I am trying to define a multimethod and its implementation in a separate file. It goes something like this:
In file 1
(ns thing.a.b)
(defn dispatch-fn [x] x)
(defmulti foo dispatch-fn)
In file 2
(ns thing.a.b.c
(:require [thing.a.b :refer [foo]])
(defmethod foo "hello" [s] s)
(defmethod foo "goodbye" [s] "TATA")
And in the main file when I am calling the method I define something like this:
(ns thing.a.e
(:require thing.a.b :as test))
.
.
.
(test/foo "hello")
When I do this I get an exception saying "No method in multimethod 'foo'for dispatch value: hello
What am I doing wrong? Or is it not possible to define implementations of multimethods in separate files?
It is possible. The problem is because thing.a.b.c namespace isn't loaded. You have to load it before using.
This is a correct example:
(ns thing.a.e
(:require
[thing.a.b.c] ; Here all your defmethods loaded
[thing.a.b :as test]))
(test/foo "hello")
Related
How can i import a function in closure? Suppose i have two files:
test.clj
test2.clj
I want to use the function from test2 in test.
I have tried the following in test, but this is not working:
(namespace user :require test2)
How am a able to import a function in another file?
Basically want to do `from lib import f in python
In file test.clj:
(ns test
(:require [test2 :as t2]))
(defn myfn [x]
(t2/somefn x)
(t2/otherfn x))
In the above example, t2 is an alias for the namespace test2. If instead you prefer to add specified symbols from the namespace use :refer:
(ns test
(:require [test2 :refer [somefn otherfn]]))
(defn myfn [x]
(somefn x)
(otherfn x))
To refer to all public symbols in the namespace, use :refer :all:
(ns test
(:require [test2 :refer :all]))
(defn myfn [x]
(somefn x)
(otherfn x))
Your namespace syntax is a bit off. I usually refer to this cheat-sheet when I need a reminder.
I think the following is the syntax you are looking for.
;; In test.clj
(ns test
(:require [test2 :refer [some-symbol-to-import]]))
I would like a macro this-ns such that it returns the namespace of the location where it is being called. For instance, if I have this code
(ns nstest.main
(:require [nstest.core :as nstest]))
(defn ns-str [x]
(-> x (.getName) name))
(defn -main [& args]
(println "The ns according to *ns*:" (ns-str *ns*))
(println "The actual ns:" (ns-str (nstest/this-ns))))
I would expect that calling lein run would produce this output:
The ns according to *ns*: user
The actual ns: nstest.main
What I came up with as implementation was the following code:
(ns nstest.core)
(defmacro this-ns []
(let [s (gensym)]
`(do (def ~s)
(-> (var ~s)
(.ns)))))
It does seem to work, but it feels very hacky. Notably, in the above example it will expand to def being invoked inside the -main function which does not feel very clean.
My question: Is there a better way to implement this-ns to obtain the namespace where this-ns is called?
here is one more variant:
(defmacro this-ns []
`(->> (fn []) str (re-find #"^.*?(?=\$|$)") symbol find-ns))
the thing is the anonymous function is compiled to a class named something like
playground.core$_main$fn__181#27a0a5a2, so it starts with the name of the actual namespace the function gets compiled in.
Can't say it looks any less hacky, then your variant, still it avoids the side effect, introduced by def in your case.
Interesting question. I would never have guessed that your code would output user for the first println statement.
The problem is that only the Clojure compiler knows the name of an NS, and that is only when a source file is being compiled. This information is lost before any functions in the NS are called at runtime. That is why we get user from the code: apparently lein calls demo.core/-main from the user ns.
