Consider this pseudo code:
(defrc name
"string"
[a :A]
[:div a])
Where defrc would be a macro, that would expand to the following
(let [a (rum/react (atom :A))]
(rum/defc name < rum/reactive []
[:div a]))
Where rum/defc is itself a macro. I came up with the code below:
(defmacro defrc
[name subj bindings & body]
(let [map-bindings# (apply array-map bindings)
keys# (keys map-bindings#)
vals# (vals map-bindings#)
atomised-vals# (atom-map vals#)]
`(let ~(vec (interleave keys# (map (fn [v] (list 'rum/react v)) (vals atomised-vals#))))
(rum/defc ~name < rum/reactive [] ~#body))))
Which almost works:
(macroexpand-all '(defrc aname
#_=> "string"
#_=> [a :A]
#_=> [:div a]))
(let* [a (rum/react #object[clojure.lang.Atom 0x727ed2e6 {:status :ready, :val nil}])] (rum/defc aname clojure.core/< rum/reactive [] [:div a]))
However when used it results in a syntax error:
ERROR: Syntax error at (clojure.core/< rum.core/reactive [] [:div a])
Is this because the inner macro is not being expanded?
Turns out the macro was working correctly but the problem occurred because < was inside the syntax quote it got expanded to clojure.core/<, and Rum simply looks for a quoted <, relevant snippet from Rum's source:
...(cond
(and (empty? res) (symbol? x))
(recur {:name x} next nil)
(fn-body? xs) (assoc res :bodies (list xs))
(every? fn-body? xs) (assoc res :bodies xs)
(string? x) (recur (assoc res :doc x) next nil)
(= '< x) (recur res next :mixins)
(= mode :mixins)
(recur (update-in res [:mixins] (fnil conj []) x) next :mixins)
:else
(throw (IllegalArgumentException. (str "Syntax error at " xs))))...
Related
I am new to Clojure and I'm learning how to write a program that can simplify logical expressions (just 'and' for now to figure out how things work first). For example:
(and-simplify '(and true)) => true
(and-simplify '(and x true)) => x
(and-simplify '(and true false x)) => false
(and-simplify '(and x y z true)) => (and x y z)
I already knew how to simplify two arguments, that everything I can do right now is:
(defn and-simplify []
(def x (and true false))
println x)
(and-simplify)
I've read this post and tried to modify my code a little bit but it doesn't seem to get me anywhere:
(defn and-simplify [&expr]
(def (and &expr))
)
What is the correct way that I should have done?
Here's my take on it.
(defn simplify-and
[[op & forms]]
(let [known-falsy? #(or (false? %) (nil? %))
known-truthy? #(and (not (symbol? %))
(not (seq? %))
(not (known-falsy? %)))
falsy-forms (filter known-falsy? forms)
unknown-forms (remove known-truthy? forms)]
(if (seq falsy-forms)
(first falsy-forms)
(case (count unknown-forms)
0 true
1 (first unknown-forms)
(cons op unknown-forms)))))
(comment (simplify-and `(and true 1 2 a)))
However, we can write a more generic simplify that uses multimethods to simplify lists, so that we can add more optimisations without modifying existing code. Here's that, with optimisations for and, or and + from clojure.core. This simplify only optimises lists based on namespace qualified names.
Check out the examples in the comment form. Hope it makes sense.
