I have two macros. The first one takes a symbol as the only parameter (because it's passed to def, which needs a symbol). The second function takes a list of symbols and should call the first with each symbol individually.
(defmacro m1 [s]
'(let [f# ... dynamic function definition ...]
(def ~s f#))
The second macro should take a list of symbols and pass them to the first, but I can't get it to work. The best I could come up with was the following:
(defmacro m2 [symbols]
`(for [s# ~symbols] (eval (read-string (str "(name.of.current.namespace/m1 " s# ")")))))
which forces the s# to be evaluated before it is passed to the first macro. It is also invoked with a list of strings, rather than a list of symbols.
This is useful for a library which I am using for which all of the functions in the library takes the same two first parameters. I am trying to create wrapper functions, in my namespace, for some of the functions, which automatically provides the first two parameter values which are common to all of them.
Any ideas to improve this?
Usually when you ask how to get two macros to cooperate, the answer is to not make them both macros. I think my blog post on macro-writing macros will help clarify. For this particular situation I'd probably combine two suggestions from the comments:
(defmacro build-simpler-functions [& names]
(cons 'do
(for [name names]
`(def ~(symbol (str "simple-" name))
(partial ~name 5 10))))) ; if you always pass 5 and 10
(build-simpler-functions f1 f2)
This expands to
(do
(def simple-f1 (clojure.core/partial f1 5 10))
(def simple-f2 (clojure.core/partial f2 5 10)))
which looks like basically what you want.
Edit: if the args you "always" pass are different for each function, you can do this:
(defmacro build-simpler-functions [& names]
(cons 'do
(for [[name & args] names]
`(def ~(symbol (str "simple-" name))
(partial ~name ~#args)))))
(build-simpler-functions [f1 5 10] [f2 "Yo dawg"]) ; expansion similar to the previous
Related
I want to make a macro for binding variables to values given a var-list and a val-list.
This is my code for it -
(defmacro let-bind (vars vals &body body)
`(let ,(loop for x in vars
for y in vals
collect `(,x ,y))
,#body))
While it works correct if called like (let-bind (a b) (1 2) ...), it doesn't seem to work when called like
(defvar vars '(a b))
(defvar vals '(1 2))
(let-bind vars vals ..)
Then I saw some effects for other of my macros too. I am a learner and cannot find what is wrong.
Basic problem: a macro sees code, not values. A function sees values, not code.
CL-USER 2 > (defvar *vars* '(a b))
*VARS*
CL-USER 3 > (defvar *vals* '(1 2))
*VALS*
CL-USER 4 > (defmacro let-bind (vars vals &body body)
(format t "~%the value of vars is: ~a~%" vars)
`(let ,(loop for x in vars
for y in vals
collect `(,x ,y))
,#body))
LET-BIND
CL-USER 5 > (let-bind *vars* *vals* t)
the value of vars is: *VARS*
Error: *VARS* (of type SYMBOL) is not of type LIST.
1 (abort) Return to top loop level 0.
You can see that the value of vars is *vars*. This is a symbol. Because the macro variables are bound to code fragments - not their values.
Thus in your macro you try to iterate over the symbol *vars*. But *vars* is a symbol and not a list.
You can now try to evaluate the symbol *vars* at macro expansion time. But that won't work also in general, since at macro expansion time *vars* may not have a value.
Your macro expands into a let form, but let expects at compile time real variables. You can't compute the variables for let at a later point in time. This would work only in some interpreted code where macros would be expanded at runtime - over and over.
If you’ve read the other answers then you know that you can’t read a runtime value from a compiletime macro (or rather, you can’t know the value it will have at runtime at compiletime as you can’t see the future). So let’s ask a different question: how can you bind the variables in your list known at runtime.
In the case where your list isn’t really variable and you just want to give it a single name you could use macroexpand:
(defun symbol-list-of (x env)
(etypecase x
(list x)
(symbol (macroexpand x env))))
(defmacro let-bind (vars vals &body body &environment env)
(let* ((vars (symbol-list-of vars env))
(syms (loop for () in vars collect gensym)))
`(destructuring-bind ,syms ,vals
(let ,(loop for sym in syms for bar in vars collect (list var sym)) ,#body))))
This would somewhat do what you want. It will symbol-macroexpand the first argument and evaluate the second.
