I have been looking at the source for defmacro which uses "let" in its definition:
(def
^{:doc "Like defn, but the resulting function name is declared as a
macro and will be used as a macro by the compiler when it is
called."
:arglists '([name doc-string? attr-map? [params*] body]
[name doc-string? attr-map? ([params*] body)+ attr-map?])
:added "1.0"}
defmacro (fn [&form &env
name & args]
(let [prefix (loop [p (list name) args args]
However, "let" is defined as a macro itself:
(defmacro let
"binding => binding-form init-expr
Evaluates the exprs in a lexical context in which the symbols in
the binding-forms are bound to their respective init-exprs or parts
therein."
{:added "1.0", :special-form true, :forms '[(let [bindings*] exprs*)]}
[bindings & body]
(assert-args
(vector? bindings) "a vector for its binding"
(even? (count bindings)) "an even number of forms in binding vector")
`(let* ~(destructure bindings) ~#body))
Can someone explain how this works as I can't understand how "defmacro" can be defined in terms of things which need "defmacro" to already be defined. (if that makes sense :)
This is possible because before defining defmacro function in core.clj there is already a definition of let at this location (which gets redefined later). Macros are just normal functions and the var they are bound to has meta data key :macro with value true so that at compile time the compiler can differentiate between a macro (which execute at compile time) with a function, without this meta key there is no way to differentiate between a macro and a function because macro itself is a function that happens to process S-expressions.
recusrive macros work fine and occur in many place in both the clojure language core and in other programs. macros are just functions that return S-Expressions, so they can be recursive just as functions can. In the case of let in your example it's actually caling let* which is a different function (its fine to have * in a functions name), so although recursive macros are fine, this doesn't happen to be an example of them
Related
My goal is to create a macro such that (defdummy MyProtocol MyImplementation) will create a record type as defrecord does. It would also be acceptable to just generate an implementation of MyProtocol like reify does. This record type should implement all methods of the given protocol, but just return nil for each method. e.g:
(defprotocol Annoying
(beep [x] "Make a sound"))
;; This form...
(defdummy Annoying FakeAnnoyer)
;; Should expand to this
(defrecord FakeAnnoyer []
Annoying
(beep [x] nil))
I see that when we evaluate the protocol, we have some methods that can be used to generate the implementations of the protocol methods: :sigs, :arglists, etc. However, because macros receive their arguments unevaluated, I can't access this information from inside the macro. If a macro user passes a protocol MyProtocol to my macro, I just see the symbol 'MyProtocol. It seems to me that this information is statically known at compile time, but I don't know if I can find it out when generating protocol methods.
The only way that I can think of is either to use eval or to expand to some of the inner workings of defrecord. Is there any other way?
You can use the resolve function. For instance, here is a basic implementation of your defdummy macro:
(defmacro defdummy [protocol record]
`(defrecord ~record []
~protocol
~#(for [[_ {:keys [name arglists]}] (:sigs #(resolve protocol))]
`(~name ~#arglists nil))))
Example:
(defprotocol Annoying
(beep [x] "Make a sound"))
(macroexpand-1 '(defdummy Annoying FakeAnnoyer))
;;=> (clojure.core/defrecord FakeAnnoyer [] Annoying (beep [x] nil))
If you run the following it only prints "foo":
(if true (println "foo") (println "bar"))
;foo
However if you pass macros in as the arguments to the if, it seems to evaluate both macros immediately.
(defmacro foo
[]
(println "foo"))
(defmacro bar
[]
(println "bar"))
(if true (foo) (bar))
;foo
;bar
How do you avoid running both macros and just evaluate the one for the true condition?
EDIT:
The main source of my confusion is that this would work in common lisp. I don't know how common lisp handles compiling macros differently but I'm guessing it has to do with more freedom not being on the JVM.
To clarify the actual code was to create forms of a certain type without quoting them as arguments. The code was being iterated on and was going to get more complicated, but in the current state was along the lines of:
(defmacro foo
[arg1 arg2]
`(quote (~arg1 ~arg2 [])))
(defmacro bar
[arg1 arg2]
`(quote (~(first arg1) ~arg2 [~(second arg1)])))
(defmacro foobar
[arg1 arg2]
`(if (list? '~arg1)
(bar ~arg1 ~arg2)
(foo ~arg1 ~arg2)))
(foobar FOO "bar")
;; doesn't work since macros get compiled and you can't pass FOO as an argument to bar
(foobar (FOO "bar") "foobar")
;; (FOO "bar" ["foobar"])
I wanted both macros to not be expanded before evaluating the if since passing something other than a list to bar would cause an exception. I now understand that's not possible because expansion happens compile time. I've switched to using multimethods--which is more verbose, but more polymorphic I suppose.
Alan has the right of it (in terms of the immediate fix needed), but to speak to the reasoning:
Macros are evaluated at compile time, not runtime. It's patently impossible for something only known at runtime to control what's compiled, because all compilation is necessarily finished before runtime starts.