(defn- known-falsy? [form]
(or (false? form) (nil? form)))
(defn- known-truthy? [form]
(and (not (symbol? form))
(not (seq? form))
(not (known-falsy? form))))
(declare simplify)
(defmulti simplify-list first)
(defmethod simplify-list :default [form] form)
(defmethod simplify-list 'clojure.core/and
[[op & forms]]
(let [forms (mapv simplify forms)
falsy-forms (filter known-falsy? forms)
unknown-forms (remove known-truthy? forms)]
(if (seq falsy-forms)
(first falsy-forms)
(case (count unknown-forms)
0 true
1 (first unknown-forms)
(cons op unknown-forms)))))
(defmethod simplify-list 'clojure.core/or
[[op & forms]]
(let [forms (mapv simplify forms)
truthy-forms (filter known-truthy? forms)
unknown-forms (remove known-falsy? forms)]
(if (seq truthy-forms)
(first truthy-forms)
(case (count unknown-forms)
0 nil
1 (first unknown-forms)
(cons op unknown-forms)))))
(defmethod simplify-list 'clojure.core/+
[[op & forms]]
(let [{nums true non-nums false} (group-by number? (mapv simplify forms))
sum (apply + nums)]
(if (seq non-nums)
(cons op (cons sum non-nums))
sum)))
(defn simplify
"takes a Clojure form with resolved symbols and performs
peephole optimisations on it"
[form]
(cond (set? form) (into #{} (map simplify) form)
(vector? form) (mapv simplify form)
(map? form) (reduce-kv (fn [m k v] (assoc m (simplify k) (simplify v)))
{} form)
(seq? form) (simplify-list form)
:else form))
(comment
(simplify `(+ 1 2))
(simplify `(foo 1 2))
(simplify `(and true (+ 1 2 3 4 5 foo)))
(simplify `(or false x))
(simplify `(or false x nil y))
(simplify `(or false x (and y nil z) (+ 1 2)))
)
Is there a short form/macro that allows me to do
(defn f [a b c]
{a b c})
instead of
(defn f [a b c]
{:a a :b b :c c})
(defmacro as-map [& syms]
(zipmap (map keyword syms) syms))
Usage:
(def a 42)
(def b :foo)
(as-map a b)
;;-> {:a 42 :b :foo}
Note that to support namespaced keywords, you'd have to drop support for ns aliases if you want to keep it as short:
(defmacro as-map [& syms]
(zipmap (map keyword syms) (map (comp symbol name) syms)))
Usage:
(def a 42)
(def b :foo)
(as-map example/a foo-of/b)
;;-> {:example/a 42 :foo-of/b :foo}
Advice: Likely not a good idea, saves you a few keyboard hits at the cost of readability and expressivity and flexibility in naming local bindings.
This shows the steps. Remove the println's for actual use:
(ns clj.core
(:gen-class))
(defmacro hasher [& args]
(let [keywords (map keyword args)
values args
keyvals-list (interleave keywords values)
]
(println "keywords " keywords)
(println "values " values)
(println "keyvals-list " keyvals-list)
`(hash-map ~#keyvals-list)
)
)
(def a 1)
(def b 2)
(println \newline "result: " (hasher a b))
> lein run
keywords (:a :b)
values (a b)
keyvals-list (:a a :b b)
result: {:b 2, :a 1}
This is an old snippet of mine I've had kicking around for a while.
(declare ^:private restructure*)
(defn ^:private restructure-1 [m [e k]]
(cond
(= :strs e) (reduce #(assoc %1 (name %2) %2) m k)
(= :keys e) (reduce #(assoc %1 (keyword (namespace %2) (name %2)) %2) m k)
:else (assoc m k (restructure* e))))
(defn ^:private restructure* [form]
(if-not (map? form)
form
(as-> {} v
(reduce restructure-1 v form)
`(hash-map ~#(mapcat identity v)))))
(defmacro restructure [form]
(restructure* form))
The idea is that it provides the complement of clojure.core/destructure which goes from a destructuring form to bindings, this captures bindings and constructs a datastructure.
(let [x 1 y 2 z 3]
(restructure {:keys [x y z]}))
;; => {:x 1 :y 2 :z 3}
Here is an example from joy of clojure chapter 8:
(defn contextual-eval [ctx expr]
(let [new-expr
`(let [~#(mapcat (fn [[k v]]
[k `'~v])
ctx)]
~expr)]
(pprint new-expr)
(eval new-expr)))
(pprint (contextual-eval '{a 1 b 2} '(+ a b)))
I find the ``'` part quite perplexing, what's it for?
