Related
I'm trying to create a simple memo defun. How can I prevent evaluating of args form in this code?
(defmacro defun/memo (name args &rest body)
`(let ((memo (make-hash-table :test 'equalp)))
(defun ,name ,args
(if (gethash (loop for x in ,args collect x) memo)
(gethash (loop for x in ,args collect x) memo)
(let ((result (progn ,#body)))
(setf (gethash (loop for x in ,args collect x) memo) result)
result)))))
Error:
; in: DEFUN ADD
; (X Y)
;
; caught STYLE-WARNING:
; undefined function: X
;
; compilation unit finished
; Undefined function:
; X
(defmacro defun/memo (name args &rest body)
You generally declare body with &body body, not &rest body.
Variable capture
`(let ((memo (make-hash-table :test 'equalp)))
The memo symbol is going to end in the generated code. If body contains references to memo, for example a symbol that was lexically bound outside of a call to defun/memo, then it will use your variable. You should use a fresh symbol instead, generated inside the macro with gensym (outside of backquotes). For example, you could do the following to avoid evaluating expr twice:
(let ((var-expr (gensym)))
`(let ((,var-expr ,expr))
(+ ,var-expr ,var-expr)))
Loop
(if (gethash (loop for x in ,args collect x) memo)
(gethash (loop for x in ,args collect x) memo)
(let ((result (progn ,#body)))
(setf (gethash (loop for x in ,args collect x) memo) result)
result)))))
What is the following supposed to do?
(loop for x in ,args collect x)
Let's say you define a function with (defun/memo test (a b c) ...), you will inject the literal list of arguments in the above, which will result in code that contains:
(loop for x in (a b c) collect x)
As you saw, the code is now trying to call function a with arguments b and c.
What if you quoted args, in your macro?
(loop for x in ',args collect x)
Then, you would obtain:
(loop for x in '(a b c) collect x)
And now, you are just copying a literal list. When the above generated code is run, it will only build a fresh list (a b c). Is that what you need?
What you want is to take all the arguments of your function, i.e. the list of values you were given. The loop could be replaced by:
(list ,#args)
Which would expand as:
(list a b c)
And here you have all your values, in a list.
But Common Lisp already provides a way to get all arguments as a list:
(defun foo (&rest args)
;; args is bound to a list of values
)
Your generated function could do the same.
Gethash
Also, (if (gethash ...) (gethash ...) other) can be written (or (gethash ...) other). This has the benefits of evaluating the call to gethash only once.
More importantly (thanks #Sylwester), since you are writing a generic macro, you cannot know in advance if nil will be a possible returned value. Having a nil value would make the result recomputed each time, given how the if/or is written. You need to use the secondary return value from gethash to check if the element existed:
(multiple-value-bind (value exists-p) (gethash ...)
(if exists-p
value
(setf (gethash ...) ...)))
Also, if your cached function return multiple values, you may want to grab them all with multiple-value-list and returns them with values-list.
Setf
By the way, the following code:
(let ((result expr))
(setf place result)
result)
... has little reason not to be written as:
(setf place expr)
The return value of setf is required to be the new value. In some cases it could lead to bad style, but here that would be fine.
Often when I try to write a macro, I run up against the following difficulty: I need one form that is passed to the macro to be evaluated before being processed by a helper function that is invoked while generating the macro's expansion. In the following example, we are only interested in how we could write a macro to emit the code we want, and not in the uselessness of the macro itself:
Imagine (bear with me) a version of Common Lisp's lambda macro, where only the number of arguments is important, and the names and order of the arguments are not. Let's call it jlambda. It would be used like so:
(jlambda 2
...body)
where 2 is the arity of the function returned. In other words, this produces a binary operator.
Now imagine that, given the arity, jlambda produces a dummy lambda-list which it passes to the actual lambda macro, something like this:
(defun build-lambda-list (arity)
(assert (alexandria:non-negative-integer-p arity))
(loop for x below arity collect (gensym)))
(build-lambda-list 2)
==> (#:G15 #:G16)
The expansion of the above call to jlambda will look like this:
(lambda (#:G15 #:16)
(declare (ignore #:G15 #:16))
…body))
Let's say we need the jlambda macro to be able to receive the arity value as a Lisp form that evaluates to a non-negative integer (as opposed to receiving a non-negative integer directly) eg:
(jlambda (+ 1 1)
...body)
The form (+ 1 1) needs to be evaluated, then the result needs to be passed to build-lambda-list and that needs to be evaluated, and the result of that is inserted into the macro expansion.
