HDM: Definition of default semantics for size_function

[jeromekelleher]

The parser should output fully a resolved model, with non-null values for every field that effects the model. Thus, we need to specify the semantics of what the default values should be. We could do something like:

The value of an Epoch's size_function should be resolved after the epoch's start_size and end_size values have been resolved. If the start_size and end_size are equal, the size function should be resolved to "constant", or otherwise to exponential.

However, I think it's simpler if we just default to exponential, and leave it up to the implementation to realise that the growth rate is 0. They will need to do this anyway, unless we make it part of the spec that parsers should raise an error if the size function is exponential and the start and end sizes are the same. This seems like it would end up being fragile and annoying to me, though.

[jeromekelleher]

Actually, come to think of it, what's the point of the "constant" size function then? It doesn't really serve any purpose, since you can always specify that the start and end sizes are the same.

This would simplify our lives a bit, since we could say that the only currently supported value is exponential and anything else is an error. We also then don't have to check if the size_function is constant but with different start and end sizes.

[grahamgower]

I think there was some discussion about "linear" being a reasonable size_function. It seems strange to me that we would have this keyword, yet allow only "exponential". Having the keyword at all indicates we think it will be useful, but then allowing only one value prevents the keyword from being used.

[grahamgower]

We decided on the behaviour here, it just needs to be described in the HDM.

demes-spec/reference_implementation/parser.py

Lines 212 to 216 in 94da0a3

	if self.size_function is None:
	if self.start_size == self.end_size:
	self.size_function = "constant"
	else:
	self.size_function = "exponential"

And the demes-python implementation has a precise description of how "exponential", "constant" and "linear" are interpreted:

https://github.com/popsim-consortium/demes-python/blob/1ddd4048a78b0ef628e35d45bdbac35c7379edf8/demes/demes.py#L166-L189