In Emacs Lisp, lexical environment are represented by an alist, mapping symbols to their value. It can be passed to evaluators as a second argument of 'eval' function.
(eval '(+ 3 var)
'((var . 4)))
→ 7
However, I can't figure out how to pass functions, not variables,
to the evaluator.
For example, either of the following expressions shows an error.
(eval '(func 3)
'((func . (lambda (x) (+ 4 x)))))
→ error: (void-function func)
(eval '(func 3)
'((func . (closure (t) (x) (+ 4 x)))))
→ error: (void-function func)
Any help is appreciated.
How about this :
(eval '(apply func (list 3))
'((func . (lambda (x) (+ 4 x)))))
Here's how you can do it:
(defun my-eval (exp var-bindings fun-bindings)
(eval `(cl-flet ,(mapcar (lambda (x) (list (car x) `',(cdr x)))
fun-bindings)
(let ,(mapcar (lambda (x) (list (car x) `',(cdr x)))
var-bindings)
,exp))
t))
or, using eval's builtin support for var-bindings:
(defun my-eval (exp var-bindings fun-bindings)
(eval `(cl-flet ,(mapcar (lambda (x) (list (car x) `',(cdr x)))
fun-bindings)
,exp)
(or var-bindings t)))
[ BTW, note that it is not always true that in Emacs Lisp, lexical environment are represented by an alist: after byte-compilation, lexical variables don't have any name any more, they're stored on "the" stack and they're directly accessed via their position in the stack. ]
Related
I'm trying to call a function (lambda) stored within an alist. Below is a small snippet that demonstrates what I'm trying to do:
(defvar *db* '((:add (lambda (a b)
(+ a b)))
(:sub (lambda (a b)
(- a b)))))
(defun perform-operation-on-numbers (operation a b)
"Performs specified operation on the supplied numbers."
(let ((func (second (find operation
*db*
:key #'car))))
;; TODO: Call `func` on `a` and `b`
(print func)))
(perform-operation-on-numbers :add 1 2)
No matter what I do, not even funcall is able to let me call the lambda stored against :add. How should I reference the retrieved lambda as a lambda?
Your use of quote lead to your inability to use funcall.
Look:
(setf *mydb* '((:add #'+)
(:sub #'-)))
;; ((:ADD #'+) (:SUB #'-))
I can't use funcall. But:
(setf *mydb* (list (cons :add #'+)
(cons :sub #'-)))
;; ((:ADD . #<FUNCTION +>) (:SUB . #<FUNCTION ->))
;;
;; ^^^^ "FUNCTION" ? That's better! <----------
;;
I can (funcall (cdr (first *MYDB*)) 2)
Then the succinct notation is with back-quote and comma.
As pointed out by other answers, you are manipulating code as data, where the forms below (lambda ...) are unevaluated. But even with your data:
(defvar *db* '((:add (lambda (a b)
(+ a b)))
(:sub (lambda (a b)
(- a b)))))
You can use funcall or apply, if you first use COERCE:
If the result-type is function, and object is a lambda expression, then the result is a closure of object in the null lexical environment.
For example, let's access the form associated with :add:
CL-USER> (second (assoc :add *db*))
(LAMBDA (A B) (+ A B))
The value is an unevaluated form.
You can coerce it to a function:
CL-USER> (coerce (second (assoc :add *db*)) 'function)
#<FUNCTION (LAMBDA (A B)) {536B988B}>
Maybe you want to walk the terms to check that the lambda are only using a restricted set of operations, in which case it makes sense to keep them as data. But at some point you'll want to turn these code snippets to actual functions, and you can do that with coerce:
CL-USER> (defvar *db-fns*
(loop
for (n c) in *db*
collect (list n (coerce c 'function))))
*DB-FNS*
Here you compute the functions once, and can reuse them later instead of calling coerce each time.
CL-USER> *db-fns*
((:ADD #<FUNCTION (LAMBDA (A B)) {536B9B5B}>)
(:SUB #<FUNCTION (LAMBDA (A B)) {536B9C0B}>))
(it is equivalent to calling eval on the lambda form)
That's not a function: it's a list beginning (lambda ...). If you want a function have a function, for instance by
(defvar *db* `((:add ,(lambda (a b)
(+ a b)))
(:sub ,(lambda (a b)
(- a b)))))
or, better, don't wrap the thing in some useless baggage:
(defvar *db* `((:add ,#'+
(:sub ,#'-))
Emacs-lisp is default using call-by-value, but I'm trying use its symbol mechanism to simulate call-by-reference.