(+ 1 1)
=> 2
(build-lambda-list 2)
=> (#:G17 #:18)
(jlambda (+ 1 1) ...body)
=> (lambda (#:G19 #:20)
(declare (ignore #:G19 #:20))
…body))
So here's a version of jlambda that works when the arity is provided as a number directly, but not when it's passed as a form to be evaluated:
(defun jlambda-helper (arity)
(let ((dummy-args (build-lambda-list arity)))
`(lambda ,dummy-args
(declare (ignore ,#dummy-args))
body)))
(defmacro jlambda (arity &body body)
(subst (car body) 'body (jlambda-helper arity)))
(jlambda 2 (print “hello”)) ==> #<anonymous-function>
(funcall *
'ignored-but-required-argument-a
'ignored-but-required-argument-b)
==> “hello”
“hello”
(jlambda (+ 1 1) (print “hello”)) ==> failed assertion in build-lambda-list, since it receives (+ 1 1) not 2
I could evaluate the (+ 1 1) using the sharp-dot read macro, like so:
(jlambda #.(+ 1 1) (print “hello”)) ==> #<anonymous-function>
But then the form cannot contain references to lexical variables, since they are not available when evaluating at read-time:
(let ((x 1))
;; Do other stuff with x, then:
(jlambda #.(+ x 1) (print “hello”))) ==> failure – variable x not bound
I could quote all body code that I pass to jlambda, define it as a function instead, and then eval the code that it returns:
(defun jlambda (arity &rest body)
(let ((dummy-args (build-lambda-list arity)))
`(lambda ,dummy-args
(declare (ignore ,#dummy-args))
,#body)))
(eval (jlambda (+ 1 1) `(print “hello”))) ==> #<anonymous-function>
But I can't use eval because, like sharp-dot, it throws out the lexical environment, which is no good.
So jlambda must be a macro, because I don't want the function body code evaluated until the proper context for it has been established by jlambda's expansion; however it must also be a function, because I want the first form (in this example, the arity form) evaluated before passing it to helper functions that generate the macro expansion. How do I overcome this Catch-22 situation?
EDIT
In response to #Sylwester 's question, here's an explanation of the context:
I'm writing something akin to an “esoteric programming language”, implemented as a DSL in Common Lisp. The idea (admittedly silly but potentially fun) is to force the programmer, as far as possible (I'm not sure how far yet!), to write exclusively in point-free style. To do this, I will do several things:
Use curry-compose-reader-macros to provide most of the functionality required to write in point-free style in CL
Enforce functions' arity – i.e. override CL's default behaviour that allows functions to be variadic
Instead of using a type system to determine when a function has been “fully applied” (like in Haskell), just manually specify a function's arity when defining it.
So I'll need a custom version of lambda for defining a function in this silly language, and – if I can't figure that out - a custom version of funcall and/or apply for invoking those functions. Ideally they'll just be skins over the normal CL versions that change the functionality slightly.
A function in this language will somehow have to keep track of its arity. However, for simplicity, I would like the procedure itself to still be a funcallable CL object, but would really like to avoid using the MetaObject Protocol, since it's even more confusing to me than macros.
A potentially simple solution would be to use a closure. Every function could simply close over the binding of a variable that stores its arity. When invoked, the arity value would determine the exact nature of the function application (i.e. full or partial application). If necessary, the closure could be “pandoric” in order to provide external access to the arity value; that could be achieved using plambda and with-pandoric from Let Over Lambda.
In general, functions in my language will behave like so (potentially buggy pseudocode, purely illustrative):
Let n be the number of arguments provided upon invocation of the function f of arity a.
If a = 0 and n != a, throw a “too many arguments” error;
Else if a != 0 and 0 < n < a, partially apply f to create a function g, whose arity is equal to a – n;
Else if n > a, throw a “too many arguments” error;
Else if n = a, fully apply the function to the arguments (or lack thereof).
The fact that the arity of g is equal to a – n is where the problem with jlambda would arise: g would need to be created like so:
(jlambda (- a n)
...body)
Which means that access to the lexical environment is a necessity.
This is a particularly tricky situation because there's no obvious way to create a function of a particular number of arguments at runtime. If there's no way to do that, then it's probably easiest to write a a function that takes an arity and another function, and wraps the function in a new function that requires that is provided the particular number of arguments:
(defun %jlambda (n function)
"Returns a function that accepts only N argument that calls the
provided FUNCTION with 0 arguments."
