Let's say I have a module "foo.rkt" which exports a struct foo, e.g.,
#lang racket (provide foo) (struct foo ())
In another module, I use "foo.rkt" but I'd also like to associate the binding to "struct foo" to another namespace (I don't use prefabs for various reasons, so I can't use namespace-require).
I thought that I could use namespace-attach-module as follows:
(define ns (make-base-namespace))
(namespace-attach-module (current-namespace) "foo.rkt" ns)
(eval '(foo) ns)
But that does not work, as namespace-mapped-symbols reveals that s is not bound in ns (if this is the only place to look for bindings). It does however work in the REPL. Why?
I assume the problem is to avoid instantiating the module in "foo.rkt" twice, since this leads to two incompatible structure definitions.
The function namespace-attach-module is part of the puzzle, but it only attaches
the instantiated module to the namespace ns - that is the name "foo.rkt" is now associated with the correct instantiation of "foo.rkt". It however doesn't make the bindings available in ns - that is the job of namespace-require.
Here is an example:
File: "computer.rkt"
#lang racket
(provide (struct-out computer))
(struct computer (name price) #:transparent)
File: "use-computer.rkt"
#lang racket
(require "computer.rkt") ; instatiate "computer.rkt"
(define ns (make-base-namespace))
(namespace-attach-module (current-namespace) "computer.rkt" ns) ; ns now knows the same instance
(define a-computer
(parameterize ([current-namespace ns])
(namespace-require "computer.rkt")
(eval '(computer "Apple" 2000) ns)))
(computer-name a-computer) ; works, since ns used the same instantiation of "computer.rkt"
The result of running this is:
"Apple"
Note that removing the namespace-attach-module line leads to the error:
computer-name: contract violation;
given value instantiates a different structure type with the same name
expected: computer?
given: (computer "Apple" 2000)
since without the attachment, the namespace-require will instatiate "computer.rkt" a second time leading to two incompatible structures begin declared.
Related
Common Lisp newbie here. I am having trouble understanding parameter passing in Lisp functions. For example, imagine the following function definition in Common Lisp (say, SBCL):
(defun foo (x <&-keyword goes here> args)
(let ((v (make-hash-table args)))
(setf (gethash "foo" v) x)
v))
My question is: is there a &-keyword I could specify to pass keyword arguments in foo to make-hash-table in this situation? I already tried &rest as a &-keyword, and it always gives me the following warning:
; caught WARNING:
; The function MAKE-HASH-TABLE is called with odd number of keyword arguments.
I also read about &allow-other-keys as a possible &-keyword, yet don't seem to find how can the other keys be used by make-hash-table within foo. Thus, I'm stumped. Can keyword parameters be passed to internal function calls in Common Lisp? If so, how?
What you are looking for is apply:
(defun foo (x &rest mht-arguments)
(let ((v (apply #'make-hash-table mht-arguments)))
(setf (gethash "foo" v) x)
v))
See also my other answer on how to combine that with &key.
I'm trying to create a Clojure function, that returns another function with a custom name. My attempts so far:
(defn function-with-custom-name [name] (fn name [] 42))
(function-with-custom-name "hello")
# --> #object[lang.main$function_with_custom_name$name__4660 0xa6afefa "lang.main$function_with_custom_name$name__4660#a6afefa"]
# syntactically ok, but its name is 'name' and not 'hello'
(defn function-with-custom-name [name] (fn (symbol name) [] 42))
# --> CompilerException java.lang.IllegalArgumentException: Parameter declaration symbol should be a vector, compiling:(/tmp/form-init3365336074265515078.clj:1:40)
(defn function-with-custom-name [name] (fn '(symbol name) [] 42))
# --> CompilerException java.lang.IllegalArgumentException: Parameter declaration quote should be a vector, compiling:(/tmp/form-init3365336074265515078.clj:1:40)
I understand that fn is a macro, and therefore proper quoting is probably important for the parameter, but as per above, I could not get it right, but I'm 99% sure there is a way, since (looking at the source of fn), the only criteria is that the first parameter should be recognized as a symbol.
Any hints on how to do this?
EDIT: Use-case, as asked in the comment: I'm writing a simple language interpreter in Clojure, which (among other things) lets you create functions. The functions from my language are currently represented by anonymous Clojure functions. However, it would make debugging the interpreter much easier, if the Clojure functions also did have a name.
