expression(...) as.expression(x) is.expression(x)
expression
.
expression
returns an expression, that is, a list-like
object of mode
"expression"
.
Its elements are the arguments, unsubstituted and unevaluated.
as.expression
merely changes the mode of
x
to
"expression"
.
is.expression
returns
TRUE
if
x
has mode
"expression"
and
FALSE
otherwise.
Both
is.expression
and
as.expression
are generic functions; currently
there are no methods defined for either of these.
The output from
parse
is of mode
"expression"
.
methods <- expression(lsfit(x,y), l1fit(x,y)) eval(methods[[which]]) rand.choice <- expression("Gaussian"=rnorm(1),"exp"=rexp(1),"uniform"=runif(1)) eval(rand.choice[["exp"]])