I also tried to modify the function a bit:
(defn contextual-eval [ctx expr]
(let [new-expr
`(let [~#(mapcat (fn [[k v]]
[k `~v])
ctx)]
~expr)]
(pprint new-expr)
(eval new-expr)))
(pprint (contextual-eval '{a 1 b 2} '(+ a b)))
(defn contextual-eval [ctx expr]
(let [new-expr
`(let [~#(vec (apply
concat
ctx))]
~expr)]
(pprint new-expr)
(eval new-expr)))
(pprint (contextual-eval '{a 1 b 2} '(+ a b)))
All the versions above have similar effect. Why did the author choose to use `' then?
A more detailed look:
(use 'clojure.pprint)
(defmacro epprint [expr]
`(do
(print "==>")
(pprint '~expr)
(pprint ~expr)))
(defmacro epprints [& exprs]
(list* 'do (map (fn [x] (list 'epprint x))
exprs)))
(defn contextual-eval [ctx expr]
(let [new-expr
`(let [~#(mapcat (fn [[k v]]
(epprints
(class v)
v
(class '~v)
'~v
(class `'~v)
`'~v
(class ctx)
ctx)
[k `~v])
ctx)]
~expr)]
(pprint new-expr)
(eval new-expr)))
(pprint (contextual-eval '{a (* 2 3) b (inc 11)} '(+ a b)))
This prints out the following in the repl:
==>(class v)
clojure.lang.PersistentList
==>v
(* 2 3)
==>(class '~v)
clojure.lang.PersistentList
==>'~v
~v
==>(class
(clojure.core/seq
(clojure.core/concat
(clojure.core/list 'quote)
(clojure.core/list v))))
clojure.lang.Cons
==>(clojure.core/seq
(clojure.core/concat (clojure.core/list 'quote) (clojure.core/list v)))
'(* 2 3)
==>(class ctx)
clojure.lang.PersistentArrayMap
==>ctx
{a (* 2 3), b (inc 11)}
==>(class v)
clojure.lang.PersistentList
==>v
(inc 11)
==>(class '~v)
clojure.lang.PersistentList
==>'~v
~v
==>(class
(clojure.core/seq
(clojure.core/concat
(clojure.core/list 'quote)
(clojure.core/list v))))
clojure.lang.Cons
==>(clojure.core/seq
(clojure.core/concat (clojure.core/list 'quote) (clojure.core/list v)))
'(inc 11)
==>(class ctx)
clojure.lang.PersistentArrayMap
==>ctx
{a (* 2 3), b (inc 11)}
==>new-expr
(clojure.core/let [a (* 2 3) b (inc 11)] (+ a b))
18
Again, using a single syntax quote for v seems to get the job done.
In fact, using `'v might cause you some trouble:
(defn contextual-eval [ctx expr]
(let [new-expr
`(let [~#(mapcat (fn [[k v]]
[k `'~v])
ctx)]
~expr)]
(pprint new-expr)
(eval new-expr)))
(pprint (contextual-eval '{a (inc 3) b (* 3 4)} '(+ a b)))
CompilerException java.lang.ClassCastException: clojure.lang.PersistentList cannot be cast to java.lang.Number, compiling:(/Users/kaiyin/personal_config_bin_files/workspace/typedclj/src/typedclj/macros.clj:14:22)
`'~v is a way to return
(list 'quote v)
in this case quoting the actual value of v in the let expression, not the symbol itself.
IDK The Joy Of Clojure, but apparently the authors want to prevent forms passed in ctx from being evaluated in the expanded let form. E. g. (contextual-eval '{a (+ 3 4)} 'a) will return (+ 3 4) but 7 in your versions which are both identical in behavior.
Your modified versions have the same effect only because you're trying them on very simple data. Try instead with a mapping like {'a 'x}, a context in which the binding for a is the symbol x.
user> (defn contextual-eval [ctx expr]
(let [new-expr
`(let [~#(mapcat (fn [[k v]]
[k `'~v])
ctx)]
~expr)]
(eval new-expr)))
#'user/contextual-eval
user> (contextual-eval {'a 'x} '(name a))
"x"
user> (defn contextual-eval [ctx expr]
(let [new-expr
`(let [~#(mapcat (fn [[k v]]
[k `~v])
ctx)]
~expr)]
(eval new-expr)))
#'user/contextual-eval
user> (contextual-eval {'a 'x} '(name a))
; Evaluation aborted.
