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@(#)ch8.n 6.3 (Berkeley) 04/17/91
." $Header: ch8.n,v 1.4 83/07/27 15:12:22 layer Exp $ .Lc Functions, Fclosures, and Macros 8 .sh 2 valid function objects \n(ch 1 .pp There are many different objects which can occupy the function field of a symbol object. Table 8.1, on the following page, shows all of the possibilities, how to recognize them, and where to look for documentation. .(z
| informal name object type documentation |
| interpreted list with car 8.2 |
| lambda function eq to lambda |
| interpreted list with car 8.2 |
| nlambda function eq to nlambda |
| interpreted list with car 8.2 |
| lexpr function eq to lexpr |
| interpreted list with car 8.3 |
| macro eq to macro |
| fclosure vector with vprop 8.4 |
| eq to fclosure |
| compiled binary with discipline 8.2 |
| lambda or lexpr eq to lambda |
| function |
| compiled binary with discipline 8.2 |
| nlambda function eq to nlambda |
| compiled binary with discipline 8.3 |
| macro eq to macro |
| foreign binary with discipline 8.5 |
| subroutine of \*(lqsubroutine\*(rq\*[\(dg\*] |
| foreign binary with discipline 8.5 |
| function of \*(lqfunction\*(rq\*[\(dg\*] |
| foreign binary with discipline 8.5 |
| integer function of \*(lqinteger-function\*(rq\*[\(dg\*] |
| foreign binary with discipline 8.5 |
| real function of \*(lqreal-function\*(rq\*[\(dg\*] |
| foreign binary with discipline 8.5 |
| C function of \*(lqc-function\*(rq\*[\(dg\*] |
| foreign binary with discipline 8.5 |
| double function of \*(lqdouble-c-function\*(rq\*[\(dg\*] |
| foreign binary with discipline 8.5 |
| structure function of \*(lqvector-c-function\*(rq\*[\(dg\*] |
| array array object 9 |
.sh 2 functions .pp The basic Lisp function is the lambda function. When a lambda function is called, the actual arguments are evaluated from left to right and are lambda-bound to the formal parameters of the lambda function. .pp An nlambda function is usually used for functions which are invoked by the user at top level. Some built-in functions which evaluate their arguments in special ways are also nlambdas (e.g cond, do, or). When an nlambda function is called, the list of unevaluated arguments is lambda bound to the single formal parameter of the nlambda function. .pp Some programmers will use an nlambda function when they are not sure how many arguments will be passed. Then, the first thing the nlambda function does is map eval over the list of unevaluated arguments it has been passed. This is usually the wrong thing to do, as it will not work compiled if any of the arguments are local variables. The solution is to use a lexpr. When a lexpr function is called, the arguments are evaluated and a fixnum whose value is the number of arguments is lambda-bound to the single formal parameter of the lexpr function. The lexpr can then access the arguments using the arg function. .pp When a function is compiled, .i special declarations may be needed to preserve its behavior. An argument is not lambda-bound to the name of the corresponding formal parameter unless that formal parameter has been declared .i special (see \(sc12.3.2.2). .pp Lambda and lexpr functions both compile into a binary object with a discipline of lambda. However, a compiled lexpr still acts like an interpreted lexpr. .sh 2 macros .pp An important feature of Lisp is its ability to manipulate programs as data. As a result of this, most Lisp implementations have very powerful macro facilities. The Lisp language's macro facility can be used to incorporate popular features of the other languages into Lisp. For example, there are macro packages which allow one to create records (ala Pascal) and refer to elements of those records by the field names. The .i struct package imported from Maclisp does this. Another popular use for macros is to create more readable control structures which expand into .i cond , .i or and .i and . One such example is the If macro. It allows you to write
(If (equal numb 0) then (print 'zero) (terpr)
elseif (equal numb 1) then (print 'one) (terpr)
else (print '|I give up|))
which expands to
(cond
((equal numb 0) (print 'zero) (terpr))
((equal numb 1) (print 'one) (terpr))
(t (print '|I give up|)))
.sh 3 macro forms
.pp
A macro is a function which accepts a Lisp expression as input and returns
another Lisp expression.
The action the macro takes is called macro expansion.