What you can do is make your if part of another macro, thus moving it to compile time as well.
You forgot the "syntax quote". The code should look like this:
(defmacro foo
[]
`(println "foo"))
(defmacro bar
[]
`(println "bar"))
(if true
(foo)
(bar))
;=> foo
Macros are abstraction on syntax. You use it when you see a pattern you often write, but because of argument evaluation you cannot use functions. Macros evaluate to code and there should not be side effects since you have no control over when it gets expanded (run).
foo and bar should have been functions. Then it will work as you intended.
(defn foo
[]
(println "foo"))
(defn bar
[]
(println "bar"))
(if true
(foo)
(bar)) ; prints "foo"
Using a macro when it's not needed is considered bad practice.
I'm searching a way to keep my helpers functions to the bottom of the file, without declare them all at the top.
One solution should be to write a "declare-helpers" function that grab the names of all functions create via defn- macro in the current file and wrap them in a declare call.
Here I'm asking about the better way to grab those names.
* edit *
I know this is bad practice but, the following code seems to do what I want
Note that it apply only to helpers function define with the "dehfn" macro
;define helper function
(defmacro dehfn [name & body]
`(defn- ~name ~#body))
(defmacro declare-helpers []
`(declare ~#(map symbol
(re-seq #"(?<=dehfn\s)[a-zA-Z+!\-_?0-9*~##''`/.$=]*(?=\s)"
(slurp (str "src/" *file*))))))
Now you can do that:
(declare-helpers)
(defn hello-user [name] (greet name))
(dehfn greet [name] (str "Hello my dear " name))
This is not possible. No macro can know about code written later in the file than the macro invocation, since there are no vars for it to inspect yet. Just practice reading files "upside down": Clojure is not the only language in which the public and/or important stuff is often at the bottom.
I am trying to dynamically create functions based on some static public fields of a Java class. So in one file I have something like:
(intern *ns* (symbol (get-fieldname-from-class class)) some-func)
The problem is that I want to call that particular function while it isn't defined yet.
For example, the Java class has a static int PARENTHESIZED_EXPRESSION field. From this I generate a parenthesized-expression? function. This works quite nice, but when I load a Clojure file in the REPL that uses this functions I get an
unable to resolve parenthesized-expression?
error. So I have to make sure that the symbol's are interned first. How can I do this?
I realize this is not a very functional approach, but I'm a little bit hesitant to enter almost 80 similar functions for all the fields of this class. Besides, this class might be subject to change.
I wonder if you could get by using a macro like this:
user=> (defmacro f [sym] `(defn ~(symbol (str sym "?")) [x#] (= x# ~(symbol (str "java.util.Calendar/" (str sym))))))
#'user/f
user=> (f DAY_OF_MONTH)
#'user/DAY_OF_MONTH?
user=> (DAY_OF_MONTH? java.util.Calendar/DAY_OF_WEEK)
false
user=> (DAY_OF_MONTH? java.util.Calendar/DAY_OF_MONTH)
true
user=>
On a particular namespace I am working on I am beginning to run out of function names. Is there a way to get a warning like the one I get if I override a symbol from another namespace if I reuse a symbol which is already bound to a function in the same namespace?
If this is enough of a problem that you'd be willing to replace a (set of) core macro(s), you could try this approach:
(ns huge.core
(:refer-clojure :exclude [defn]))
(defmacro defn [name & defn-tail]
(assert (nil? (resolve name))
(str "Attempting to redefine already defined Var "
"#'" (.name *ns*) "/" name))
`(clojure.core/defn ~name ~#defn-tail))
Then any attempt to redefine an existing Var with defn will fail:
user=> (defn foo [] :foo)
#'user/foo
user=> (defn foo [] :bar)
AssertionError Assert failed: Attempting to redefine already defined Var #'user/foo
(nil? (resolve name)) user/defn (NO_SOURCE_FILE:2)
You could similarly replace defmacro; in that case you would have to call clojure.core/defmacro when defining your own variant.
Plain, unadorned def is a special form and receives magic treatment from the compiler, so you could still overwrite existing Vars with it. If you would like to guard against name clashes on that flank too, you could switch to something like defvar (used to be available in clojure.contrib.def) with a similar custom assert.
This isn't quite an answer to your question but may help avoid the issue depending on how the functions in your namespace are being used. You could make them into local functions using letfn, allowing you to reuse names for functions that are only used within the context of another function.
(defn main-fn [x]
(letfn [(secondary-fn [x] (* x x))
(another-fn [x] (secondary-fn (inc x)))]
(/ (another-fn x) 4)))
Even if you restrict yourself to single-character function names, you are in no danger of running out, as there are (about) 64 thousand Unicode characters, any one of which is a valid function name.
Given that you can in fact have names that are ten thousand characters long, you are on even safer ground.