Aw: defnk pre- and post-conditions
Hi,
clojure supports keyword arguments for a quote some time now.
(defn foo
[positional1 ... positionalN {:keys [kw1 ... kwn]}]
...)
The syntax is the usual destructuring syntax for maps. So you can specify
defaults via :or etc. And even have none keyword keys.
Sincerely
Meikel
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Re: Aw: defnk pre- and post-conditions
On Tue, 26 Apr 2011 07:27:52 -0400, Meikel Brandmeyer m...@kotka.de wrote:
clojure supports keyword arguments for a quote some time now.
(defn foo
[positional1 ... positionalN {:keys [kw1 ... kwn]}]
...)
This is used extensively in Pallet but has a couple of drawbacks.
- Repetition of keywords to specify defaults
- Repetition of keywords and defaults in doc strings
- The defaulted argument values are not available as a map
- No flag for presence of specified keyword (à la CL)
Inspired by a discussion [1] on supporting shFlags in stevedore, I drafted
a defnkw [2] that allows for the following syntax:
(defnkw foo
My foo
[positional1 … positionalN
[[kw1 description1 default1]
[kw2 description2]]]
…)
and automatically generates a suffix to the doc string.
(doc foo) =
My foo
- kw1 description1. Default default1.
- kw2 description2.
The formatting of the generated doc string can obviously be improved, and
it is not obvious to me what syntax to use for naming the keyword option
map (:as). This approach would allow defaulted values to be merged back
into the option map. Support for presence flags would require an extra
element in the specification vector (e.g. [description default flag?])
An alternative would be to use a more defnk like syntax:
(defnkw foo
My foo
[positional1 … positionalN
:kw1 [description1 default1]
:kw2 description2]
…)
I would be interested in any comments.
[1] https://github.com/pallet/stevedore/pull/1
[2]
https://github.com/pallet/common/blob/feature%2Fdefn-for-kw-args/src/pallet/common/core.clj
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Hugo Duncan
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Re: Aw: defnk pre- and post-conditions
On Tue, Apr 26, 2011 at 8:26 PM, Hugo Duncan duncan.h...@gmail.com wrote:
Inspired by a discussion [1] on supporting shFlags in stevedore, I drafted
a defnkw [2] that allows for the following syntax:
(defnkw foo
My foo
[positional1 … positionalN
[[kw1 description1 default1]
[kw2 description2]]]
…)
The formatting of the generated doc string can obviously be improved, and
it is not obvious to me what syntax to use for naming the keyword option map
(:as). This approach would allow defaulted values to be merged back into the
option map. Support for presence flags would require an extra element in
the specification vector (e.g. [description default flag?])
An alternative would be to use a more defnk like syntax:
(defnkw foo
My foo
[positional1 … positionalN
:kw1 [description1 default1]
:kw2 description2]
…)
Here's an interesting alternative:
(defnkw foo
My foo
[positional1 ... positionalN
{:kw1 {:doc The kw1 argument
:default default1}
:kw2 {:doc Another
:default blah
:as wheee}
:kw2 {}} :as options]
We can specify an :as key on the entire option map.
Maps are more flexible than vectors for attributes. :doc, :default, :as are
fairly descriptive
and other attributes can easily be added. There would be defaults for each
key.
I've put a spin on the CL defun shortcut for renaming keyword args (:kw2
will be bound to wheee).
See Keyword Parameters: http://www.gigamonkeys.com/book/functions.html
This is an alternative which is much closer to CL's method:
{[:kw2 wheee] {... atrs.. ]}
On the whole it seems pretty readable, but may be misleading with the { }
syntax.
It's also markedly more verbose.
I would be interested in any comments.
[1] https://github.com/pallet/stevedore/pull/1
[2]
https://github.com/pallet/common/blob/feature%2Fdefn-for-kw-args/src/pallet/common/core.clj
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