implement dbg for if expressions

[dkuku]

Dbg currently is not showing any bindings when executed on if statement so it's very hard to guess what's going on:

if a do
  b
else
  c
end #=> 2

This pr extends dbg to support if statements:

 If condition:
 true and x #=> true
 
 If expression (do clause executed):
 if true and x do
     map[:a] * 2
 else
    map[:b]
 end #=> 10

I proposed this on elixir forum with other tweaks to dbg, but did not get any answers.

[sabiwara]

I think we might need to check the meta here - since if is not a special form unlike case & co, it can be redefined.

 import Kernel, except: [if: 2]
 import MyOtherIf

We'd need a test for this case too I think

[dkuku]

I did that but maybe there is a better way to implement it?

[whatyouhide]

@josevalim do we need to do this because if is not a special form (like case and cond)?

If we need to do this, let's do it like this

Suggested change

	if_from_kernel? =
	for {Kernel, macros} <- env.macros, {:if, 2} <- macros do
	true
	end == [true]
	if_from_kernel? = Enum.any?(env.macros, fn {mod, macros} -> mod == Kernel and {:if, 2} in macros end

[josevalim]

There is Macro.Env.lookup_import which we should use instead.

[whatyouhide]

Should we call the "do clause" the "truthy clause" instead?

[josevalim]

Honestly, I would just print the value of the condition and the value we return. It is easy to infer which clause was executed from that.

[dkuku]

I also think it's enough - I was trying to make it similar to other implementations.

[dkuku]

I also wonder if the condition should be split on operators - but this may require a bit of refactoring to this module

 if condition:
 true  #=> true
 true and x #=> true
 
 If expression:

[sabiwara]

I also wonder if the condition should be split on operators - but this may require a bit of refactoring to this module

I think it would make sense, as && and if are very close in nature. But this can perhaps be done as a follow-up pull-request?

[dkuku]

@sabiwara yes, this pr is ready.

[sabiwara]

LGTM! Thank you for the great proposal and work! 💜

[whatyouhide]

Great work @dkuku thank you! 💟

[sabiwara]

@dkuku we discussed and agreed to handle blocks (just plain :block tuples, not [do: block] ones).
If you are interested to work on this, feel free to open up a PR 🙂

[dkuku]

@sabiwara cool, I will do that