Merge operator

[yannham]

Is your feature request related to a problem? Please describe.
In a configuration-oriented language, several fundamental operations may be elegantly subsumed by the use of a merge operator à la Cue: define both data and schemas in the same language, validate data against schemas, set default values and more generally have a form of inheritance for better code reuse.

Describe the solution you'd like
Enrich the language along two axis:

• Add the following new kind of values that will have a specific semantics with respect to merge, and can be converted to standard values for any operation
  • Default values
  • Types/Contracts
  • Docstrings
• Add a merge - or a meet - operator to combine records

Additional context
This takes inspiration from the CUE programming language where types and values live in the same space, namely a bounded lattice (example of merging in CUE). Nickel allows more things (e.g general recursion and arbitrary terms as contracts) thus cannot have the same kind of strong properties, but we aim to still retain the spirit and the niceness of the feature.

[aspiwack]

What's missing for this to be closed?

[yannham]

Closed by #79.