What if you want to evaluate the first argument? Well we could try generating something that uses eval. As eval will evaluate in the null lexical environment (ie can’t refer to any external local variables), we would need to have eval generate a function to bind variables and then call another function. That is a function like (lambda (f) (let (...) (funcall f)). You would evaluate the expression to get that function and then call it with a function which does he body (but was not made by eval and so captures the enclosing scope). Note that this would mean that you could only bind dynamic variables.
What if you want to bind lexical variables? Well there is no way to go from symbol to the memory location of a variable at runtime in Common Lisp. A debugger might know how to do this. There is no way to get a list of variables in scope in a macro, although the compiler knows this. So you can’t generate a function to set a lexically bound symbol. And it would be even harder to do if you wanted to shadow the binding although you could maybe do it with some symbol-macrolet trickery if you knew every variable in scope.
But maybe there is a better way to do this for special variables and it turns out there is. It’s an obscure special form called progv. It has the same signature that you want let-bind to have except it works. link.
Imagine the following code to dynamically create a macro:
(def a (list '+ 1 2))
(def b (list '- 10 5))
(def c (list '/ 22 2))
(defmacro gg [h]
(let [k# `~h]
k#))
The intent is to pass a vector of symbols to a macro, do some evaluation on each element of the vector such that it returns a nice macro-esque form, then have the macro combine them into a nice macro and evaluate it. The above example works all except the actual evaluation.
When I run it I get:
(gg [a b c])
=> [(+ 1 2) (- 10 5) (/ 22 2)]
What is the secret to passing a symbol that is a list of symbols and getting a macro to evaluate them? I have tried lots of combinations of quoting and have yet to hit the right one.
The real purpose of this question is to build an Archimedes Ogre query based on a definition of a path through the graph. If someone has an example of that, I would be grateful.
EDIT:
(defmacro gg2 [h]
`(do ~#(map identity h)))
(macroexpand '(gg2 [a b c]))
=> (do a b c)
(gg2 [a b c])
=> (/ 22 2)
I was hoping to get 11 rather than the form.
You don't need a macro. Macros don't do what you're looking for here. What you are looking for is eval.
(def a '/)
(def b 22)
(def c 2)
(eval (list* [a b c]))
=> 11
Of course, you can write a macro which expands into (eval (list* ...)) if you want. It could just as well be a function though.
This is a very common mistake when starting out with macros; trying to write a macro which depends on the run-time value of its arguments. Macros run at compile-time, and generally the values of the symbols which you pass to a macro are not yet available when the macro is expanded.
About the use of eval, some cautions are in order. No one said it better than Paul Graham:
Generally it is not a good idea to call eval at runtime, for two reasons:
It’s inefficient: eval is handed a raw list, and either has to compile it on the spot, or evaluate it in an interpreter. Either way is slower than compiling the code beforehand, and just calling it.
It’s less powerful, because the expression is evaluated with no lexical context. Among other things, this means that you can’t refer to ordinary variables visible outside the expression being evaluated.
Usually, calling eval explicitly is like buying something in an airport gift-shop. Having waited till the last moment, you have to pay high prices for a limited selection of second-rate goods.
I have a program that takes as inputs a chunk of data and a list of rules, applying both a set of standard rules and the rules given as input to the chunk of data. The size of both inputs may vary.
I want to be able to write a list of rules like this:
(rule-generating-macro
(rule-1-name rule-1-target
(rule-action-macro (progn actions more-actions)))
(rule-2-name rule-2-target
(rule-action-macro (or (action-2) (default-action))))
;; more rules
)
Right now, rules are more verbose -- they look more like
(defvar rule-list
`((rule-1-name rule-1-target
,#(rule-action-macro (progn actions more-actions)))
(rule-2-name rule-2-target
,#(rule-action-macro (or (action-2) (default-action))))
;; more rules
)
The latter form looks uglier to me, but I can't figure out how to write a macro that can handle a variable-length &rest argument, iterate over it, and return the transformed structure. Using a defun instead of a defmacro isn't really on the table because (as hopefully the example shows) I'm trying to control evaluation of the list of rules instead of evaluating the list when my program first sees it, and once you need to control evaluation, you're in defmacro territory. In this case, the thorny point is the rule-action-macro part - getting the interpreter to read that and use its expanded value has been problematic.