EDIT2: The first edit made me think, and I came to the conclusion that I cannot use macro-based solutions for this problem, since I need to create the functions run-time (and, as far as I remember, macros can only work at compile-time). --> Changed the question title for clarity. Still, please don't delete the macro-based answers, since they give helpful insight.
You can use defmacro.
(defmacro function-with-custom-name [name]
`(fn ~(symbol name) ([] 42)))
you can also do it in runtime, without using macros, using namespace functions instead. It can give you the way to register functions from some input for example (i can't really find any good reason for this though, but maybe it's just me)
user> (defn runtime-defn [f-name f]
(intern (ns-name *ns*) (symbol f-name) f))
#'user/runtime-defn
user> (runtime-defn "my-fun" #(* 100 %))
#'user/my-fun
user> (my-fun 123)
;;=> 12300
user> (runtime-defn (read) #(* 200 %))
#input "danger!!!"
#'user/danger!!!
user> (danger!!! 1)
;;=> 200
Update:
For the simple version, you can use defmacro. For a more complicated version (such as used by the potemkin library) you need to mess around with creating a var and "intern-ing" it into the clojure namespace. This is accomplished via clojure.core/intern:
(ns tst.demo.core
(:use demo.core tupelo.test )
(:require [tupelo.core :as t] ))
(t/refer-tupelo)
(defmacro make-fn-macro-unquote [name]
(spyxx name)
`(defn ~name [] 42))
(defn make-fn-func-quote [name2]
(spyxx name2)
(intern 'tst.demo.core (symbol name2) (fn [] 43)))
(dotest
(make-fn-macro-unquote fred)
(spyxx fred)
(is= 42 (spyx (fred)))
(let [wilma-var (make-fn-func-quote "wilma")]
(spyxx wilma-var)
(is= 43 (spyx (wilma-var)))))
Look at the output:
name => clojure.lang.Symbol->fred
fred => tst.demo.core$fn__38817$fred__38818->#object[tst.demo.core$fn__38817$fred__38818 0x5f832a1 "tst.demo.core$fn__38817$fred__38818#5f832a1"]
(fred) => 42
name2 => java.lang.String->"wilma"
wilma-var => clojure.lang.Var->#'tst.demo.core/wilma
(wilma-var) => 43
Note that fred is a clojure function, while wilma-var is a clojure var. Please see this post on the relationship between a symbol like foo and a var and a function.
Note also that the macro version takes an unquoted symbol fred as input, while the function version takes a plain string in double-quotes as input.
So fred is a symbol pointing to a function, while wilma-var is a symbol pointing to a var (which then points to a function). In either case, Clojure allows us to type (fred) or (wilma-var) to make a function call, and we get either 42 or 43 as a result.
I want to get a list of public functions from another namespace so they can be exposed as commands.
A similar question for Clojure seemed close, but does not seem to work on ClojureScript.
Another question has answers for Clojurescript, but they either only show how to print them to the REPL, or return all members instead of only the publicly exposed ones.
ClojureScript does not have a resolve function. It is possible to mimic the behavior using hacks, e.g., this one:
(defn ->js [var-name]
(-> var-name
(clojure.string/replace #"/" ".")
(clojure.string/replace #"-" "_")))
(defn invoke [function-name & args]
(let [f (js/eval (->js function-name))]
(apply f args)))
The second question you linked has an answer that refers to the clojure.repl/dir function which prints
a sorted directory of public vars in a namespace.
If you can print them, you can turn them into a string with with-out-str.
Now let's assume we have a namespace called demo.core with one public function called add:
(ns demo.core)
(defn add [a b]
(println "hello from add")
(+ a b))
We can retrieve the public fns from demo.core as strings as follows:
(defn public-fns
"Returns coll of names of public fns in demo.core"
[]
(as-> (with-out-str (clojure.repl/dir demo.core)) public-fns
(clojure.string/split public-fns #"\n")
(map (fn [s] (str "demo.core/" s)) public-fns)))
So with with-out-str we turn them into a list of strings, then split on newline, then prepend the names of public functions with "demo.core".
