Crate rxeval 

rxeval — evaluate ODK/OpenRosa XForms logic.

The third piece beside rxform (spreadsheet → XForm) and rxdata (submission → typed data): the rules a form carries — relevant, constraint, required, calculate — are written in XPath, and until something evaluates them a server can only take a device’s word for what it collected.

One engine, three places. On the server it checks what arrived. Compiled to WebAssembly it drives a web form. Bound from R it lets a questionnaire’s logic be exercised before anyone goes to the field. Enketo’s engine is browser-only JavaScript and JavaRosa’s is JVM-only; neither travels.

What an expression can do. These are XForms expressions, not a scripting language: the grammar is XPath 1.0, the only inputs are the instance tree and the environment handed in, and evaluation performs no I/O, spawns nothing and cannot reach the host. A hostile expression can waste time or return a wrong answer; it cannot escape. eval_str is named for XPath evaluation, and shares nothing with the eval of interpreted languages.

Nothing here guesses. An expression this crate does not understand is an error, never a default. A form engine that returns false for a relevant it failed to parse hides a question and reports success, and the damage shows up in the data weeks later.

Re-exports

pub use eval::evaluate;

pub use eval::Context;

pub use eval::Environment;

pub use eval::Fixed;

pub use eval::Value;

pub use parser::parse;

pub use parser::Expr;

pub use portability::check_form;

pub use portability::Breaks;

pub use portability::Issue;

pub use rules::Binding;

pub use rules::Clock;

pub use rules::Form;

pub use rules::Rules;

pub use rules::Violation;

pub use rules::ViolationKind;

pub use session::Outcome;

pub use session::Session;

pub use tree::Instance;

pub use tree::NodeId;

Modules

eval

Evaluating a parsed expression against an instance.

functions

The function library: XPath 1.0’s, plus the OpenRosa extensions.

parser

XPath 1.0 expression parser.

portability

Will this form mean the same thing on a tablet and in a browser?

rules

The rules a form carries, and what they say about a filled instance.

session

A form being filled in.

tree

The instance tree XPath runs over.

Functions

eval_str

Parse and evaluate one expression against an instance, with . at the root. The convenience path; a form engine keeps parsed expressions.