The problem is that in your version, by neglecting the quote, you are double-evaluating the values bound to your symbols: x shouldn't be evaluated, because the value is actually the symbol x. You get away with this double evaluation in your simple test cases, because 1 evaluates to itself: (eval (eval (eval 1))) would work fine too. But doing that with most data structures is wrong, because they have non-trivial evaluation semantics.
Note also that the following expressions are identical in all cases, so there's never a reason to write any of them but the first one:
x
`~x
`~`~x
```~`~~`~~x
If you syntax-quote and then immediately un-quote, you haven't accomplished anything. So, if you ever find yourself writing a quote followed by an unquote, this should be a big red flag that you are doing something wrong.
I know how to extract one attribute using zip-xml/attr, but how to extract multiple attributes?
e.g I have the following
<table>
<column name="col1" type="varchar" length="8"/>
<column name="col2" type="varchar" length="16"/>
<column name="col3" type="int" length="16"/>
<table>
And the expected result is. A silly way is to call zip-xml/attr for each attribute, but is there any elegant way to do that?
[["co11" "varchar" 8] [["co12" "varchar" 16] [["co13" "int" 16]
My advice is to use a tree-walking function to extract the interesting data from the XML tree. clojure.walk has several of these, but here I use tree-seq from core clojure to just produce a seq of nodes and work on that. This function takes two functions - a branch? predicate which checks if a node can have children and a children function which gets them. I use :content for both, as tags with no nested tags produce nil, which is a falsey value and so it works also as a predicate.
(->> (clojure.xml/parse "res/doc.xml") ;;source file for your xml
(tree-seq :content :content) ;; Produce a seq by walking the tree
(filter #(= :column (:tag %))) ;;Take only :column tags
(mapv (comp vec vals :attrs)))
;;Collect the values of the :attrs maps into vectors
;;and collect those into a vector with mapv
Your desired output had unmatched square brackets, but I assume it should be like
[["col1" "varchar" "8"] ["col2" "varchar" "16"] ["col3" "int" "16"]]
which was my return value. However, this is potentially brittle - you're relying on the maps returned by clojure.xml/parse preserving the ordering of the attributes in the XML in order to know what the data means. That's not really part of the contract of maps. As an implementation detail it creates clojure.lang.PersistentStructMaps which apparently do have this feature, but it might not always be so.
Alternatively you could use just (mapv :attrs) to keep the whole of the map in there.
The right solution depends on how large and complex the XML is and to some extent, what you know about its structure. If it needs to be very generic, then you need to have quite a lot of logic to navigate the nodes etc. However, if it is a known format and you know what nodes you are interested in, its pretty straight-forward.