(lambda (&rest args)
(unless (eql n (length args))
(error "Wrong number of arguments."))
(funcall function)))
Once you have that, it's easy to write the macro around it that you'd like to be able to:
(defmacro jlambda (n &body body)
"Produces a function that takes exactly N arguments and and evalutes
the BODY."
`(%jlambda ,n (lambda () ,#body)))
And it behaves roughly the way you'd want it to, including letting the arity be something that isn't known at compile time.
CL-USER> (let ((a 10) (n 7))
(funcall (jlambda (- a n)
(print 'hello))
1 2 3))
HELLO
HELLO
CL-USER> (let ((a 10) (n 7))
(funcall (jlambda (- a n)
(print 'hello))
1 2))
; Evaluation aborted on #<SIMPLE-ERROR "Wrong number of arguments." {1004B95E63}>.
Now, you might be able to do something that invokes the compiler at runtime, possibly indirectly, using coerce, but that won't let the body of the function be able to refer to variables in the original lexical scope, though you would get the implementation's wrong number of arguments exception:
(defun %jlambda (n function)
(let ((arglist (loop for i below n collect (make-symbol (format nil "$~a" i)))))
(coerce `(lambda ,arglist
(declare (ignore ,#arglist))
(funcall ,function))
'function)))
(defmacro jlambda (n &body body)
`(%jlambda ,n (lambda () ,#body)))
This works in SBCL:
CL-USER> (let ((a 10) (n 7))
(funcall (jlambda (- a n)
(print 'hello))
1 2 3))
HELLO
CL-USER> (let ((a 10) (n 7))
(funcall (jlambda (- a n)
(print 'hello))
1 2))
; Evaluation aborted on #<SB-INT:SIMPLE-PROGRAM-ERROR "invalid number of arguments: ~S" {1005259923}>.
While this works in SBCL, it's not clear to me whether it's actually guaranteed to work. We're using coerce to compile a function that has a literal function object in it. I'm not sure whether that's portable or not.
NB: In your code you use strange quotes so that (print “hello”) doesn't actually print hello but the whatever the variable “hello” evaluates to, while (print "hello") does what one would expect.
My first question is why? Usually you know how many arguments you are taking compile time or at least you just make it multiple arity. Making an n arity function only gives you errors when passwd with wrong number of arguments as added feature with the drawback of using eval and friends.
It cannot be solved as a macro since you are mixing runtime with macro expansion time. Imagine this use:
(defun test (last-index)
(let ((x (1+ last-index)))
(jlambda x (print "hello"))))
The macro is expanded when this form is evaluated and the content replaced before the function is assigned to test. At this time x doesn't have any value whatsoever and sure enough the macro function only gets the symbols so that the result need to use this value. lambda is a special form so it again gets expanded right after the expansion of jlambda, also before any usage of the function.
There is nothing lexical happening since this happens before the program is running. It could happen before loading the file with compile-file and then if you load it will load all forms with the macros already expanded beforehand.
With compile you can make a function from data. It is probably as evil as eval is so you shouldn't be using it for common tasks, but they exist for a reason:
;; Macro just to prevent evaluation of the body
(defmacro jlambda (nexpr &rest body)
`(let ((dummy-args (build-lambda-list ,nexpr)))
(compile nil (list* 'lambda dummy-args ',body))))
So the expansion of the first example turns into this:
(defun test (last-index)
(let ((x (1+ last-index)))
(let ((dummy-args (build-lambda-list x)))
(compile nil (list* 'lambda dummy-args '((print "hello")))))))
This looks like it could work. Lets test it:
(defparameter *test* (test 10))
(disassemble *test*)
;Disassembly of function nil
;(CONST 0) = "hello"
;11 required arguments <!-- this looks right
;0 optional arguments
;No rest parameter
;No keyword parameters
;4 byte-code instructions:
;0 (const&push 0) ; "hello"
;1 (push-unbound 1)
;3 (calls1 142) ; print
;5 (skip&ret 12)
;nil
Possible variations
I've made a macro that takes a literal number and makes bound variables from a ... that can be used in the function.
