77 lines
2.1 KiB
ReStructuredText
77 lines
2.1 KiB
ReStructuredText
==================
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Hy representations
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==================
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.. versionadded:: 0.13.0
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``hy.contrib.hy-repr`` is a module containing two functions.
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To import them, say::
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(import [hy.contrib.hy-repr [hy-repr hy-repr-register]])
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To make the Hy REPL use it for output, invoke Hy like so::
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$ hy --repl-output-fn=hy.contrib.hy-repr.hy-repr
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.. _hy-repr-fn:
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hy-repr
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-------
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Usage: ``(hy-repr x)``
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This function is Hy's equivalent of Python's built-in ``repr``.
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It returns a string representing the input object in Hy syntax.
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.. code-block:: hy
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=> (hy-repr [1 2 3])
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'[1 2 3]'
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=> (repr [1 2 3])
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'[1, 2, 3]'
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Like ``repr`` in Python, ``hy-repr`` can round-trip many kinds of
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values. Round-tripping implies that given an object ``x``,
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``(eval (read-str (hy-repr x)))`` returns ``x``, or at least a value
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that's equal to ``x``.
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.. _hy-repr-register-fn:
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hy-repr-register
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----------------
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Usage: ``(hy-repr-register the-type fun)``
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``hy-repr-register`` lets you set the function that ``hy-repr`` calls to
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represent a type.
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.. code-block:: hy
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=> (defclass C)
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=> (hy-repr-register C (fn [x] "cuddles"))
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=> (hy-repr [1 (C) 2])
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'[1 cuddles 2]'
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If the type of an object passed to ``hy-repr`` doesn't have a registered
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function, ``hy-repr`` will search the type's method resolution order
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(its ``__mro__`` attribute) for the first type that does. If ``hy-repr``
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doesn't find a candidate, it falls back on ``repr``.
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Registered functions often call ``hy-repr`` themselves. ``hy-repr`` will
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automatically detect self-references, even deeply nested ones, and
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output ``"..."`` for them instead of calling the usual registered
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function. To use a placeholder other than ``"..."``, pass a string of
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your choice to the keyword argument ``:placeholder`` of
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``hy-repr-register``.
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.. code-block:: hy
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(defclass Container [object]
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(defn __init__ (fn [self value]
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(setv self.value value))))
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(hy-repr-register Container :placeholder "HY THERE" (fn [x]
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(+ "(Container " (hy-repr x.value) ")")))
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(setv container (Container 5))
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(setv container.value container)
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(print (hy-repr container)) ; Prints "(Container HY THERE)"
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