Here is a simple example:
-> (def first (macro (x) (cons 'car (cdr x)))) first -> (first '(a b c)) a -> (apply 'first '(first '(a b c))) (car '(a b c))The first input line defines a macro called .i first . Notice that the macro has one formal parameter, x. On the second input line, we ask the interpreter to evaluate (first '(a b c)). .i Eval sees that .i first has a function definition of type macro, so it evaluates .i first 's definition, passing to .i first , as an argument, the form .i eval itself was trying to evaluate: (first '(a b c)). The .i first macro chops off the car of the argument with .i cdr , cons' a .i car at the beginning of the list and returns (car '(a b c)), which .i eval evaluates. The value .i a is returned as the value of (first '(a b c)). Thus whenever .i eval tries to evaluate a list whose car has a macro definition it ends up doing (at least) two operations, the first of which is a call to the macro to let it macro expand the form, and the other is the evaluation of the result of the macro. The result of the macro may be yet another call to a macro, so .i eval may have to do even more evaluations until it can finally determine the value of an expression. One way to see how a macro will expand is to use .i apply as shown on the third input line above. .sh +0 defmacro .pp The macro .i defmacro makes it easier to define macros because it allows you to name the arguments to the macro call. For example, suppose we find ourselves often writing code like (setq stack (cons newelt stack). We could define a macro named push to do this for us. One way to define it is:
-> (de\kAf push
\h'|\nAu'(macro (x) (list 'setq (caddr x) (list 'cons (cadr x) (caddr x)))))
push
then (push newelt stack) will expand to the form mentioned above.
The same macro written using defmacro would be:
->\kA (defmacro push (value stack)
\h'|\nAu'(list 'setq ,stack (list 'cons ,value ,stack)))
push
Defmacro allows you to name the arguments of the macro call, and makes the
macro definition look more like a function definition.
.sh +0 the backquote character macro
.pp
The default syntax for
.Fr
has four characters with associated character macros.
One is semicolon for comments.
Two others are the backquote and comma which are
used by the backquote character
macro.
The fourth is the sharp sign macro described in the next section.
.pp
The backquote macro is used to create lists where many of the elements are
fixed (quoted).
This makes it very useful for creating macro definitions.
In the simplest case, a backquote acts just like a single quote:
->`(a b c d e)
(a b c d e)
If a comma precedes an element of a backquoted list then that element is
evaluated and its value is put in the list.
->(setq d '(x y z)) (x y z) ->`(a b c ,d e) (a b c (x y z) e)If a comma followed by an at sign precedes an element in a backquoted list, then that element is evaluated and spliced into the list with .i append .
->`(a b c ,@d e)
(a b c x y z e)
Once a list begins with a backquote, the commas may appear anywhere in the
list as this example shows:
->`(a b (c d ,(cdr d)) (e f (g h ,@(cddr d) ,@d)))
(a b (c d (y z)) (e f (g h z x y z)))
It is also possible and sometimes even useful to use the
backquote macro within itself.
As a final demonstration of the backquote macro, we shall define the
first and push macros using all the power at our disposal: defmacro
and the backquote macro.
->(defmacro first (list) `(car ,list)) first ->(defmacro push (value stack) `(setq ,stack (cons ,value ,stack))) stack.sh +0 sharp sign character macro .pp The sharp sign macro can perform a number of different functions at read time. The character directly following the sharp sign determines which function will be done, and following Lisp s-expressions may serve as arguments. .sh +1 conditional inclusion .lp If you plan to run one source file in more than one environment then you may want to some pieces of code to be included or not included depending on the environment. The C language uses \*(lq#ifdef\*(lq and \*(lq#ifndef\*(rq for this purpose, and Lisp uses \*(lq#+\*(rq and \*(lq#-\*(rq. The environment that the sharp sign macro checks is the (status features) list which is initialized when the Lisp system is built and which may be altered by (sstatus feature foo) and (sstatus nofeature bar) The form of conditional inclusion is
.tl ''#+when what'' where .i when is either a symbol or an expression involving symbols and the functions .i and , .i or , and .i not . The meaning is that .i what will only be read in if .i when is true. A symbol in .i when is true only if it appears in the .i (status features) list. .Eb ; suppose we want to write a program which references a file ; and which can run at ucb, ucsd and cmu where the file naming conventions ; are different. ; -> (de\kAfun howold (name) \h'|\nAu'\kC(terpr) \h'|\nCu'\kB(load #\kA+(or ucb ucsd) "/usr/lib/lisp/ages.l" \h'|\nAu'#+cmu "/usr/lisp/doc/ages.l") \h'|\nBu'\kA(patom name) \h'|\nBu'\kA(patom " is ") \h'|\nAu'\kB(print (cdr (assoc name agefile))) \h'|\nBu'\kA(patom "years old") \h'|\nAu'(terpr)) .Ee The form
