RFC: do not redefine def macro and use attribute tags

[x4lldux]

I've made a mistake making the original pull request (#1) from master branch. This is a fix for this so I can still work on this lib, adding new features.

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I've seen your talk yesterday - nice one! DbC is a great concept especial in Elixir/Erlang in creating fault tolerant systems.

This pull request is meant to be of a Request For Comment and initiating a discussion. My implementation, removes the need to redefine def macro (redefining core functions/macros is somewhat iffy and can have side effects, especial if another module does the same thing). Additionally, adding a contract to a functions feels more like it's a meta information about that function, so attributes are more appropriate way of assigning a contract to a function (tagging it with a contract).

By doing this that way, (in the future) we can extend this by adding another attribute for defining how to inform about a broken condition:

@requires contract tank.in_valve == :closed && tank.out_valve == :open
@ensures  contract empty?(result) && result.in_valve == :closed && result.out_valve == :closed
@on_break :error_tupel
def empty(tank) do

which will return a {:error, result}. Or maybe a information what part of contract was broken.

Unfortunately, since attributes require values, their arguments are evaluated first. This is why macro contract is needed, to return AST form of a conditions . It basically works as a quote, but this can change if we'll add more information when raising on broken conditions (maybe similar information to what is returned by ExCase.assert macro).

Pros:

• not redefining a core Elixir element - def macro
• tagging functions with contracts feels more correct than calling macros before a definition of a function
• introspection - Module.info :attributes returns contract_predicates attribute containing information about each tagged functions

Cons:

• breaking changes (use of attributes instead of macros)
• need of contract macro

Also, I think we should use singular form ensure and require.

Cheers!