Then, using our earlier created invoke function, we can obtain add and invoke it with the arguments 1 and 2:
(invoke (first (public-fns)) 1 2)
;; "hello from add"
;; => 3
This is all very hacky, and it might break in advanced compilation, but it works.
Is it possible to bind the this context in ClojureScript?
Right now, I am stuck passing in this to a higher-order function, as in:
(defn generateTransactFunction [this]
(fn [item] (do stuff with this and item)))
This does not feel optimal! I am just learning ClojureScript, so I assume there's something I am missing.
EDIT:
Looks like partial can do the job, as in:
(defn abc [this arg1 arg2] ())
and passing
(partial abc this)
As you mentioned you can use partial if your this parameter at the beginning of the function's parameter list and remaining params will be bound later.
For cases where your this argument position prevents you from using partial you can use anonymous function literal which will be more consise than function literal ((fn [args...] body)):
(defn abc [arg1 arg2 this] ...)
(do-sth #(abc %1 %2 this))
In Clojure, given a class name as a string, I need to create a new instance of the class. In other words, how would I implement new-instance-from-class-name in
(def my-class-name "org.myorg.pkg.Foo")
; calls constructor of org.myorg.pkg.Foo with arguments 1, 2 and 3
(new-instance-from-class-name my-class-name 1 2 3)
I am looking for a solution more elegant than
calling the Java newInstance method on a constructor from the class
using eval, load-string, ...
In practice, I will be using it on classes created using defrecord. So if there is any special syntax for that scenario, I would be quite interested.
There are two good ways to do this. Which is best depends on the specific circumstance.
The first is reflection:
(clojure.lang.Reflector/invokeConstructor
(resolve (symbol "Integer"))
(to-array ["16"]))
That's like calling (new Integer "16") ...include any other ctor arguments you need in the to-array vector. This is easy, but slower at runtime than using new with sufficient type hints.
The second option is as fast as possible, but a bit more complicated, and uses eval:
(defn make-factory [classname & types]
(let [args (map #(with-meta (symbol (str "x" %2)) {:tag %1}) types (range))]
(eval `(fn [~#args] (new ~(symbol classname) ~#args)))))
(def int-factory (make-factory "Integer" 'String))
(int-factory "42")
The key point is to eval code that defines an anonymous function, as make-factory does. This is slow -- slower than the reflection example above, so only do it as infrequently as possible such as once per class. But having done that you have a regular Clojure function that you can store somewhere, in a var like int-factory in this example, or in a hash-map or vector depending on how you'll be using it. Regardless, this factory function will run at full compiled speed, can be inlined by HotSpot, etc. and will always run much faster than the reflection example.
When you're specifically dealing with classes generated by deftype or defrecord, you can skip the type list since those classes always have exactly two ctors each with different arities. This allows something like:
(defn record-factory [recordname]
(let [recordclass ^Class (resolve (symbol recordname))
max-arg-count (apply max (map #(count (.getParameterTypes %))
(.getConstructors recordclass)))
args (map #(symbol (str "x" %)) (range (- max-arg-count 2)))]
(eval `(fn [~#args] (new ~(symbol recordname) ~#args)))))
(defrecord ExampleRecord [a b c])
(def example-record-factory (record-factory "ExampleRecord"))
(example-record-factory "F." "Scott" 'Fitzgerald)
Since 'new' is a special form, I'm not sure there you can do this without a macro. Here is a way to do it using a macro:
user=> (defmacro str-new [s & args] `(new ~(symbol s) ~#args))
#'user/str-new
user=> (str-new "String" "LOL")
"LOL"
Check out Michal's comment on the limitations of this macro.
Here is a technique for extending defrecord to automatically create well-named constructor functions to construct record instances (either new or based on an existing record).
http://david-mcneil.com/post/765563763/enhanced-clojure-records
In Clojure 1.3, defrecord will automatically defn a factory function using the record name with "->" prepended. Similarly, a variant that takes a map will be the record name prepended with "map->".
user=> (defrecord MyRec [a b])
user.MyRec
user=> (->MyRec 1 "one")
#user.MyRec{:a 1, :b "one"}
user=> (map->MyRec {:a 2})
#user.MyRec{:a 2, :b nil}
A macro like this should work to create an instance from the string name of the record type:
(defmacro newbie [recname & args] `(~(symbol (str "->" recname)) ~#args))