Domain-Specific Languages in R by Thomas Mailund

Author:Thomas Mailund
Language: eng
Format: epub
Publisher: Apress, Berkeley, CA

## [1] 5

However, this will fail if we refer to the promise that needs the variable before we compute it.

h <- function(x, y = 2 * w) {

res <- x + y

w <- 2

res

}

h(1)

## Error in h(1): object 'w' not found

We have to be careful if a promise depends on a variable that we update during a computation. Note that a promise is evaluated only once; after the evaluation, the variable that used to hold it now holds the result of the evaluation and no longer the promise expression. If we change variables that occurred in the promise, we do not update the value that the variable now holds.

h <- function(x, y = 2 * w) {

w <- 1

res <- x + y

w <- 2

res

}

h(1)

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