.tl ''#-when what'' is equivalent to
.tl ''#+(not when) what'' .sh +0 fixnum character equivalents .lp When working with fixnum equivalents of characters, it is often hard to remember the number corresponding to a character. The form
.tl ''#/c'' is equivalent to the fixnum representation of character c. .Eb ; a function which returns t if the user types y else it returns nil. ; -> (de\kBfun yesorno nil \h'|\nBu'(progn \kA(ans) \h'|\nAu'\kB(setq ans (tyi)) \h'|\nBu'(cond \kA((equal ans #/y) t) \h'|\nAu'(t nil)))) .Ee .sh +0 read time evaluation .lp Occasionally you want to express a constant as a Lisp expression, yet you don't want to pay the penalty of evaluating this expression each time it is referenced. The form
.tl ''#.expression'' evaluates the expression at read time and returns its value. .Eb ; a function to test if any of bits 1 3 or 12 are set in a fixnum. ; -> (de\kCfun testit (num) \h'|\nCu'(cond \kA(\kB(zerop (boole 1 num #.(+ (lsh 1 1) (lsh 1 3) (lsh 1 12)))) \h'|\nBu'nil) \h'|\nAu'(t t))) .Ee .sh 2 fclosures .pp Fclosures are a type of functional object. The purpose is to remember the values of some variables between invocations of the functional object and to protect this data from being inadvertently overwritten by other Lisp functions. Fortran programs usually exhibit this behavior for their variables. (In fact, some versions of Fortran would require the variables to be in COMMON). Thus it is easy to write a linear congruent random number generator in Fortran, merely by keeping the seed as a variable in the function. It is much more risky to do so in Lisp, since any special variable you picked, might be used by some other function. Fclosures are an attempt to provide most of the same functionality as closures in Lisp Machine Lisp, to users of .Fr . Fclosures are related to closures in this way:
(fclosure '(a b) 'foo) <==>
(let ((a a) (b b)) (closure '(a b) 'foo)) .sh 3 an example
.sz -2 .hl % lisp Franz Lisp, Opus 38.60 ->(defun code (me count) (print (list 'in x)) (setq x (+ 1 x)) (cond ((greaterp count 1) (funcall me me (sub1 count)))) (print (list 'out x))) code ->(defun tester (object count) (funcall object object count) (terpri)) tester ->(setq x 0) 0 ->(setq z (fclosure '(x) 'code)) fclosure[8] -> (tester z 3) (in 0)(in 1)(in 2)(out 3)(out 3)(out 3) nil ->x 0 .hl.sz +2 .pp The function fclosure creates a new object that we will call an fclosure, (although it is actually a vector). The fclosure contains a functional object, and a set of symbols and values for the symbols. In the above example, the fclosure functional object is the function code. The set of symbols and values just contains the symbol `x' and zero, the value of `x' when the fclosure was created. .lp When an fclosure is funcall'ed: .ip 1) The Lisp system lambda binds the symbols in the fclosure to their values in the fclosure. .ip 2) It continues the funcall on the functional object of the fclosure. .ip 3) Finally, it un-lambda binds the symbols in the fclosure and at the same time stores the current values of the symbols in the fclosure. .pp Notice that the fclosure is saving the value of the symbol `x'. Each time a fclosure is created, new space is allocated for saving the values of the symbols. Thus if we execute fclosure again, over the same function, we can have two independent counters:
.sz -2 .hl -> (setq zz (fclosure '(x) 'code)) fclosure[1] -> (tester zz 2) (in 0)(in 1)(out 2)(out 2) -> (tester zz 2) (in 2)(in 3)(out 4)(out 4) -> (tester z 3) (in 3)(in 4)(in 5)(out 6)(out 6)(out 6) .hl.sz +2 .sh 3 useful functions .pp Here are some quick some summaries of functions dealing with closures. They are more formally defined in \(sc2.8.4. To recap, fclosures are made by (fclosure 'l_vars 'g_funcobj). l_vars is a list of symbols (not containing nil), g_funcobj is any object that can be funcalled. (Objects which can be funcalled, include compiled Lisp functions, lambda expressions, symbols, foreign functions, etc.) In general, if you want a compiled function to be closed over a variable, you must declare the variable to be special within the function. Another example would be: .