if_bytecode.R

# Let's build this expression by hand
# val <- 0
# if (this) {
#   val <- 1
# }
constant_pool <- list(
  NULL,
  as.symbol("val"), # we need a symbol val
  as.symbol("this"), # we also need a symbol this,
  0, 1 # next we need the two values 0 and 1
)
code <- as.integer(c(
  12L, # I believe this is the byte code version
  compiler:::LDCONST.OP, 3L, # first let's load the 0 at index 3
  compiler:::SETVAR.OP, 1L, # assign last value to symbol at position 1
  compiler:::POP.OP, # remove value 0 from the stack
  compiler:::GETVAR.OP, 2L, # load variable this
  compiler:::BRIFNOT.OP, 0L, 17L, # if last val is true, then proceed, else goto 17
  compiler:::LDCONST.OP, 4L, # load the 1 at index 4 from pool
  compiler:::SETVAR.OP, 1L, # set that value to symbol at index 1
  compiler:::GOTO.OP, 18L, # now go to invisible return (i.e. skip next line)
  compiler:::LDNULL.OP, # 17: else target; load null on the stack
  compiler:::INVISIBLE.OP, # 18: make the result invisible
  compiler:::RETURN.OP # return
))
bc <- .Internal(mkCode(code, constant_pool))
this <- FALSE; eval(bc)
val
#> [1] 0
rm(val); this <- TRUE; eval(bc)
val
#> [1] 1