If you are not using the arguments why not make a macro that does this:
(defmacro jlambda2 (&rest body)
`(lambda (&rest #:rest) ,#body))
The result takes any number of arguments and just ignores it:
(defparameter *test* (jlambda2 (print "hello")))
(disassemble *test*)
;Disassembly of function :lambda
;(CONST 0) = "hello"
;0 required arguments
;0 optional arguments
;Rest parameter <!-- takes any numer of arguments
;No keyword parameters
;4 byte-code instructions:
;0 (const&push 0) ; "hello"
;1 (push-unbound 1)
;3 (calls1 142) ; print
;5 (skip&ret 2)
;nil
(funcall *test* 1 2 3 4 5 6 7)
; ==> "hello" (prints "hello" as side effect)
EDIT
Now that I know what you are up to I have an answer for you. Your initial function does not need to be runtime dependent so all functions indeed have a fixed arity, so what we need to make is currying or partial application.
;; currying
(defmacro fixlam ((&rest args) &body body)
(let ((args (reverse args)))
(loop :for arg :in args
:for r := `(lambda (,arg) ,#body)
:then `(lambda (,arg) ,r)
:finally (return r))))
(fixlam (a b c) (+ a b c))
; ==> #<function :lambda (a) (lambda (b) (lambda (c) (+ a b c)))>
;; can apply multiple and returns partially applied when not enough
(defmacro fixlam ((&rest args) &body body)
`(let ((lam (lambda ,args ,#body)))
(labels ((chk (args)
(cond ((> (length args) ,(length args)) (error "too many args"))
((= (length args) ,(length args)) (apply lam args))
(t (lambda (&rest extra-args)
(chk (append args extra-args)))))))
(lambda (&rest args)
(chk args)))))
(fixlam () "hello") ; ==> #<function :lambda (&rest args) (chk args)>
;;Same but the zero argument functions are applied right away:
(defmacro fixlam ((&rest args) &body body)
`(let ((lam (lambda ,args ,#body)))
(labels ((chk (args)
(cond ((> (length args) ,(length args)) (error "too many args"))
((= (length args) ,(length args)) (apply lam args))
(t (lambda (&rest extra-args)
(chk (append args extra-args)))))))
(chk '()))))
(fixlam () "hello") ; ==> "hello"
If all you want is lambda functions that can be applied either partially or fully, I don't think you need to pass the amount of parameters explicitly. You could just do something like this (uses Alexandria):
(defmacro jlambda (arglist &body body)
(with-gensyms (rest %jlambda)
`(named-lambda ,%jlambda (&rest ,rest)
(cond ((= (length ,rest) ,(length arglist))
(apply (lambda ,arglist ,#body) ,rest))
((> (length ,rest) ,(length arglist))
(error "Too many arguments"))
(t (apply #'curry #',%jlambda ,rest))))))
CL-USER> (jlambda (x y) (format t "X: ~s, Y: ~s~%" x y))
#<FUNCTION (LABELS #:%JLAMBDA1046) {1003839D6B}>
CL-USER> (funcall * 10) ; Apply partially
#<CLOSURE (LAMBDA (&REST ALEXANDRIA.0.DEV::MORE) :IN CURRY) {10038732DB}>
CL-USER> (funcall * 20) ; Apply fully
X: 10, Y: 20
NIL
CL-USER> (funcall ** 100) ; Apply fully again
X: 10, Y: 100
NIL
CL-USER> (funcall *** 100 200) ; Try giving a total of 3 args
; Debugger entered on #<SIMPLE-ERROR "Too many arguments" {100392D7E3}>
Edit: Here's also a version that lets you specify the arity. Frankly, I don't see how this could possibly be useful though. If the user cannot refer to the arguments, and nothing is done with them automatically, then, well, nothing is done with them. They might as well not exist.