(l (fclosure '(a b) #'(lambda (x) (plus x a))) .)l Here, the #' construction will make the compiler compile the lambda expression. .pp There are times when you want to share variables between fclosures. This can be done if the fclosures are created at the same time using fclosure-list. The function fclosure-alist returns an assoc list giving the symbols and values in the fclosure. The predicate fclosurep returns t iff its argument is a fclosure. Other functions imported from Lisp Machine Lisp are .i symeval-in-fclosure, .i let-fclosed, and .i set-in-fclosure. Lastly, the function fclosure-function returns the function argument. .sh 3 internal structure .pp Currently, closures are implemented as vectors, with property being the symbol fclosure. The functional object is the first entry. The remaining entries are structures which point to the symbols and values for the closure, (with a reference count to determine if a recursive closure is active). .sh 2 foreign subroutines and functions .pp .Fr has the ability to dynamically load object files produced by other compilers and to call functions defined in those files. These functions are called .i foreign functions.* .(f *This topic is also discussed in Report PAM-124 of the Center for Pure and Applied Mathematics, UCB, entitled ``Parlez-Vous Franz? An Informal Introduction to Interfacing Foreign Functions to Franz LISP'', by James R. Larus .)f There are seven types of foreign functions. They are characterized by the type of result they return, and by differences in the interpretation of their arguments. They come from two families: a group suited for languages which pass arguments by reference (e.g. Fortran), and a group suited for languages which pass arguments by value (e.g. C). .lp There are four types in the first group: .ip subroutine This does not return anything. The Lisp system always returns t after calling a subroutine. .ip function This returns whatever the function returns. This must be a valid Lisp object or it may cause the Lisp system to fail. .ip integer-function This returns an integer which the Lisp system makes into a fixnum and returns. .ip real-function This returns a double precision real number which the Lisp system makes into a flonum and returns. .lp There are three types in the second group: .ip c-function This is like an integer function, except for its different interpretation of arguments. .ip double-c-function This is like a real-function. .ip vector-c-function This is for C functions which return a structure. The first argument to such functions must be a vector (of type vectori), into which the result is stored. The second Lisp argument becomes the first argument to the C function, and so on .lp A foreign function is accessed through a binary object just like a compiled Lisp function. The difference is that the discipline field of a binary object for a foreign function is a string whose first character is given in the following table: .(b
| letter type |
| s subroutine |
| f function |
| i integer-function |
| r real-function. |
| c c-function |
| v vector-c-function |
| d double-c-function |
.sz -2 .hl These are the C coded functions % cat ch8auxc.c /* demonstration of c coded foreign integer-function */ /* the following will be used to extract fixnums out of a list of fixnums */ struct listoffixnumscell { struct listoffixnumscell *cdr; int *fixnum; }; struct listcell { struct listcell *cdr; int car; }; cfoo(a,b,c,d) int *a; double b[]; int *c[]; struct listoffixnumscell *d; { printf("a: %d, b[0]: %f, b[1]: %f\n", *a, b[0], b[1]); printf(" c (first): %d c (second): %d\n", *c[0],*c[1]); printf(" ( %d %d ... )\n ", *(d->fixnum), *(d->cdr->fixnum)); b[1] = 3.1415926; return(3); } struct listcell * cmemq(element,list) int element; struct listcell *list; { for( ; list && element != list->car ; list = list->cdr); return(list); } These are the Pascal coded functions % cat ch8auxp.p type pinteger = ^integer; realarray = array[0..10] of real; pintarray = array[0..10] of pinteger; listoffixnumscell = record cdr : ^listoffixnumscell; fixnum : pinteger; end; plistcell = ^listcell; listcell = record cdr : plistcell; car : integer; end; function pfoo ( var a : integer ; var b : realarray; var c : pintarray; var d : listoffixnumscell) : integer; begin writeln(' a:',a, ' b[0]:', b[0], ' b[1]:', b[1]); writeln(' c (first):', c[0]^,' c (second):', c[1]^); writeln(' ( ', d.fixnum^, d.cdr^.fixnum^, ' ...) '); b[1] := 3.1415926; pfoo := 3 end ; { the function pmemq looks for the Lisp pointer given as the first argument in the list pointed to by the second argument. Note that we declare " a : integer " instead of " var a : integer " since we are interested in the pointer value instead of what it points to (which could be any Lisp object) } function pmemq( a : integer; list : plistcell) : plistcell; begin while (list <> nil) and (list^.car <> a) do list := list^.cdr; pmemq := list; end ; The files are compiled % cc -c ch8auxc.c 1.0u 1.2s 0:15 14% 30+39k 33+20io 147pf+0w % pc -c ch8auxp.p 3.0u 1.7s 0:37 12% 27+32k 53+32io 143pf+0w % lisp Franz Lisp, Opus 38.60First the files are loaded and we set up one foreign function binary. We have two functions in each file so we must choose one to tell cfasl about. The choice is arbitrary.
