Jensen's Device: Difference between revisions

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=={{header|R}}==

R uses a [[wp:Evaluation_strategy#Call_by_need|call by need]] evaluation strategy where function inputs

are evaluated on demand and then cached; functions can bypass the normal argument evaluation by using functions <tt>substitute</tt> and <tt>match.call</tt> to access the parse tree of the as-yet-unevaluated arguments, and using <tt>parent.frame</tt> to access the scope of the caller. There are some proposed

are lazily evaluated and then cached; functions can implement

nonstandard evaluation such as call-by-name by using the "substitute" primitive to

access the parse tree of the unevaluated input. There are some proposed

[http://developer.r-project.org/nonstandard-eval.pdf conventions] to do this in a way that is less confusing to the user

of a function. ~~The~~ ~~following~~ ~~ignores~~ ~~any~~ ~~conventions~~ to ~~mimic~~ the ~~effect~~

of a function; however, ignoring conventions we can come disturbingly close to the ALGOL call-by-name semantics.

of the original ALGOL where "i" is truly assigned in the calling context.

⚫

<lang R>sum <- function(var, lo, hi, term)

⚫

eval(substitute({

⚫

sum <- function(var, lo, hi, term) {

s <- 0

.temp <- 0;

for (i in lo:hi) {

for (var in lo:hi) {

.temp <- .temp + term

eval.parent(substitute(x <- i, list(x=substitute(var), i=i)))

}

⚫

~~s <- s +~~ eval~~.parent~~(substitute(~~term))~~

}

.temp

}, as.list(match.call()[-1])),

s

enclos=parent.frame())

}

~~print(~~sum(i, 1, 100, 1/i))

sum(i, 1, 100, 1/i) #prints 5.187378

</lang>

##and because of enclos=parent.frame(), the term can involve variables in the caller's scope:

x <- -1

sum(i, 1, 100, i^x) #5.187378</lang>

=={{header|REXX}}==