4.3 Expressions for Values

Oz provides expressions to describe values. There may be many alternative expressions describing the same value. We start with variable free expressions that determine some value completely.

• Integers are described as 0, 1, ~1, 2, 3 etc and floats by 0.0, 1.0, ~1.1 etc.

• Atoms are described by words starting with lower case letter like thisIsAnAtom or by a word in backwards quotes like 'case', 'true' and 'ThisIsAnAtom'.

• The Booleans and the unit are described by the keywords true, false, unit.

• Typical description for tuples and records are the following:

  plus(5 times(5 ~10))
 
address(street:'Talstrasse'  
        name:unit(first:hans  
                  second:kamp))
 
det(phon:a number:singular)

  In the first tuple, we have left out the features; it's a syntactically suggared version of plus(1:5
  2:times(1:5 2:~10)).

  The values of a record at some feature can be selected by using the selection function that is denoted by a dot. For instance, the atom singular is described by the expression

  det(phon:a number:singular).number

• Typical descriptions of lists are: 1|2|3|nil, [1 2 3], and nil. Note however that [ ] does not describe the empty list!

• A description of a procedure computing the square function is:

  fun{$ X} X*X end

  The symbol $ simply means that this procedure is anonymous, i.e. is not yet given a name. The syntax for the application of procedures uses curly brackets. For instance, the number 9 is described by the following application whose evaluation computes the square of 3:

  {fun{$ X} X*X end 3}