Constructing quines (self-reproducing functions)

问题:

Has anyone constructed a quine ("A program that generates a copy of its own source text as its complete output": http://www.nyx.net/~gthompso/quine.htm) in R? (The [quine] tag pulls up lots of examples in Python, Java, ... but apparently none in R.)

f <- function() { body() }

comes close:

> f()
{
    body()
}

but lacks the name of the function.

How about the shortest possibility? Most obfuscated?

edit: from the variety of answers below, it seems that there are a variety of ways to define self-referentiality and the environment in which it must occur:

• within the R environment: function -> function (@bill_080)
• within the OS/shell environment: program -> program [more or less equivalent to program -> text]: (@kohske)
• other: function -> text (@JoshUlrich, @James, problem as defined above)

Notes:

• The thread from R-help pointed out by @Spacedman (which seems to emphasize obfuscation over brevity) suggests that identical(quine,quine()) is a good test case, although it's tricky because environments get carried along: identical(quine,quine(),ignore.environment=TRUE) might be easier.
• A recent (Oct 2015) blog post provides another answer ...

回答1:

This is the shortest I can come up with:

> "f" <- function() call("<-", "f", f)
> f()
"f" <- function () 
call("<-", "f", f)

回答2:

Here is a real Quine, a program (not a function) that generates a copy of its own source text as its complete output.

On console,

# y1.R is a quine program
$ cat y1.R
(function(x){cat(x);cat('(');cat(intToUtf8(0x0022));cat(x);cat(intToUtf8(0x0022));cat(')')})("(function(x){cat(x);cat('(');cat(intToUtf8(0x0022));cat(x);cat(intToUtf8(0x0022));cat(')')})")

# execute y1.R and show output
$ /usr/bin/R --vanilla --slave < y1.R
(function(x){cat(x);cat('(');cat(intToUtf8(0x0022));cat(x);cat(intToUtf8(0x0022));cat(')')})("(function(x){cat(x);cat('(');cat(intToUtf8(0x0022));cat(x);cat(intToUtf8(0x0022));cat(')')})")

# save the output of the execution of y1
$ /usr/bin/R --vanilla --slave < y1.R > y2.R

# compare input and output -- exactly same.
$ diff y1.R y2.R

probably this is not the shortest one.

UPDATED:

and slightly shorter version:

(function(x){cat(x,'(',d<-intToUtf8(0x0022),x,d,')',sep='')})("(function(x){cat(x,'(',d<-intToUtf8(0x0022),x,d,')',sep='')})")

回答3:

Using what body does as inspiration, call can be used to reproduce the calling command:

f <- function () 
{
    call("<-", as.name(as.character(sys.calls()[[1]])), sys.function(sys.parent()))
}

Which outputs:

> f()
f <- function () 
{
    call("<-", as.name(as.character(sys.calls()[[1]])), sys.function(sys.parent()))
}

回答4:

If you want a function that returns a function.....maybe this?

junk <- function(...) {
  function(...) {
    structure(junk(...))
  }
}

The output is:

> junk()

function(...) {
    structure(junk(...))
  }
<environment: 01ef8e50>


> boo <- junk(999)
> boo

function(...) {
    structure(junk(...))
  }
<environment: 020e1048>


>dput(boo)

function (...) 
{
    structure(junk(...))
}

回答5:

While I'm not sure if this "counts" from a quine perspective (I stumbled across this question while trying to verify if it does), the script

function(){}

will output function(){}. This works on the same principle as Joshua Ulrich's answer, just pared down to the essentials.