Render parens on zero arity fun definitions

[lpil]

https://github.com/lexmag/elixir-style-guide#fun-parens

[rgreenjr]

I'll start work on this one.

[rgreenjr]

This looks like it will be fairly straightforward (famous last words).

However I've run into a problem. The semantic tests in Semantic.ExfmtProjectTest fail anytime parentheses are added to zero arity function definitions.

For example, changing the macro in Exfmt.Ast.Infix from:

defmacro infix_ops do
    @infix_ops
end

to:

defmacro infix_ops() do
    @infix_ops
end

produces this error:

** (Exfmt.SemanticsError) Error: The semantic meaning of the source code differs
     between the input and the formatted output! We are unable
     to continue as formatting may break your code.

I'll dig into this, but wanted to post this in case you knew of an easy fix.

[rgreenjr]

The AST produced by a zero arity function definition without parentheses:

quote do
  def one do
    1
  end
end

{:def, [context: Elixir, import: Kernel],
 [{:one, [context: Elixir], Elixir}, [do: 1]]}

differs more than I expected from the same function definition with parentheses:

quote do
  def one() do
    1
  end
end

{:def, [context: Elixir, import: Kernel],
 [{:one, [context: Elixir], []}, [do: 1]]}

The semantic check is getting tripped up by this difference.

[OvermindDL1]

'def' is just a macro invocation, and the change is from going from a 'binding' ast to a 'call' ast, which is definitely a big change. Internally the 'def' is a macro (as might other things be too) that can accept both (design fault in my opinion...).

[rgreenjr]

Thanks @OvermindDL1, that's very helpful.