(defmacro jlambda (arity &body body)
(with-gensyms (rest %jlambda n)
`(let ((,n ,arity))
(named-lambda ,%jlambda (&rest ,rest)
(cond ((= (length ,rest) ,n)
,#body)
((> (length ,rest) ,n)
(error "Too many arguments"))
(t (apply #'curry #',%jlambda ,rest)))))))
CL-USER> (jlambda (+ 1 1) (print "hello"))
#<CLOSURE (LABELS #:%JLAMBDA1085) {1003B7913B}>
CL-USER> (funcall * 2)
#<CLOSURE (LAMBDA (&REST ALEXANDRIA.0.DEV::MORE) :IN CURRY) {1003B7F7FB}>
CL-USER> (funcall * 5)
"hello"
"hello"
Edit2: If I understood correctly, you might be looking for something like this (?):
(defvar *stack* (list))
(defun jlambda (arity function)
(lambda ()
(push (apply function (loop repeat arity collect (pop *stack*)))
*stack*)))
CL-USER> (push 1 *stack*)
(1)
CL-USER> (push 2 *stack*)
(2 1)
CL-USER> (push 3 *stack*)
(3 2 1)
CL-USER> (push 4 *stack*)
(4 3 2 1)
CL-USER> (funcall (jlambda 4 #'+)) ; take 4 arguments from the stack
(10) ; and apply #'+ to them
CL-USER> (push 10 *stack*)
(10 10)
CL-USER> (push 20 *stack*)
(20 10 10)
CL-USER> (push 30 *stack*)
(30 20 10 10)
CL-USER> (funcall (jlambda 3 [{reduce #'*} #'list])) ; pop 3 args from
(6000 10) ; stack, make a list
; of them and reduce
; it with #'*
I'm trying to understand the following two snippets of code:
(defun make-adder1 (n) `(lambda (x) (+ ,n x)))
(defun make-adder2 (n) (lexical-let ((n n)) (lambda (x) (+ n x))))
These both seem to produce callables:
(funcall (make-adder1 3) 5) ;; returns 8
(funcall (make-adder2 3) 5) ;; returns 8
These both work. I have two main questions:
1) I don't understand the disparity in "quoting level" between the two approaches. In the first case, the lambda expression is quoted, which means the "symbol itself" is returned instead of the value. In the second case, it seems like the statement with the lambda will get evaluated, so the value of the lambda will be returned. Yet, these both work with funcall. When using funcall on a defun'ed function, it has to be quoted. Is lexical-let doing some kind of quoting automatically? Isn't this, kind of surprising?
2) Reading other posts on this topic, I'm given to understand that the first approach will break down under certain circumstances and deviate from what one would expect from working with lambdas and higher order functions in other languages, because elisp has dynamic scoping by default. Can someone give a concrete example of code that makes this difference apparent and explain it?
In the first example there is no variable n in the resulting function, which is just (lambda (x) (+ 3 x)). It does not need lexical binding because there is no free variable in the lambda, i.e., no variable that needs to be kept in a binding of a closure. If you don't need the variable n to be available, as a variable in uses of the function, i.e., if its value at function definition time (=3) is all you need, then the first example is all you need.
(fset 'ad1 (make-adder1 3))
(symbol-function 'ad1)
returns:
(lambda (x) (+ 3 x))
The second example creates what is, in effect, a function that creates and applies a complicated closure.
(fset 'ad2 (make-adder2 3))
(symbol-function 'ad2)
returns
(lambda (&rest --cl-rest--)
(apply (quote (closure ((--cl-n-- . --n--) (n . 3) t)
(G69710 x)
(+ (symbol-value G69710) x)))
(quote --n--)
--cl-rest--))
A third option is to use a lexical-binding file-local variable and use the most straightforward definition. This creates a simple closure.
;;; foo.el --- toto -*- lexical-binding: t -*-
(defun make-adder3 (n) (lambda (x) (+ n x)))
(fset 'ad3 (make-adder3 3))
(symbol-function 'ad3)
returns:
(closure ((n . 3) t) (x) (+ n x))
(symbol-function 'make-adder1)
returns:
(lambda (n)
(list (quote lambda)
(quote (x))
(cons (quote +) (cons n (quote (x))))))
(symbol-function 'make-adder2)
returns:
(closure (t)
(n)
(let ((--cl-n-- (make-symbol "--n--")))
(let* ((v --cl-n--)) (set v n))
(list (quote lambda)
(quote (&rest --cl-rest--))
(list (quote apply)
(list (quote quote)
(function
(lambda (G69709 x)
(+ (symbol-value G69709) x))))
(list (quote quote) --cl-n--)
(quote --cl-rest--)))))
(symbol-function 'make-adder3)
returns
(closure (t) (n) (function (lambda (x) (+ n x))))
I'm trying different binding models for macro lambda lists.
Edit: in fact the lambda list for my test macros is always (&rest ...). Which means that I'm 'destructuring' the argument list and not the lambda list. I try to get a solution that works for combining optional with key arguments or rest/body with key arguments - both combinations don't work in the Common Lisp standard implementation.
So I have different functions giving me a list of bindings having the same syntax as used by 'let'.
E.g:
(build-bindings ...) => ((first 1) middle (last "three"))
Now I thought to use a simple macro inside my test macros feeding such a list to 'let'.
This is trivial if I have a literal list:
(defmacro let-list (_list &rest _body)
`(let ,_list ,#_body))
(let-list ((a 236)) a) => 236
But that's the same as a plain 'let'.
What I'd like to have is the same thing with a generated list.
So e.g.
(let-list (build-bindings ...)