How can I create a macro that handles a variable-length argument so that I can write rule lists in a concise way?
defmacro will happily accept a &rest argument
(see Defining Macros for Emacs Lisp and Macro Lambda Lists for Common Lisp).
Then you can do pretty much anything you want with it in the macro body - e.g., iterate over it. Remember, macro is much more than just backquote!
E.g.:
(defmacro multidefvar (&rest vars)
(let ((forms (mapcar (lambda (var) `(defvar ,var)) vars)))
`(progn ,#forms)))
(macroexpand '(multidefvar a b c d))
==> (PROGN (DEFVAR A) (DEFVAR B) (DEFVAR C) (DEFVAR D))
This question is a based on a limitation of this answer.
If I have a macro that uses splicing unquote like this:
(defmacro instantiate [klass values]
`(new ~klass ~#values))
It will only work if values is a literal sequence or seq-able.
If it is passed a var holding a sequence like:
(def v [1 2 3])
(macroexpand '(instantiate Person v))
Then the output would be an error indicating that v is not a sequence.
Even a function call would be interpreted as a list:
(defn vf [] [1 2 3])
(macroexpand '(instantiate Person (vf)))
user=>(new Person vf)
My question is: Is there any way to use the splicing unquote in Clojure macros in those two cases where the sequence to be spliced isn't a literal?
Macros receive their arguments unevaluated, so the behavior you are seeing is as intended.
Macros are expanded at compile time, not run time. The values of any variables passed into a macro may not be available at compile time, so dirty hacks like using eval will not work in the general case. Don't create macros that require such tricks.
splicing quotes save a lot of time in almost all cases, except when they dont work, then you need to do things the old fashioned way...
(defmacro instantiate [klass values]
`(new ~klass ~#values))
could become
(defmacro instantiate [klass values]
(concat (list 'new klass) (if (seq? values)
values
(list values))))
user=> (macroexpand '(instantiate asdf (1 2 3)))
(new asdf 1 2 3)
user=> (macroexpand '(instantiate asdf 1))
(new asdf 1)
I am trying to implement a huge Java interface with numerous (~50) getter and setter methods (some with irregular names). I thought it would be nice to use a macro to reduce the amount of code. So instead of
(def data (atom {:x nil}))
(reify HugeInterface
(getX [this] (:x #data))
(setX [this v] (swap! data assoc :x v)))
I want to be able to write
(def data (atom {:x nil}))
(reify HugeInterface
(set-and-get getX setX :x))
Is this set-and-get macro (or something similar) possible? I haven't been able to make it work.
(Updated with a second approach -- see below the second horizontal rule -- as well as some explanatory remarks re: the first one.)
I wonder if this might be a step in the right direction:
(defmacro reify-from-maps [iface implicits-map emit-map & ms]
`(reify ~iface
~#(apply concat
(for [[mname & args :as m] ms]
(if-let [emit ((keyword mname) emit-map)]
(apply emit implicits-map args)
[m])))))
(def emit-atom-g&ss
{:set-and-get (fn [implicits-map gname sname k]
[`(~gname [~'this] (~k #~(:atom-name implicits-map)))
`(~sname [~'this ~'v]
(swap! ~(:atom-name implicits-map) assoc ~k ~'v))])})
(defmacro atom-bean [iface a & ms]
`(reify-from-maps ~iface {:atom-name ~a} ~emit-atom-g&ss ~#ms))
NB. that the atom-bean macro passes the actual compile-time value of emit-atom-g&ss on to reify-from-maps. Once a particular atom-bean form is compiled, any subsequent changes to emit-atom-g&ss have no effect on the behaviour of the created object.
