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Jensen's Device: Difference between revisions

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Jensen's Device (view source)

Revision as of 21:41, 5 May 2011

209 bytes added ,  13 years ago
→‎{{header|R}}
(omit from Go)
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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.; however, Theignoring followingconventions ignoreswe anycan conventionscome todisturbingly mimicclose to the effectALGOL 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) {▼
<lang R>
s <- s + eval.parent(substitute(term)){▼
▲sum <- function(var, lo, hi, term) {
s .temp <- 0;
for (ivar 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)) #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}}==
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