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Why does StrictYAML make you define a schema in Python - a Turing-complete language?

StrictYAML defines schemas in Python (i.e. Turing-complete) code. For example:

Map({"name": Str(), "email": Str()})

Instead of:

type:       map
    type:      str
    required:  yes
    type:      str
    required:  yes

There are some trade offs here:

Schema definition in a non-Turing-complete language like YAML makes the schema programming language independent and gives it more potential for being read and understood by non-programmers.

However, schema definition in a non-Turing-complete language also restricts and makes certain use cases impossible or awkward.

Some use cases I came across included:

  • Being able to import pycountry's country list and restrict "country: " to valid country names.
  • Being able to implement a schema that validated date/time scalar values against the specific date/time parser I wanted.
  • Being able to revalidate sections of the document on a 'second pass' that used new data - e.g. a list in one part of the document is restricted to items which come from another part.