In the source code I find this pattern repeated over and over.
As an example, in the elixir-1.5.3/lib/elixir/lib/enum.ex I find:

  @spec reduce(t, acc, reducer) :: result
  def reduce(enumerable, acc, fun)

The “def reduce…” has no “do” block. Almost all specs in this
file follow this pattern. Some do not.

Is this a “template” of some sort?

I’ve looked but can’t find an explanation.

What is this pattern called?

What changes when a spec has this and some specs do not?

Is this “def” without a “do” related to a “spec” at all? If not,
then what does this syntax provide?

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It’s called a body less clause, and in this case used only to name params for the docs, as they are used only as patterns in the following clauses and the names given by the algorithm were not very good.

This kind of vodyless clause is optional.

If though you do use default arguments in functions that have multiple clauses a bodyless clause specifying the default arguments is mandatory.

8 Likes

Thanks! Now that I know the name, “bodyless clause”, I
found something about this bugger.

None of my Elixir books or searches of “def without do”
or similar guesses offered any illumination.

Thanks again! :slight_smile: :slight_smile:

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It also helps the compiler when you have multiple definitions with default values. For example

    def foo(fn,acc)

    def foo(:ok, acc \\ []) do
         :ok 
    end

    def foo(:error, acc \\ nil) do
        :error
    end

Try it without the bodyless clause and it should display a warning. (It should, but I’m on mobile so I can’t be sure).

I don’t know exactly why, I just discovered it a few days ago.

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You can only have one default clause per function, so if you have many heads then you put it on the bodyless clause.

4 Likes

Most interesting OvermindDL1.

I get warning messages that finally error out.

Interesting to try various compiler suggested mods.

I don’t thoroughly understand it either, but thanks for the tip. :slight_smile:

1 Like