(format t "first: ~s~%" first)
last)
with (build-bindings ...), evaluated in the same lexical scope as the call (let-list ...), returning
((first 1) middle (last "three"))
the expansion of the macro should be
(let
((first 1) middle (last "three"))
(format t "first: ~s~%" first)
last)
and should print 1 and return "three".
Any idea how to accomplish that?
Edit (to make the question more general):
If I have a list of (symbol value) pairs, i.e. same syntax that let requires for it's list of bindings, e.g. ((one 1) (two 'two) (three "three")), is there any way to write a macro that creates lexical bindings of the symbols with the supplied values for it's &rest/&body parameter?
This is seems to be a possible solution which Joshua pointed me to:
(let ((list_ '((x 23) (y 6) z)))
(let
((symbols_(loop for item_ in list_
collect (if (listp item_) (car item_) item_)))
(values_ (loop for item_ in list_
collect (if (listp item_) (cadr item_) nil))))
(progv symbols_ values_
(format t "x ~s, y ~s, z ~s~%" x y z))))
evaluates to:
;Compiler warnings :
; In an anonymous lambda form: Undeclared free variable X
; In an anonymous lambda form: Undeclared free variable Y
; In an anonymous lambda form: Undeclared free variable Z
x 23, y 6, z NIL
I could also easily rearrange my build-bindings functions to return the two lists needed.
One problem is, that the compiler spits warnings if the variables have never been declared special.
And the other problem that, if the dynamically bound variables are also used in a surrounding lexical binding, they a shadowed by the lexical binding - again if they have never been declared special:
(let ((x 47) (y 11) (z 0))
(let ((list_ '((x 23) (y 6) z)))
(let
((symbols_(loop for item_ in list_
collect (if (listp item_) (car item_) item_)))
(values_ (loop for item_ in list_
collect (if (listp item_) (cadr item_) nil))))
(progv symbols_ values_
(format t "x ~s, y ~s, z ~s~%" x y z)))))
evaluates to:
x 47, y 11, z 0
A better way could be:
(let ((x 47) (y 11) (z 0))
(locally
(declare (special x y))
(let ((list_ '((x 23) (y 6) z)))
(let
((symbols_(loop for item_ in list_
collect (if (listp item_) (car item_) item_)))
(values_ (loop for item_ in list_
collect (if (listp item_) (cadr item_) nil))))
(progv symbols_ values_
(format t "x ~s, y ~s, z ~s~%" x y z))))))
evaluates to:
;Compiler warnings about unused lexical variables skipped
x 23, y 6, z NIL
I can't see at the moment whether there are other problems with the dynamic progv bindings.
But the whole enchilada of a progv wrapped in locally with all the symbols declared as special cries for a macro again - which is again not possible due to same reasons let-list doesn't work :(
The possiblilty would be a kind of macro-lambda-list destructuring-hook which I'm not aware of.
I have to look into the implementation of destructuring-bind since that macro does kind of what I'd like to do. Perhaps that will enlight me ;)
So a first (incorrect) attempt would look something like this:
(defun build-bindings ()
'((first 1) middle (last "three")))
(defmacro let-list (bindings &body body)
`(let ,bindings
,#body))
Then you could try doing something like:
(let-list (build-bindings)
(print first))
That won't work, of course, because the macro expansion leaves the form (build-bindings) in the resulting let, in a position where it won't be evaluated:
CL-USER> (pprint (macroexpand-1 '(let-list (build-bindings)
(print first))))
(LET (BUILD-BINDINGS)
(PRINT FIRST))
Evaluation during Macroexpansion time
The issue is that you want the result of build-bindings at macroexpansion time, and that's before the code as a whole is run. Now, in this example, build-bindings can be run at macroexpansion time, because it's not doing anything with any arguments (remember I asked in a comment what the arguments are?). That means that you could actually eval it in the macroexpansion:
(defmacro let-list (bindings &body body)
`(let ,(eval bindings)
,#body))
CL-USER> (pprint (macroexpand-1 '(let-list (build-bindings)
(print first))))
(LET ((FIRST 1) MIDDLE (LAST "three"))
(PRINT FIRST))
Now that will work, insofar as it will bind first, middle, and last to 1, nil, and "three", respectively. However, if build-bindings actually needed some arguments that weren't available at macroexpansion time, you'd be out of luck. First, it can take arguments that are available at macroexpansion time (e.g., constants):
(defun build-bindings (a b &rest cs)
`((first ',a) (middle ',b) (last ',cs)))
CL-USER> (pprint (macroexpand-1 '(let-list (build-bindings 1 2 3 4 5)
(print first))))
(LET ((FIRST '1) (MIDDLE '2) (LAST '(3 4 5)))
(PRINT FIRST))
You could also have some of the variables appear in there:
(defun build-bindings (x ex y why)
`((,x ,ex) (,y ,why)))
CL-USER> (pprint (macroexpand-1 '(let-list (build-bindings 'a 'ay 'b 'bee)
(print first))))
(LET ((A AY) (B BEE))
(PRINT FIRST))
What you can't do, though, is have the variable names be determined from values that don't exist until runtime. E.g., you can't do something like:
(let ((var1 'a)
(var2 'b))
(let-list (build-bindings var1 'ay var2 'bee)
(print first))
because (let-list (build-bindings …) …) is macroexpanded before any of this code is actually executed. That means that you'd be trying to evaluate (build-bindings var1 'ay var2 'bee) when var1 and var2 aren't bound to any values.