For example,
(setq lexical-binding nil)
(defun cbr (x)
(message "cbr (symbol-name x) %s" (symbol-name x))
(message "cbr (symbol-value x) %s" (symbol-value x))
(set x 2))
(let ((a 1))
(cbr 'a)
a)
;; cbr (symbol-name x) a
;; cbr (symbol-value x) 1
;; 2
It works well, because the result of let expression is 2, so it is indeed the call-by-reference behavior.
However, if I change the name from a to x:
(let ((x 1))
(cbr 'x)
x)
;; cbr (symbol-name x) x
;; cbr (symbol-value x) x
;; 1
Now it doesn't work as expected anymore.
Why?
Notice that it even can not get the correct symbol-name in cbr.
I think I have known what happen.
The second program returns 1, because the symbol x is captured by cbr's param x. When the body of cbr is evaluated, there are two bindings in the environment: one is the let binding x = 1, the other is x = x which is created by cbr's application. The symbol x in the (set x 2) uses the later one.
A workaround of this question is:
(let ((gen-x (gensym)))
(set gen-x 1)
(cbr gen-x)
(symbol-value gen-x))
;; cbr (symbol-name x) g36
;; cbr (symbol-value x) 1
;; 2
What should be clear from this is that relying on dynamic scope and symbol-value is a disaster: you need gensyms all over the place. Relying on dynamic scope for anything is generally a disaster, except in the specific, rare but extremely useful, case where you actually want dynamic scope.
But solving this problem trivial, even in elisp, with lexical scope. Here is one simple approach:
(defmacro ref (form)
;; Construct a reference to a form
(unless lexical-binding
(error "doomed"))
(let ((<setting> (gensym)))
`(lambda (&rest ,<setting>) ;hack for &optional (v nil vp)
(cond
((null ,<setting>)
,form)
((null (cdr ,<setting>))
(setf ,form (car ,<setting>)))
(t
(error "mutant"))))))
(defun ref-value (ref)
(funcall ref))
(defun set-ref-value (ref value)
;; should be (setf ref-value), but elisp
(funcall ref value))
And now, for instance, given:
(defun outer (v)
(let ((x 1))
(princ (format "x is first %s\n" x))
(inner (ref x) v)
(princ (format "and x is now %s\n" x))
x))
(defun inner (ref v)
(princ (format " ref is first %s\n" (ref-value ref)))
(set-ref-value ref v)
(princ (format " and ref is now %s\n" (ref-value ref))))
Then
ELISP> (outer 4)
x is first 1
ref is first 1
and ref is now 4
and x is now 4
4 (#o4, #x4, ?\C-d)
This is probably a stupid question, but I'm walking through the PG lisp book, and I wanted to step through some example macros that he provides with actual values, for instance:
(defmacro our-let (binds &body body)
`(
(lambda ,(
mapcar #'(lambda (x) (if (consp x) (car x) x)) binds
)
,#body
)
,#(mapcar #'(lambda (x) (if (consp x) (cadr x) nil)) binds)
)
)
I naively tried to run (trace our-let) and then (our-let ((x 1) (y 2)) (+ x y)) but I'm getting an error, can't use encapsulation to trace anonymous function #<FUNCTION (MACRO-FUNCTION OUR-LET) {22675BBB}>. Also not sure how to best put print statements into the lambdas. What's the best way to debug this macro/output how it's processing inputs?
EDIT(1): I had the incorrect formatting for macroexpand, which works.
Actually being able to trace macros is not very common in Common Lisp implementations. Compilers will typically expand the macro forms during compilation.
A few implementations support it though - which makes sense when they also support a Lisp interpreter, which runs the actual source. Among those are LispWorks and CLISP.
Here using the code from Sylwester in CLISP:
i i i i i i i ooooo o ooooooo ooooo ooooo
I I I I I I I 8 8 8 8 8 o 8 8
I \ `+' / I 8 8 8 8 8 8
\ `-+-' / 8 8 8 ooooo 8oooo
`-__|__-' 8 8 8 8 8
| 8 o 8 8 o 8 8
------+------ ooooo 8oooooo ooo8ooo ooooo 8
Welcome to GNU CLISP 2.49.93+ (2018-02-18) <http://clisp.org/>
Copyright (c) Bruno Haible, Michael Stoll 1992-1993
Copyright (c) Bruno Haible, Marcus Daniels 1994-1997
Copyright (c) Bruno Haible, Pierpaolo Bernardi, Sam Steingold 1998
Copyright (c) Bruno Haible, Sam Steingold 1999-2000
Copyright (c) Sam Steingold, Bruno Haible 2001-2018
Type :h and hit Enter for context help.
[1]> (defmacro our-let ((&rest bindings) &body body)
(let ((names (mapcar #'(lambda (x) (if (consp x) (car x) x)) bindings))
(exprs (mapcar #'(lambda (x) (if (consp x) (cadr x) nil)) bindings)))
`((lambda ,names ,#body) ,#exprs)))
OUR-LET
[2]> (trace our-let)
;; Tracing macro OUR-LET.
