Statements and assignments¶

A rule is usually one expression. DPM-XL also allows a script: statements separated by ;, with two kinds of assignment.

Statement	Meaning
`expression`	An expression, with an optional `with` context.
`name := expression`	A temporary assignment: `name` holds the expression's records for later statements.
`{target} <- expression`	A persistent assignment: the value is computed and returned in `RuleResult.assignments`. Targets are cell selections such as `{tT, r0010, c0010}` or parameters such as `{vX}`.

Statements run in order; the last one gives the rule's records and verdict. The last statement needs no ;, and a trailing ; is allowed.

from dpmxl import MemoryProvider, Number, TableLayout, evaluate_rule

provider = MemoryProvider(
    [TableLayout("T", rows=("0010", "0020"), columns=("0010", "0020"))],
    {
        ("T", "0010", "0010", None, ()): Number.of(10, 0),
        ("T", "0020", "0010", None, ()): Number.of(4, 0),
        ("T", "0010", "0020", None, ()): Number.of(1, 0),
        ("T", "0020", "0020", None, ()): Number.of(4, 0),
    },
)
script = "d := with {tT, c*}: {r0010} - {r0020}; d > 0"
result = evaluate_rule(script, provider)
print(
    result.status, [(dict(outcome.key), outcome.passed) for outcome in result.outcomes]
)

failed [({'c': '0010'}, True), ({'c': '0020'}, False)]

A temporary keeps the records of its expression, keys included. In later statements its name wins over a report parameter of that name and loses to a key component inside a clause. The rule's context, temporaries and assignments do not reach operations referenced with {o_…}, which are evaluated on their own.

Persistent assignments¶

result = evaluate_rule(
    "{tT, r0010, c0010} <- {tT, r0020, c0010} * 2; {tT, r0010, c0010} = 10", provider
)
print(result.status)
for target, recordset in result.assignments.items():
    print(target, [record.value for record in recordset.records.values()])

passed
{tT, r0010, c0010} [Number(value=Decimal('8'), tolerance=Decimal('0'))]

Nothing is written to the report: the last statement still reads the reported 10. The assignment's value is for the caller, keyed by the target's normalized DPM-XL text.

Parsing scripts¶

parse_program parses a script into a Program; parse accepts one expression (with an optional trailing ;) and says so when given a script. evaluate_rule, the columnar fallback and dpm-xl check accept scripts.

Outside clauses <- is the assignment, so compare with a negated operand as a < -b. Inside […] clauses, as in the grammar, <- reads < -.

The columnar evaluator hands every script with assignments to the reference evaluator. The language reference gives the syntax of programs and statements.