join {teal.code}

Join qenv objects

Description

Checks and merges two qenv objects into one qenv object.

Usage

join(x, y)

Arguments

x

(qenv)

y

(qenv)

Details

Any common code at the start of the qenvs is only placed once at the start of the joined qenv. This allows consistent behavior when joining qenvs which share a common ancestor. See below for an example.

There are some situations where join() cannot be properly performed, such as these three scenarios:

1. Both qenv objects contain an object of the same name but are not identical.

   Example:

   x <- eval_code(qenv(), expression(mtcars1 <- mtcars))
   y <- eval_code(qenv(), expression(mtcars1 <- mtcars['wt']))
   
   z <- join(x, y)
   # Error message will occur
   

   In this example, mtcars1 object exists in both x and y objects but the content are not identical. mtcars1 in the ⁠x qenv⁠ object has more columns than mtcars1 in the ⁠y qenv⁠ object (only has one column).

2. join() will look for identical ⁠@id⁠ values in both qenv objects. The index position of these ⁠@id⁠s must be the same to determine the evaluation order. Otherwise, join() will throw an error message.

   Example:

   common_q <- eval_code(qenv(), expression(v <- 1))
   x <- eval_code(
     common_q,
     "x <- v"
   )
   y <- eval_code(
     common_q,
     "y <- v"
   )
   z <- eval_code(
     y,
     "z <- v"
   )
   q <- join(x, y)
   join_q <- join(q, z)
   # Error message will occur
   
   # Check the order of evaluation based on the id slot
   shared_ids <- intersect(q@id, z@id)
   match(shared_ids, q@id) # Output: 1 3
   match(shared_ids, z@id) # Output: 1 2
   

   The error occurs because the index position of identical ⁠@id⁠ between the two objects is not the same.

3. The usage of temporary variable in the code expression could cause join() to fail.

   Example:

   common_q <- qenv()
   x <- eval_code(
     common_q,
     "x <- numeric(0)
      for (i in 1:2) {
        x <- c(x, i)
      }"
   )
   y <- eval_code(
     common_q,
     "y <- numeric(0)
      for (i in 1:3) {
        y <- c(y, i)
      }"
   )
   q <- join(x,y)
   # Error message will occur
   
   # Check the value of temporary variable i in both objects
   x@env$i # Output: 2
   y@env$i # Output: 3
   

   join() fails to provide a proper result because of the temporary variable i exists in both objects but has different value. To fix this, we can set i <- NULL in the code expression for both objects.

   common_q <- qenv()
   x <- eval_code(
     common_q,
     "x <- numeric(0)
      for (i in 1:2) {
        x <- c(x, i)
      }
      # dummy i variable to fix it
      i <- NULL"
   )
   y <- eval_code(
     common_q,
     "y <- numeric(0)
      for (i in 1:3) {
        y <- c(y, i)
      }
      # dummy i variable to fix it
      i <- NULL"
   )
   q <- join(x,y)
   

Value

qenv object.

Examples

q <- qenv()
q1 <- eval_code(q, expression(iris1 <- iris, mtcars1 <- mtcars))
q2 <- q1
q1 <- eval_code(q1, "iris2 <- iris")
q2 <- eval_code(q2, "mtcars2 <- mtcars")
qq <- join(q1, q2)
get_code(qq)

common_q <- eval_code(q, quote(x <- 1))
y_q <- eval_code(common_q, quote(y <- x * 2))
z_q <- eval_code(common_q, quote(z <- x * 3))
join_q <- join(y_q, z_q)
# get_code only has "x <- 1" occurring once
get_code(join_q)