I used clojure.zip to create a zipper from the XML file and then use clojure.data.zip.xml to extract the nodes/paths I was interested in. I then defined simple helper functions to process specific nodes. This was pretty much my first bit of clojure and I've not yet gone back to it to re-factor it and refine/clarify some of my very rough clojure idioms based on what I've learnt since, but in the spirit of an example being worth 1000 words, here it is -
(ns arcis.models.nessus
(:use [taoensso.timbre :only [trace debug info warn error fatal]])
(:require [arcis.util :as util]
[arcis.models.db :as db]
[clojure.java.io :as io]
[clojure.xml :as xml]
[clojure.zip :as zip]
[clojure.data.zip.xml :as zx]))
(def nessus-host-keys [:hostname :host_fqdn
:system_type :operating_system
:operating_system_unsupported])
(def used-nessus-host-keys (conj nessus-host-keys
:host_start :host_end
:items :traceroute_hop_0 :traceroute_hop_1
:traceroute_hop_2 :traceroute_hop_3
:traceroute_hop_4 :traceroute_hop_5
:traceroute_hop_6 :traceroute_hop_7
:traceroute_hop_8 :traceroute_hop_9
:traceroute_hop_10 :traceroute_hop_11
:traceroute_hop_12 :traceroute_hop_13
:traceroute_hop_14 :traceroute_hop_15
:traceroute_hop_16 :traceroute_hop_17
:host_ip :patch_summary_total_cves
:cpe_0 :cpe_1 :cpe_2 :cpe_3 :cpe_4 :cpe_5
:cpe_6 :cpe_7 :cpe_8 :cpe_9))
(def nessus-item-keys [:port :svc_name :protocol :severity :plugin_id
:plugin_output])
(def used-nessus-item-keys (conj nessus-item-keys
:plugin_details
:plugin_name
:plugin_family))
(def nessus-plugin-keys [:plugin_id :plugin_name :plugin_family :fname
:script_version :plugin_type :exploitability_ease
:vuln_publication_date :cvss_temporal_data
:solution :cvss_temporal_score :risk_factor
:description :cvss_vector :synopsis
:patch_publication_date :exploit_available
:plugin_publication_date :plugin_modification_date
:cve :bid :exploit_framework_canvas :edb_id
:exploit_framework_metasploit :exploit_framework_core
:metasploit_name :canvas_package :osvdb :cwe
:cvss_temporal_vector :cvss_base_score :cpe
:exploited_by_malware])
(def used-nessus-plugin-keys (conj nessus-plugin-keys
:xref :see_also :cert
:attachment :iava :stig_severity :hp
:secunia :iawb :msft))
(def show-unprocessed true)
(defn log-unprocessed [title vls]
(if (and show-unprocessed
(seq vls))
(println (str "Unprocessed " title ": " vls))))
;;; parse nessus report
(defn parse-xref [xref]
{:xref (first (:content xref))})
(defn parse-see-also [see-also]
{:see_also (first (:content see-also))})
(defn parse-plugin [plugin]
{(util/db-keyword (name (:tag plugin))) (first (:content plugin))})
(defn parse-contents [cont]
(let [xref (mapv parse-xref (filter #(= (:tag %) :xref) cont))
see-also (mapv parse-see-also (filter #(= (:tag %) :see-also) cont))
details (reduce merge {}
(map parse-plugin
(remove #(or (= (:tag %) :xref)
(= (:tag %) :see-also)) cont)))]
(assoc details
:see_also see-also
:xref xref)))
(defn fix-item-keywords [item]
(let [ks (keys item)]
(into {}
(for [k ks]
[(util/db-keyword (name k))
(k item)]))))
(defn parse-item [item]
(let [attrs (fix-item-keywords (:attrs item))
contents (parse-contents (:content item))]
(assoc attrs
:plugin_output (:plugin_output contents)
:plugin_details (assoc (dissoc contents :plugin_output)
:plugin_id (:plugin_id attrs)
:plugin_family (:plugin_family attrs)))))
(defn parse-properties [props]
(into {}
(for [p props]
[(util/db-keyword (:name (:attrs p)))
(first (:content p))])))
(defn parse-host [h]
(let [items (map first (zx/xml-> h :ReportItem))
properties (:content (first (zx/xml1-> h :HostProperties)))]
