hy/docs/language/model_patterns.rst

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2018-05-29 01:15:47 +02:00
==============
Model Patterns
==============
The module ``hy.model-patterns`` provides a library of parser combinators for
parsing complex trees of Hy models. Model patterns exist mostly to help
implement the compiler, but they can also be useful for writing macros.
A motivating example
--------------------
The kind of problem that model patterns are suited for is the following.
Suppose you want to validate and extract the components of a form like:
.. code-block:: clj
(setv form '(try
(foo1)
(foo2)
(except [EType1]
(foo3))
(except [e EType2]
(foo4)
(foo5))
(except []
(foo6))
(finally
(foo7)
(foo8))))
You could do this with loops and indexing, but it would take a lot of code and
be error-prone. Model patterns concisely express the general form of an
expression to be matched, like what a regular expression does for text. Here's
a pattern for a ``try`` form of the above kind:
.. code-block:: clj
(import [funcparserlib.parser [maybe many]])
(import [hy.model-patterns [*]])
(setv parser (whole [
(sym "try")
(many (notpexpr "except" "else" "finally"))
(many (pexpr
(sym "except")
(| (brackets) (brackets FORM) (brackets SYM FORM))
(many FORM)))
(maybe (dolike "else"))
(maybe (dolike "finally"))]))
You can run the parser with ``(.parse parser form)``. The result is:
.. code-block:: clj
(,
['(foo1) '(foo2)]
[
'([EType1] [(foo3)])
'([e EType2] [(foo4) (foo5)])
'([] [(foo6)])]
None
'((foo7) (foo8)))
which is conveniently utilized with an assignment such as ``(setv [body
except-clauses else-part finally-part] result)``. Notice that ``else-part``
will be set to ``None`` because there is no ``else`` clause in the original
form.
Usage
-----
Model patterns are implemented as funcparserlib_ parser combinators. We won't
reproduce funcparserlib's own documentation, but here are some important
built-in parsers:
- ``(+ ...)`` matches its arguments in sequence.
- ``(| ...)`` matches any one of its arguments.
- ``(>> parser function)`` matches ``parser``, then feeds the result through
``function`` to change the value that's produced on a successful parse.
- ``(skip parser)`` matches ``parser``, but doesn't add it to the produced
value.
- ``(maybe parser)`` matches ``parser`` if possible. Otherwise, it produces
the value ``None``.
- ``(some function)`` takes a predicate ``function`` and matches a form if it
satisfies the predicate.
The best reference for Hy's parsers is the docstrings (use ``(help
hy.model-patterns)``), but again, here are some of the more important ones:
- ``FORM`` matches anything.
- ``SYM`` matches any symbol.
- ``(sym "foo")`` or ``(sym ":foo")`` matches and discards (per ``skip``) the
named symbol or keyword.
- ``(brackets ...)`` matches the arguments in square brackets.
- ``(pexpr ...)`` matches the arguments in parentheses.
Here's how you could write a simple macro using model patterns:
.. code-block:: clj
(defmacro pairs [&rest args]
(import [funcparserlib.parser [many]])
(import [hy.model-patterns [whole SYM FORM]])
(setv [args] (->> args (.parse (whole [
(many (+ SYM FORM))]))))
`[~@(->> args (map (fn [x]
(, (name (get x 0)) (get x 1)))))])
(print (pairs a 1 b 2 c 3))
; => [["a" 1] ["b" 2] ["c" 3]]
A failed parse will raise ``funcparserlib.parser.NoParseError``.
.. _funcparserlib: https://github.com/vlasovskikh/funcparserlib