The only way to save the NS information so it is accessible at runtime (vs compile time) is to force an addition to the NS under a known name, as you did with your def in the macro. This is similar to Sean's trick (from Carcingenicate's link):
(def ^:private my-ns *ns*) ; need to paste this into *each* ns
The only other approach I could think of was to somehow get the Java call stack, so we could find out who called our "get-ns" function. Of course, Java provides a simple way to examine the call stack:
(ns demo.core
(:use tupelo.core)
(:require
[clojure.string :as str]))
(defn caller-ns-func []
(let [ex (RuntimeException. "dummy")
st (.getStackTrace ex)
class-names (mapv #(.getClassName %) st)
class-name-this (first class-names)
class-name-caller (first
(drop-while #(= class-name-this %)
class-names))
; class-name-caller is like "tst.demo.core$funky"
[ns-name fn-name] (str/split class-name-caller #"\$")]
(vals->map ns-name fn-name)))
and usage:
(ns tst.demo.core
(:use demo.core tupelo.core tupelo.test)
(:require
[clojure.string :as str]
[demo.core :as core]))
(defn funky [& args]
(spyx (core/caller-ns-func)))
(dotest
(funky))
with result:
(core/caller-ns-func) => {:ns-name "tst.demo.core", :fn-name "funky"}
And we didn't even need a macro!
There are some functions that read source code of a function like: source and source-fn.
Is there any way or function that returns source code of the Clojure file when a namespace is provided?
Such as: (all-source 'my-ns)
Returns such as:
(ns my-ns
(:require [kezban.core :refer :all]
[leiningen.c.util :as util]))
(defn my-fn
[]
)
...
I think I found a way(it works if the ns has at least one var):
(defn source-clj
[ns]
(require ns)
(some->> ns
ns-publics
vals
first
meta
:file
(.getResourceAsStream (RT/baseLoader))
IOUtils/toString))
I am trying to do the following:
(ns ns-test.core
(:use [ns-test.a :as a]
[ns-test.b :as b]))
(def test-map {:key "a"})
(defmulti print-ns :key)
(defmethod print-ns "a" [input-map]
(a/foo input-map))
(defmethod print-ns "b" [input-map]
(b/foo input-map))
with namespaces a and b that look like this:
(ns ns-test.a)
(defn foo [x]
(println x "I'm in namespace A."))
and
(ns ns-test.b)
(defn foo [x]
(println x "I'm in namespace B."))
but when I try to load these classes into the REPL, I get this:
user=> (use 'ns-test.core :reload)
CompilerException java.lang.IllegalStateException: foo already refers to: #'ns-test.a/foo in namespace: ns-test.core, compiling:(ns_test/core.clj:1:1)
Why does this conflict between a/foo and b/foo exist, and how can I prevent it? (Isn't the whole point of namespaces and namespace qualification to allow me to have two different functions of the same name?)
You probably wanted to :require the namespaces a and b instead of :use. :use interns the namespace symbols to the current namespace, thus the conflict.
How do I call a function in one Clojure namespace, bene-csv.core from another namespace, bene-cmp.core? I've tried various flavors of :require and :use with no success.
Here is the function in bene-csv:
(defn ret-csv-data
"Returns a lazy sequence generated by parse-csv.
Uses open-csv-file which will return a nil, if
there is an exception in opening fnam.
parse-csv called on non-nil file, and that
data is returned."
[fnam]
(let [ csv-file (open-csv-file fnam)
csv-data (if-not (nil? csv-file)
(parse-csv csv-file)
nil)]
csv-data))
Here is the header of bene-cmp.core:
(ns bene-cmp.core
.
.
.
(:gen-class)
(:use [clojure.tools.cli])
(:require [clojure.string :as cstr])
(:use bene-csv.core)
(:use clojure-csv.core)
.
.
.
The calling function -- currently a stub -- in (bene-cmp.core)
defn fetch-csv-data
"This function merely loads the two csv file arguments."
[benetrak-csv-file gic-billing-file]
(let [benetrak-csv-data ret-csv-data]))
If I modify the header of bene-cmp.clj
(:require [bene-csv.core :as bcsv])
and change the call to ret-csv-data
(defn fetch-csv-data
"This function merely loads the two csv file arguments."
[benetrak-csv-file gic-billing-file]
(let [benetrak-csv-data bcsv/ret-csv-data]))
I get this error
Caused by: java.lang.RuntimeException: No such var: bcsv/ret-csv-data
So, how do I call fetch-csv-data?
Thank You.
You need to invoke the function, not just reference the var.
If you have this in your ns:
(:require [bene-csv.core :as bcsv])
Then you need to put parentheses around the namespace/alias qualified var to invoke it:
(let [benetrak-csv-data (bcsv/ret-csv-data arg)]
; stuff
)