When turning a language object into a string (e.g. using dput, deparse,
or just
printing it), R usually inserts parentheses so that the printed string
is an
accurate representation of the underlying AST.
Example (good):
(expr = substitute(a * 10, list(a = quote(if (TRUE) 1 else 0))))
(if (TRUE) 1 else 0) * 10
eval(expr)
[1] 10
The representation is incorrect, however, when unary operators around
language
constructs like "if", "for" etc. are involved.
Example (bad):
(expr = substitute(-a * 10, list(a = quote(if (TRUE) 1 else 0))))
-if (TRUE) 1 else 0 * 10
The parentheses around the "if"-construct are missing. The expected
output
(the true representation of the expression) is, in fact:
-(if (TRUE) 1 else 0) * 10
as can be tested when evaluating it (which shows that this is not a bug
in
"substitute"):
eval(expr)
[1] -10
The deparsed string on the other hand evaluates to:
-if (TRUE) 1 else 0 * 10
[1] -1
Even when using deparse with control="all", which comes, per its help
file,
"closest to making 'deparse()' an inverse of 'parse()'", this bug
persists:
deparse(expr, control="all")
[1] "quote(-if (TRUE) 1 else 0 * 10)"
Only some language constructs appear to be affected. Unary operators
applied to
function calls, other unary operators, or binary operators render as
expected:
(expr = substitute(-a * 10, list(a = quote(!0))))
-(!0) * 10
I am using the R version 3.4.0 (2017-04-21) platform
x86_64-redhat-linux-gnu
package from Fedora 25.
Regards,
Martin Binder
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