(assoc (parse-properties properties)
:hostname (zx/attr h :name)
:items (mapv parse-item items))))
(defn parse-hosts [hosts]
(mapv parse-host hosts))
(defn parse-file [f]
(let [root (zip/xml-zip (xml/parse (io/file f)))
report-xml (zx/xml1-> root :Report)
hosts (zx/xml-> report-xml :ReportHost)]
{:report_name (zx/attr report-xml :name)
:policy (zx/text (zx/xml1-> root :Policy :policyName))
:hosts (parse-hosts hosts)}))
;;; insert nessus records into db
(defn mk-host-rec [scan-id host]
(let [[id err] (db/get-sequence-nextval "host_seq")]
(if (nil? err)
(assoc (util/build-map host nessus-host-keys)
:ipv4 (:host_ip host)
:scan_start (util/from-nessus-date (:scan_start host))
:scan_end (util/from-nessus-date (:scan_end host))
:total_cves (:patch_summary_total_cves host)
:id id
:scan_id scan-id)
nil)))
(defn insert-patches [p]
(when (seq p)
(db/insert-nessus-host-patch (first p))
(recur (rest p))))
(defn insert-host-patch [id host]
(let [p-keys (filter #(re-find #"patch_summary_*" %) (map name (keys host)))
recs (map (fn [s]
{:id (first (db/get-sequence-nextval "patch_seq"))
:host_id id
:summary ((keyword (str "patch_summary_txt_" s)) host)
:cve_num ((keyword (str "patch_summary_cve_num_" s)) host)
:cves ((keyword (str "patch_summary_cves_" s)) host)})
(filter seq
(map #(second (re-find #"patch_summary_txt_(.*)" %))
p-keys)))]
(insert-patches recs)
(util/remove-keys host (map keyword p-keys))))
(defn mk-item-rec [host-id item]
(let [[id err] (db/get-sequence-nextval "item_seq")]
(assoc (util/build-map item nessus-item-keys)
:host_id host-id
:id id)))
(defn insert-item [host-id item]
(let [rec (mk-item-rec host-id item)
not-done (keys (util/remove-keys item used-nessus-item-keys))]
(log-unprocessed "Item Keys" not-done)
(db/insert-nessus-report-item rec)
(:plugin_id item)))
(defn mk-plugin-rec [item]
(let [rec (util/build-map (:plugin_details item) nessus-plugin-keys)
not-used (keys (util/remove-keys (:plugin_details item)
used-nessus-plugin-keys))]
(log-unprocessed "Plugin Keys" not-used)
(assoc rec
:vuln_publication_date (util/from-nessus-date
(:vuln_publication_date rec))
:patch_publication_date (util/from-nessus-date
(:patch_publication_date rec))
:plugin_publication_date (util/from-nessus-date
(:plugin_publication_date rec))
:plugin_modification_date (util/from-nessus-date
(:plugin_modificaiton_date rec)))))
(defn insert-xref [plugin-id xrefs]
(when (seq xrefs)
(let [xref {:id (first (db/get-sequence-nextval "xref_seq"))
:plugin_id plugin-id
:xref (:xref (first xrefs))}]
(db/insert-nessus-xref xref)
(recur plugin-id (rest xrefs)))))
(defn insert-see-also [plugin-id see-also]
(when (seq see-also)
(let [sa {:id (first (db/get-sequence-nextval "ref_seq"))
:plugin_id plugin-id
:reference (:see_also (first see-also))}]
(db/insert-nessus-ref sa)
(recur plugin-id (rest see-also)))))
(defn insert-plugin [item]
(let [rec (mk-plugin-rec item)
xref (:xref (:plugin_details item))
see-also (:see_also (:plugin_details item))]
(if (seq xref)
(insert-xref (:plugin_id rec) xref))
(if (seq see-also)
(insert-see-also (:plugin_id rec) see-also))
(db/upsert-nessus-plugin rec)))
(defn insert-items [host-id items plugin-set]
(if (empty? items)
plugin-set
(let [p (insert-item host-id (first items))]
(if-not (contains? plugin-set p)
(insert-plugin (first items)))
(recur host-id (rest items) (conj plugin-set p)))))
(defn insert-host [scan-id host plugin-set]
(if-let [h-rec (mk-host-rec scan-id host)]
(let [[v err] (db/insert-nessus-host h-rec)
items (:items host)]
(if (nil? err)
(let [host2 (insert-host-patch (:id h-rec) host)]