(OUR-LET)
[3]> (dotimes (i 3)
(our-let ((x (* i 10)))
(+ x 3)))
1. Trace: (OUR-LET ((X (* I 10))) (+ X 3))
1. Trace: OUR-LET ==> ((LAMBDA (X) (+ X 3)) (* I 10))
1. Trace: (OUR-LET ((X (* I 10))) (+ X 3))
1. Trace: OUR-LET ==> ((LAMBDA (X) (+ X 3)) (* I 10))
1. Trace: (OUR-LET ((X (* I 10))) (+ X 3))
1. Trace: OUR-LET ==> ((LAMBDA (X) (+ X 3)) (* I 10))
NIL
[4]>
How you debug it:
(macroexpand-1 '(our-let ((x 1) (y 2)) (+ x y)))
; ==> ((lambda (X Y) (+ X Y)) 1 2)
; ==> t
BTW your formatting is not good. Here is how it can look:
(defmacro our-let (binds &body body)
`((lambda ,(mapcar #'(lambda (x) (if (consp x) (car x) x)) binds)
,#body)
,#(mapcar #'(lambda (x) (if (consp x) (cadr x) nil)) binds)))
Or I would prefer:
(defmacro our-let ((&rest bindings) &body body)
(let ((names (mapcar #'(lambda (x) (if (consp x) (car x) x)) bindings))
(exprs (mapcar #'(lambda (x) (if (consp x) (cadr x) nil)) bindings)))
`((lambda ,names ,#body) ,#exprs)))
A nice thing about CL is that its designers thought quite hard about some things. In particular it turns out that you can trace macroexpansion portably in CL, thanks to *macroexpand-hook*. The code at the end of this answer uses it to trace macroexpansion It makes some attempt to cooperate with anything else which might be talking to *macroexpand-hook*, and to avoid recursive tracing, but it's not very well tested. There are controls for how much should be printed which have default values which are 'much less than everything'.
Here is an example of this in LispWorks:
> (macroexpand-traced-p)
nil
> (trace-macroexpand)
t
> (defun foo (x) x)
(defun foo (x) ...)
-> (dspec:def (defun foo) (dspec:install-defun 'foo # ...))
(dspec:def (defun foo) (dspec:install-defun 'foo # ...))
-> (compiler-let (#) (compiler::top-level-form-name # #))
(compiler::top-level-form-name (defun foo)
(dspec:install-defun 'foo # ...))
-> (compiler::tlf-name-binding (compiler-let #)
(dspec:install-defun 'foo # ...))
(compiler::tlf-name-binding (compiler-let #)
(dspec:install-defun 'foo # ...))
-> (compiler-let (# #) (dspec:install-defun 'foo # ...))
(dspec:location)
-> ':listener
foo
As you can see you get a lot of internal expansions which are probably not interesting. To deal with this there is support for filtering the output so you don't see macroexpansions which may not be interesting to you, of which there are a lot.
Here is a filter function which tries to only show expansions where the thing being expanded is visible in the current package:
(defun trace-macroexpand-trace-this-package-p (macro-function macro-form
environment)
(declare (ignore macro-function environment))
(and (consp macro-form)
(symbolp (first macro-form))
(let ((name (first macro-form)))
(eq (find-symbol (symbol-name name) *package*) name))))
And here is the some output for that:
> (setf *trace-macroexpand-trace-p* #'trace-macroexpand-trace-this-package-p)
(setf *trace-macroexpand-trace-p*
#'trace-macroexpand-trace-this-package-p)
-> (let* (#) (setq *trace-macroexpand-trace-p* #:|Store-Var-1102|))
#<Function trace-macroexpand-trace-this-package-p 4060000844>
> (defun foo (x) x)
(defun foo (x) ...)
-> (dspec:def (defun foo) (dspec:install-defun 'foo # ...))
foo
As you can see you only now get 'interesting' macroexpansions. Cleverer filters could be defined, of course.
Here is the code:
(eval-when (:load-toplevel :compile-toplevel :execute)
;; macroexpansion tracing really wants to be off when compiling this
;; code as exciting things may happen during the evaluation of
;; DEFVAR &c otherwise.
(when (fboundp 'trace-macroexpand)
(ignore-errors ;don't barf
(trace-macroexpand nil))))
(defvar *trace-macroexpand-print-length* 3
"The value of *PRINT-LENGTH* used when tracing macroexpansions")
(defvar *trace-macroexpand-print-level* 2
"The value of *PRINT-LEVEL* used when tracing macroexpansions")
(defvar *trace-macroexpand-trace-p* (constantly t)
"Should we trace a given macroexpansion?
