Passing information between primitives

Since each primitive is evaluated in its own scope, it is not possible (or even desirable) to pass simple variables between separate primitives. Two means are provided for doing this:

• Using the %_PRIMITIVE_NAME_ hash. The argument hash for each primitive is visible to all other primitives at the same level. This is because primitives are translated into:

  my %_PRIMITIVE_1_ = read_arguments( ... );
  {
    _PRIMITIVE_1_
  }
  my %_PRIMITIVE_2_ = read_arguments( ... );
  {
    _PRIMITIVE_2_
  }
  

  This works but has a number of problems:
  • The system breaks if the previous primitive is removed from the recipe.⁸
  • The system breaks if the primitive is renamed.
  • If the primitive from which data are required is in a different scope from the current primitive this will not work.
  • It feels too much like a global variable...

• Store the information with the current frame or group object. Both frame and group objects can use the uhdr() method to store arbitrary data (including references) in a hash. This is the recommended way of transferring data between primitives when the information relates to the current frame or group. By convention, the hdr() method should be used for storing FITS-like data. Checks should be made for the existence of data in the hash before using it.

The first method using the primitive hash only allows information to be passed within the current recipe whereas the frame header allows the information to be retained for subsequent frame processing.