17.2.1 Satisfying a Two-Argument Test

When an object O is being considered iteratively against each element E_i of a sequence S by an operator F listed in Figure 17–2, it is sometimes useful to control the way in which the presence of O is tested in S is tested by F. This control is offered on the basis of a function designated with either a :test or :test-not argument.

  adjoin           nset-exclusive-or  search            
  assoc            nsublis            set-difference    
  count            nsubst             set-exclusive-or  
  delete           nsubstitute        sublis            
  find             nunion             subsetp           
  intersection     position           subst             
  member           pushnew            substitute        
  mismatch         rassoc             tree-equal        
  nintersection    remove             union             
  nset-difference  remove-duplicates                    

  Figure 17–2: Operators that have Two-Argument Tests to be Satisfied

The object O might not be compared directly to E_i. If a :key argument is provided, it is a designator for a function of one argument to be called with each E_i as an argument, and yielding an object Z_i to be used for comparison. (If there is no :key argument, Z_i is E_i.)

The function designated by the :key argument is never called on O itself. However, if the function operates on multiple sequences (e.g., as happens in set-difference), O will be the result of calling the :key function on an element of the other sequence.

A :test argument, if supplied to F, is a designator for a function of two arguments, O and Z_i. An E_i is said (or, sometimes, an O and an E_i are said) to satisfy the test

if this :test function returns a generalized boolean representing true.

A :test-not argument, if supplied to F, is designator for a function of two arguments, O and Z_i. An E_i is said (or, sometimes, an O and an E_i are said) to satisfy the test

if this :test-not function returns a generalized boolean representing false.

If neither a :test nor a :test-not argument is supplied, it is as if a :test argument of #'eql was supplied.

The consequences are unspecified if both a :test and a :test-not argument are supplied in the same call to F.