(log-unprocessed "Host Keys" (keys (util/remove-keys
host2 used-nessus-host-keys)))
(insert-items (:id h-rec) items plugin-set))
plugin-set))
plugin-set))
(defn insert-hosts
([id hosts]
(insert-hosts id hosts #{}))
([id hosts plugins]
(if (empty? hosts)
plugins
(let [plugin-set (insert-host id (first hosts) plugins)]
(recur id (rest hosts) plugin-set)))))
(defn mk-scan-record [id report]
{:id id
:name (:report_name report)
:scan_dt (util/to-sql-date)
:policy (:policy report)
:entered_dt (util/to-sql-date)})
(defn store-report [update-plugins report]
(let [[id err] (db/get-sequence-nextval "nscan_seq")
scan-rec (mk-scan-record id report)]
(if (nil? err)
(let [[v e] (db/insert-nessus-scan scan-rec)]
(if (nil? e)
(if update-plugins
(let [plugin-list (set (first (db/select-nessus-plugin-ids)))]
[(insert-hosts id (:hosts report) plugin-list) nil])
[(insert-hosts id (:hosts report)) nil])
[v e]))
[id err])))
(defn process-nessus-report [update-plugins filename]
(let [report (parse-file filename)]
(println (str "Report: " (:report_name report)
"\nPolicy: " (:policy report)
"\nHost Records: " (count (:hosts report))))
(store-report update-plugins report)))
Magos's answer using tree-seq is perfectly fine, but there's no reason to abandon zippers; filtering using zippers is more succinct and the arguably the "clojure" way. (note this example uses data.xml ([org.clojure/data.xml "0.0.8"]) instead of clojure.xml).
(require '[clojure.data.zip.xml :as zf])
(require '[clojure.zip :as z])
(def ex
"<table>
<column name=\"col1\" type=\"varchar\" length=\"8\"/>
<column name=\"col2\" type=\"varchar\" length=\"16\"/>
<column name=\"col3\" type=\"int\" length=\"16\"/>
</table>")
(let [x (z/xml-zip (clojure.data.xml/parse-str ex))]
(->> (zf/xml-> x :column) ;;equivalent to (->> treeseq ... filter)
flatten
(keep :attrs)
(map vals)))
;>>> (("col1" "varchar" "8") ("col2" "varchar" "16") ("col3" "int" "16"))
But the xml-> macro simply applies functions in order, so you can do the following:
(let [x (z/xml-zip (clojure.data.xml/parse-str ex))]
(->> (zf/xml-> x :column #(keep :attrs %))
(map vals)))
;>>> (("col1" "varchar" "8") ("col2" "varchar" "16") ("col3" "int" "16"))
I want to create a function (thunk) that will return successive elements in a list. What is the best way to do this? I wrote this code based on an apparently flawed understanding of how local variables in clojure work:
(defn reader-for [commands]
(with-local-vars
[stream commands]
(fn []
(let
[r (var-get stream)]
(if (empty? r)
nil
(let
[cur (first r)
_ (var-set stream (rest r))]
cur))))))
In this code I get:
#<CompilerException java.lang.IllegalStateException: Var null/null is unbound. (Chapel.clj:1)>
which seems to suggest that with-local-vars is dynamically scoped. Is that true? Is there any lexically scoped alternative? Thanks for any help.
If you require mutable state, use one of the clojure reference types:
user=> (defn reader-for [coll]
(let [a (atom coll)]
(fn []
(let [x (first #a)]
(swap! a next)
x))))
#'user/reader-for
user=> (def f (reader-for [1 2 3]))
#'user/f
user=> (f)
1
user=> (f)
2
user=> (f)
3
user=> (f)
nil
Also, let is for lexical scoping, binding is for dynamic scoping.
Edit: the thread-safe version as pointed out by Alan.
(defn reader-for [coll]
(let [r (ref coll)]
#(dosync
(let [x (first #r)]
(alter r next)
x))))
And just for fun, a thread-safe version with atoms (don't do this):
(defn reader-for [coll]
(let [a (atom coll)]
(fn []
(let [ret (atom nil)]
(swap! a (fn [[x & xs]]
(compare-and-set! ret nil x)
xs))
#ret))))