Added additional guards for Keywords.merge/3.

[janderland]

Added guards to ensure only lists can be passed to Keywords.merge/3.

I thought about adding this change after realizing the merge/3 method had inconsistent behavior...

Keyword.merge([a: 1, b: 2], nil)
# ** (FunctionClauseError)

Keyword.merge([], nil)
# nil

What do you think?

[yordis]

@janderland it seems that it should always expect two Keywordlist instead of simply return one or the other one.

This is an odd behavior from API perspective to be honest.

[eksperimental]

I agree with the changes, but if you want to be correct, the first two clauses should be removed all together, and let the third clause deal with it.

The current implementation and the proposed one, will not pass this test:

iex> Keyword.merge([:this, :is, :not, :a, :kw, :list], [])
[:this, :is, :not, :a, :kw, :list]

[michalmuskala]

Those two clauses are an important optimisation for when merging with an empty keyword. In general, the Keyword module does not guarantee in only accepts valid keywords - it might accept other things as well, but it must accept valid keywords. It checks when it is practical or does not impose performance issues, but not otherwise.

[josevalim]

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