An example macroexpansion from the REPL (with some line breaks and indentation added for clarity):
user> (-> '(atom-bean HugeInterface data
(set-and-get setX getX :x))
macroexpand-1
macroexpand-1)
(clojure.core/reify HugeInterface
(setX [this] (:x (clojure.core/deref data)))
(getX [this v] (clojure.core/swap! data clojure.core/assoc :x v)))
Two macroexpand-1s are necessary, because atom-bean is a macro which expands to a further macro call. macroexpand would not be particularly useful, as it would expand this all the way to a call to reify*, the implementation detail behind reify.
The idea here is that you can supply an emit-map like emit-atom-g&ss above, keyed by keywords whose names (in symbolic form) will trigger magic method generation in reify-from-maps calls. The magic is performed by the functions stored as functions in the given emit-map; the arguments to the functions are a map of "implicits" (basically any and all information which should be accessible to all method definitions in a reify-from-maps form, like the name of the atom in this particular case) followed by whichever arguments were given to the "magic method specifier" in the reify-from-maps form. As mentioned above, reify-from-maps needs to see an actual keyword -> function map, not its symbolic name; so, it's only really usable with literal maps, inside other macros or with help of eval.
Normal method definitions can still be included and will be treated as in a regular reify form, provided keys matching their names do not occur in the emit-map. The emit functions must return seqables (e.g. vectors) of method definitions in the format expected by reify: in this way, the case with multiple method definitions returned for one "magic method specifier" is relatively simple. If the iface argument were replaced with ifaces and ~iface with ~#ifaces in reify-from-maps' body, multiple interfaces could be specified for implementation.
Here's another approach, possibly easier to reason about:
(defn compile-atom-bean-converter [ifaces get-set-map]
(eval
(let [asym (gensym)]
`(fn [~asym]
(reify ~#ifaces
~#(apply concat
(for [[k [g s]] get-set-map]
[`(~g [~'this] (~k #~asym))
`(~s [~'this ~'v]
(swap! ~asym assoc ~k ~'v))])))))))
This calls on the compiler at runtime, which is somewhat expensive, but only needs to be done once per set of interfaces to be implemented. The result is a function which takes an atom as an argument and reifies a wrapper around the atom implementing the given interfaces with getters and setters as specified in the get-set-map argument. (Written this way, this is less flexible than the previous approach, but most of the code above could be reused here.)
Here's a sample interface and a getter/setter map:
(definterface IFunky
(getFoo [])
(^void setFoo [v])
(getFunkyBar [])
(^void setWeirdBar [v]))
(def gsm
'{:foo [getFoo setFoo]
:bar [getFunkyBar setWeirdBar]})
And some REPL interactions:
user> (def data {:foo 1 :bar 2})
#'user/data
user> (def atom-bean-converter (compile-atom-bean-converter '[IFunky] gsm))
#'user/atom-bean-converter
user> (def atom-bean (atom-bean-converter data))
#'user/atom-bean
user> (.setFoo data-bean 3)
nil
user> (.getFoo atom-bean)
3
user> (.getFunkyBar data-bean)
2
user> (.setWeirdBar data-bean 5)
nil
user> (.getFunkyBar data-bean)
5
The point is reify being a macro itself which is expanded before your own set-and-get macro - so the set-and-get approach doesn't work. So, instead of an inner macro inside reify, you need a macro on the "outside" that generates the reify, too.
Since the trick is to expand the body before reify sees it, a more general solution could be something along these lines:
(defmacro reify+ [& body]
`(reify ~#(map macroexpand-1 body)))
You can also try to force your macro to expand first:
(ns qqq (:use clojure.walk))
(defmacro expand-first [the-set & code] `(do ~#(prewalk #(if (and (list? %) (contains? the-set (first %))) (macroexpand-all %) %) code)))
(defmacro setter [setterf kw] `(~setterf [~'this ~'v] (swap! ~'data assoc ~kw ~'v)))
(defmacro getter [getterf kw] `(~getterf [~'this] (~kw #~'data)))
(expand-first #{setter getter}
(reify HugeInterface
(getter getX :x)
(setter setX :x)))