specify location of ‘empty’ method

[davidchambers]

Commit message:

An instance method is more flexible than a static method. It facilitates

> Identity([1, 2, 3]).empty()
Identity([])

since Identity#empty can dispatch to the inner value's empty method.

[scott-christopher]

I find empty most useful when an instance is not available, so I would still want a way to obtain an empty value statically.

e.g. The typical implementation of Const.of requires an empty value of the inner type, which makes it difficult to wire up if an instance is required.

Const.of = (_) => Const(OtherType.empty())

I'd probably go a step further and prefer the empty instance to be a plain property value rather than a nullary method. A property descriptor getter function could still be used if laziness is desired.

I'm not necessarily a fan of requiring it live on the constructor property though either. Perhaps that's an indication that the static-land approach might be more suitable for things such as this.

[CrossEye]

I find empty most useful when an instance is not available, so I would still want a way to obtain an empty value statically.

That's not where I started, but it's where I ended up in the long discussion in #88. I'm afraid this is one of those square peg/round hole situations that are bound to come up when you try to merge static and dynamic typing. I don't know that there is a good solution.

[davidchambers]

I'd probably go a step further and prefer the empty instance to be a plain property value rather than a nullary method.

This was proposed in #82. This would mean that the Identity type could not satisfy Monoid, I believe. Does this bother you, @scott-christopher?

The typical implementation of Const.of requires an empty value of the inner type, which makes it difficult to wire up if an instance is required.

Const.of = (_) => Const(OtherType.empty())

Hmm, I see the problem.

It seems we have four options:

1. Require an instance method only, making the Const type sad. 😢
2. Require a static method only, making the Identity type sad. 😢
3. Continue to support both, but state that the instance method should be used if both are present.
4. Do nothing.

Does anyone object to the third option? It's an improvement on the status quo, and shouldn't make anyone sad.

[SimonRichardson]

3 sounds reasonable

On Tue, 13 Sep 2016, 13:05 David Chambers, notifications@github.com wrote:

I'd probably go a step further and prefer the empty instance to be a
plain property value rather than a nullary method.

This was proposed in #82
#82. This would mean
that the Identity type could not satisfy Monoid, I believe. Does this
bother you, @scott-christopher https://github.com/scott-christopher?

The typical implementation of Const.of requires an empty value of the
inner type, which makes it difficult to wire up if an instance is required.

Const.of = (_) => Const(OtherType.empty())

Hmm, I see the problem.

It seems we have four options:

1. Require an instance method only, making the Const type sad. 😢
2. Require a static method only, making the Identity type sad. 😢
3. Continue to support both, but state that the instance method should
   be used if both are present.
4. Do nothing.

Does anyone object to the third option? It's an improvement on the status
quo, and shouldn't make anyone sad.

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[davidchambers]

I'll open a new pull request shortly.