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crumb: Python Premise
The syntax of a python premise.
Python Premise Syntax
python_premise ::= pattern '=' python_exp
| pattern 'in' python_exp
| 'check' python_exp
Each of these clauses results in a python expression being executed. Their
meaning is as follows:
pattern_ '=' python_exp
*python_exp* is evaluated and the result matched
with pattern_. If the result does not match,
the clause fails and backtracking_ is initiated.
The clause always fails on backtracking_, meaning that it only produces
a single result (contrasted with ``in``).
pattern_ 'in' python_exp
*python_exp* is evaluated and the first element
from the resulting *iterable* is matched
with pattern_. On backtracking_, successive
elements from the result are matched with
pattern_. When the result is exhausted, the clause fails.
This has the effect of offering each element of the result, one at a
time, to the subsequent premise clauses. Each element is thus acted on
*python_exp* is evaluated. If the result is
python "true" the clause succeeds, otherwise
it fails. The clause always fails on backtracking_.
.. _backtracking: ../overview/rules/backward_chaining.html#backtracking
.. _pattern: pattern.html