Common Lisp does all its macroexpansion first, and then evaluates code. That means that values that aren't available until runtime are not available at macroexpansion time.
Compilation (and Macroexpansion) at Runtime
Now, even though I said that Common Lisp does all its macroexpansion first, and then evaluates code, the code above actually uses eval at macroexpansion to get some extra evaluation earlier. We can do things in the other direction too; we can use compile at runtime. That means that we can generate a lambda function and compile it based on code (e.g., variable names) provided at runtime. We can actually do this without using a macro:
(defun %dynamic-lambda (bindings body)
(flet ((to-list (x) (if (listp x) x (list x))))
(let* ((bindings (mapcar #'to-list bindings))
(vars (mapcar #'first bindings))
(vals (mapcar #'second bindings)))
(apply (compile nil `(lambda ,vars ,#body)) vals))))
CL-USER> (%dynamic-lambda '((first 1) middle (last "three"))
'((list first middle last)))
;=> (1 NIL "three")
This compiles a lambda expression that is created at runtime from a body and a list of bindings. It's not hard to write a macro that takes some fo the quoting hassle out of the picture:
(defmacro let-list (bindings &body body)
`(%dynamic-lambda ,bindings ',body))
CL-USER> (let-list '((first 1) middle (last "three"))
(list first middle last))
;=> (1 NIL "three")
CL-USER> (macroexpand-1 '(let-list (build-bindings)
(list first middle last)))
;=> (%DYNAMIC-LAMBDA (BUILD-BINDINGS) '((LIST FIRST MIDDLE LAST)))
CL-USER> (flet ((build-bindings ()
'((first 1) middle (last "three"))))
(let-list (build-bindings)
(list first middle last)))
;=> (1 NIL "three")
This gives you genuine lexical variables from a binding list created at runtime. Of course, because the compilation is happening at runtime, you lose access to the lexical environment. That means that the body that you're compiling into a function cannot access the "surrounding" lexical scope. E.g.:
CL-USER> (let ((x 3))
(let-list '((y 4))
(list x y)))
; Evaluation aborted on #<UNBOUND-VARIABLE X {1005B6C2B3}>.
Using PROGV and special variables
If you don't need lexical variables, but can use special (i.e., dynamically scoped) variables instead, you can establish bindings at runtime using progv. That would look something like:
(progv '(a b c) '(1 2 3)
(list c b a))
;;=> (3 2 1)
You'll probably get some warnings with that if run it, because when the form is compiled, there's no way to know that a, b, and c are supposed to be special variables. You can use locally to add some special declarations, though:
(progv '(a b c) '(1 2 3)
(locally
(declare (special a b c))
(list c b a)))
;;=> (3 2 1)
Of course, if you're doing this, then you have to know the variables in advance which is exactly what you were trying to avoid in the first place. However, if you're willing to know the names of the variables in advance (and your comments seem like you might be okay with that), then you can actually use lexical variables.
Lexical variables with values computed at run time
If you're willing to state what the variables will be, but still want to compute their values dynamically at run time, you can do that relatively easily. First, lets write the direct version (with no macro):
;; Declare three lexical variables, a, b, and c.