If this is bound to a function that function will be called with the
same three arguments that *MACROEXPAND-HOOK* takes, and should return
true if the expansion is to be printed. Otherwise it should be true
if expansion is to be printed, false otherwise.")
(defvar *traced-macroexpand-hook*
;; the old value of *MACROEXPAND-HOOK*, used to restore it and to
;; know if we should trace. Not user-adjustable.
nil)
(defun trace-macroexpand (&optional (tracep t))
"Trace or untrace macroexpansion.
If called with no argument, or an argument which is true, ensure that
macroexpansion is on. If it was already on return NIL, otherwise
return T.
If called with an argument which is NIL then ensure macroexpansion is
not traced. If it was traced return T else return NIL."
(if tracep
(if *traced-macroexpand-hook*
nil
(let ((hook *macroexpand-hook*))
(flet ((macroexpand-hook (macro-function macro-form environment)
(if (if (functionp *trace-macroexpand-trace-p*)
(funcall *trace-macroexpand-trace-p*
macro-function macro-form environment)
*trace-macroexpand-trace-p*)
(let ((expanded-form (funcall hook macro-function
macro-form environment))
(*print-length* *trace-macroexpand-print-length*)
(*print-level* *trace-macroexpand-print-level*)
(*print-pretty* t))
(format *debug-io* "~&~S~% -> ~S~%" macro-form expanded-form)
expanded-form)
(funcall hook macro-function macro-form environment))))
(setf *traced-macroexpand-hook* hook
*macroexpand-hook* #'macroexpand-hook)
t)))
(if *traced-macroexpand-hook*
(progn
(setf *macroexpand-hook* *traced-macroexpand-hook*
*traced-macroexpand-hook* nil)
t)
nil)))
(defun macroexpand-traced-p ()
"Is macroexpansion currently traced?"
(if *traced-macroexpand-hook* t nil))
Here is one way to trace the macro that should work in any Common Lisp:
(eval-when (:compile-toplevel :load-toplevel :execute)
(defun our-let-expander (binds body)
`((lambda ,(mapcar #'(lambda (x) (if (consp x) (car x) x)) binds)
,#body)
,#(mapcar #'(lambda (x) (if (consp x) (cadr x) nil)) binds))))
(defmacro our-let (binds &body body)
(our-let-expander binds body))
Now we just (trace our-let-expander).
In my experience, complicated macros often end up handled via expander helper functions like this anyway, for various reasons.
(One big reason is this: as soon as you have two or more syntactic interfaces to the same expansion logic, you don't want to copy and paste that logic into multiple defmacro forms, but have it in a function.)
P.S. note the reformatting of the backquote form. Do not separate opening parentheses from what follows, and have closing parentheses on lines by themselves.
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 would like to create a function that allows to:
(incf vara varb varc vard)
Instead of
(incf vara)
(incf varb)
(incf varc)
(incf vard)
What I do not understand is how to be able to send more arguments, how to define that in a function?
(defun inc (&rest arg)
(interactive)
(mapcar 'incf arg)
)
This increases the argument, but ofcourse does not save them back into the variables.
How to go about this?
If you want to be able to write this form as (my-incf a b c) without quoting the variable names a, b, and c, make it a macro rather than a function:
(defmacro incf+ (&rest vars)
`(progn
,#(mapcar (lambda (var) `(incf ,var)) vars)))
Check that it expands into the right code using macroexpand:
(macroexpand '(incf+ var1 var2 var3))
;; => (progn (incf var1) (incf var2) (incf var3))
Because variables in Emacs Lisp have dynamic scope by default, you can accomplish almost the same thing with a function which takes quoted variable names as arguments. But the macro version has the advantage that, since it expands into code in the place when it was called, it will work with lexically bound variables as well. symbol-value only works with dynamically bound variables.
You can test this by putting the following in a file and loading it (in Emacs 24 or higher):
;; -*- lexical-binding: t -*-
(defun incf+fun (&rest vars)
(mapc #'(lambda (var) (incf (symbol-value var))) vars))
(defun incf-macro-test ()
(let ((a 5) (b 7) (c 11))
(incf+ a b c)
(list a b c)))
(defun incf-function-test ()
(let ((a 5) (b 7) (c 11))
(incf+fun 'a 'b 'c)
(list a b c)))
Evaluating (incf-macro-test) will return (6 8 12), but (incf-function-test) will enter the debugger with a (void-variable a) error.
It should work:
(require 'cl)
(setq a 1)
(setq b 2)
(defun inc (&rest arg)
(interactive)
(mapc (lambda (x) (incf (symbol-value x))) arg))
(inc 'a 'b)
(message "%s %s" a b) => (2 3)
You have to quote each argument otherwise (inc a b) becomes (inc 1 2) before executing inc.