On Wed, Aug 8, 2012 at 11:48 AM, Brian Marick <[email protected]> wrote:
> I'm looking for medium-scale examples of using function-generating functions.
>
> Such examples might be ones that ... use closures to avoid the need to have
> some particular argument passed from function to function (which looks like
> the `this` in an instance method).
I try and use a greedy parser combinator as the next jump, and as
example of hiding arguments. String parsing is a small, yet
non-trivial example, that doesn't require domain knowledge. Something
like:
(defn result [value]
(fn [string]
[value string]))
(defn pred [predicate]
(fn [string]
(if (predicate (first string))
[(first string) (rest string)])))
(defn orp [f g]
(fn [string]
(or (f string) (g string))))
(defn bind [parser f]
(fn [string]
(if-let [[result s2] (parser string)]
((f result) s2))))
(defn many [parser]
(let [f (bind parser
(fn [h]
(bind (many parser)
(fn [rst]
(result (cons h rst))))))]
(orp f
(result []))))
(def letter (pred #(if % (Character/isLetter %))))
(def word
(bind (many letter)
(fn [w] (result (apply str w)))))
(word "foo")
;=> ["foo" ()]
The closest I see to an implicit this is:
((bind word
(fn [w1]
(bind (pred #(= % \space))
(fn [_]
(bind word
(fn [w2] (result [w1 w2]))))))) "foo bar baz")
;=> [["foo" "bar"] (\space \b \a \z)]
Here word and the space predicate are called on the string, but its
only ever mentioned as the argument. However, it is kinda ugly
without a macro to hide all the bind/fn pairs.
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