(let (a b c)
;; Iterate through a list of bindings (as for LET)
;; and based on the name in the binding, assign the
;; corresponding value to the lexical variable that
;; is identified by the same symbol in the source:
(dolist (binding '((c 3) (a 1) b))
(destructuring-bind (var &optional value)
(if (listp binding) binding (list binding))
(ecase var
(a (setf a value))
(b (setf b value))
(c (setf c value)))))
;; Do something with the lexical variables:
(list a b c))
;;=> (1 NIL 3)
Now, it's not too hard to write a macrofied version of this. This version isn't perfect, (e.g., there could be hygiene issues with names, and declarations in the body won't work (because the body is being spliced in after some stuff). It's a start, though:
(defmacro computed-let (variables bindings &body body)
(let ((assign (gensym (string '#:assign-))))
`(let ,variables
(flet ((,assign (binding)
(destructuring-bind (variable &optional value)
(if (listp binding) binding (list binding))
(ecase variable
,#(mapcar (lambda (variable)
`(,variable (setf ,variable value)))
variables)))))
(map nil #',assign ,bindings))
,#body)))
(computed-let (a b c) '((a 1) b (c 3))
(list a b c))
;;=> (1 NIL 3)
One way of making this cleaner would be to avoid the assignment altogether, and the computed values to provide the values for the binding directly:
(defmacro computed-let (variables bindings &body body)
(let ((values (gensym (string '#:values-)))
(variable (gensym (string '#:variable-))))
`(apply #'(lambda ,variables ,#body)
(let ((,values (mapcar #'to-list ,bindings)))
(mapcar (lambda (,variable)
(second (find ,variable ,values :key 'first)))
',variables)))))
This version creates a lambda function where the arguments are the specified variables and the body is the provided body (so the declarations in the body are in an appropriate place), and then applies it to a list of values extracted from the result of the computed bindings.
Using LAMBDA or DESTRUCTURING-BIND
since I'm doing some "destructuring" of the arguments (in a bit a different way), I know which arguments must be present or have which
default values in case of missing optional and key arguments. So in
the first step I get a list of values and a flag whether an optional
or key argument was present or defaulted. In the second step I would
like to bind those values and/or present/default flag to local
variables to do some work with them
This is actually starting to sound like you can do what you need to by using a lambda function or destructuring-bind with keyword arguments. First, note that you can use any symbol as a keyword argument indicator. E.g.:
(apply (lambda (&key
((b bee) 'default-bee b?)
((c see) 'default-see c?))
(list bee b? see c?))
'(b 42))
;;=> (42 T DEFAULT-SEE NIL)
(destructuring-bind (&key ((b bee) 'default-bee b?)
((c see) 'default-see c?))
'(b 42)
(list bee b? see c?))
;;=> (42 T DEFAULT-SEE NIL)
So, if you just make your function return bindings as a list of keyword arguments, then in the destructuring or function application you can automatically bind corresponding variables, assign default values, and check whether non-default values were provided.
Acting a bit indirectly:
a solution that works for combining optional with key arguments or
rest/body with key arguments
Have you considered the not-entirely-uncommon paradigm of using a sub-list for the keywords?
e.g.
(defmacro something (&key (first 1) second) &body body) ... )
or, a practical use from Alexandria:
(defmacro with-output-to-file ((stream-name file-name
&rest args
&key (direction nil direction-p)
&allow-other-keys)
&body body)
I'm reading Paul Graham's ANSI Common Lisp. In the chapter about macros he shows the following example:
(defmacro in (obj &rest choices)
(let ((insym (gensym)))
`(let ((,insym ,obj))
(or ,#(mapcar #'(lambda (c) `(eql ,insym ,c))
choices)))))
(Returns true if the first argument is equal to any of the other arguments)
He holds that it can't be written as a function. Wouldn't this function have the same functionality?
(defun in (obj &rest choices)
(reduce (lambda (x y)
(or x (eql y obj)))
choices
:initial-value nil))
The difference I see is that the macro will only evaluate arguments till it finds an eql argument. Is that it?
The point is, that the macro version evaluates the arguments lazily (it expands into an OR) stopping if a match is found. This cannot be achieved with a function, since a funcall will always evaluate all arguments first.
> (macroexpand '(in 42
(long-computation-1)
(long-computation-2)
(long-computation-3)))
(LET ((#:G799 42))
(OR (EQL #:G799 (LONG-COMPUTATION-1))
(EQL #:G799 (LONG-COMPUTATION-2))
(EQL #:G799 (LONG-COMPUTATION-3))))
To get the same effect you would need to write:
(defun in (obj &rest choices)
(reduce (lambda (x y)
(or x (eql (funcall y) obj)))
choices
:initial-value nil))
and use it this way:
(in 42
(function long-computation-1)
(function long-computation-2)
(function long-computation-3))