Function check_expression 

pub fn check_expression(
    source: &str,
    types: &Map<String, Value>,
) -> Result<Value, String>

Type-check an expression against the names it may read (a kind’s fields, a binding’s value): the type, or the error (the parse error, or the first type error). Types are spelled as JSON ("number", [T], a row object, {"?": T} for a value that may be missing).