-> (cfasl 'ch8auxc.o '_cfoo 'cfoo "integer-function") /usr/lib/lisp/nld -N -A /usr/local/lisp -T 63000 ch8auxc.o -e _cfoo -o /tmp/Li7055.0 -lc #63000-"integer-function" -> (cfasl 'ch8auxp.o '_pfoo 'pfoo "integer-function" "-lpc") /usr/lib/lisp/nld -N -A /tmp/Li7055.0 -T 63200 ch8auxp.o -e _pfoo -o /tmp/Li7055.1 -lpc -lc #63200-"integer-function" Here we set up the other foreign function binary objects -> (getaddress '_cmemq 'cmemq "function" '_pmemq 'pmemq "function") #6306c-"function"We want to create and initialize an array to pass to the cfoo function. In this case we create an unnamed array and store it in the value cell of testarr. When we create an array to pass to the Pascal program we will use a named array just to demonstrate the different way that named and unnamed arrays are created and accessed.
-> (setq testarr (array nil flonum-block 2)) array[2] -> (store (funcall testarr 0) 1.234) 1.234 -> (store (funcall testarr 1) 5.678) 5.678 -> (cfoo 385 testarr (hunk 10 11 13 14) '(15 16 17)) a: 385, b[0]: 1.234000, b[1]: 5.678000 c (first): 10 c (second): 11 ( 15 16 ... ) 3Note that cfoo has returned 3 as it should. It also had the side effect of changing the second value of the array to 3.1415926 which check next.
-> (funcall testarr 1)
3.1415926
In preparation for calling pfoo we create an array.
-> (array test flonum-block 2) array[2] -> (store (test 0) 1.234) 1.234 -> (store (test 1) 5.678) 5.678 -> (pfoo 385 (getd 'test) (hunk 10 11 13 14) '(15 16 17)) a: 385 b[0]: 1.23400000000000E+00 b[1]: 5.67800000000000E+00 c (first): 10 c (second): 11 ( 15 16 ...) 3 -> (test 1) 3.1415926 Now to test out the memq's -> (cmemq 'a '(b c a d e f)) (a d e f) -> (pmemq 'e '(a d f g a x)) nil .hl.sz +2 .pp The Fortran example will be much shorter since in Fortran you can't follow pointers as you can in other languages. The Fortran function ffoo is given three arguments: a fixnum, a fixnum-block array and a flonum. These arguments are printed out to verify that they made it and then the first value of the array is modified. The function returns a double precision value which is converted to a flonum by lisp and printed. Note that the entry point corresponding to the Fortran function ffoo is _ffoo_ as opposed to the C and Pascal convention of preceding the name with an underscore.
.sz -2 .hl % cat ch8auxf.f double precision function ffoo(a,b,c) integer a,b(10) double precision c print 2,a,b(1),b(2),c 2 format(' a=',i4,', b(1)=',i5,', b(2)=',i5,' c=',f6.4) b(1) = 22 ffoo = 1.23456 return end % f77 -c ch8auxf.f ch8auxf.f: ffoo: 0.9u 1.8s 0:12 22% 20+22k 54+48io 158pf+0w % lisp Franz Lisp, Opus 38.60 -> (cfasl 'ch8auxf.o '_ffoo_ 'ffoo "real-function" "-lF77 -lI77") /usr/lib/lisp/nld -N -A /usr/local/lisp -T 63000 ch8auxf.o -e _ffoo_ -o /tmp/Li11066.0 -lF77 -lI77 -lc #6307c-"real-function" -> (array test fixnum-block 2) array[2] -> (store (test 0) 10) 10 -> (store (test 1) 11) 11 -> (ffoo 385 (getd 'test) 5.678) a= 385, b(1)= 10, b(2)= 11 c=5.6780 1.234559893608093 -> (test 0) 22 .hl