Binding and constraining

[spitz-dan-l]

Great little project! I found this through your post to the python-ideas list.

This issue is about the ability to both constrain a matched value and bind it at the same time.

For instance, say I want to destructure a list in head/tail style. I want to bind the first element to the head attribute of my Binding, and the rest of the list to the tail attribute. Currently there doesn't seem to be a way to do the tail part.

from destructure import Binding, match

o = Binding()

schema = [o.head, ...]
data = [1, 2, 3, 4]
guard = lambda: len(data) > 0

match(schema, data, guard) # this will match what I want, but won't bind o.tail

Very similar scenario for other constraints. You can constrain a value with e.g. ellipses, a type, a substructure, but you can't also bind that value.

Seems like there ought to be an extension that enables this.

[selik]

I wrote this before I was familiar with type annotations. There might be a way to use type annotations to allow a binding and constraint for the same value. It could look like this:

o = Binding()
schema = o.x : str
match(schema, 'yo')
# o.x == 'yo'

Would that be helpful?