4.8 Testing Equality

The equality test == checks whether two Oz values are equal. It is a function of type

$==: {\rm value} \times {\rm value} \rightarrow {\rm bool}$

. For instance, a record without subrecords is identified with its label.

{Inspect atom==atom()}
{Inspect {IsRecord atom}}
 
{Inspect {And true==true()
          {And unit==unit()
           false==false()}}}
 
{Inspect "hiho"==[104 105 104 111]}

The function == for testing equality can be used to built expressions. Note the == is very much different from the operator = in assigment statements. The latter operator = binds a variable (usually on the left) to a value (usually on the right). The former equality test == returns a Boolean value, but only if both input arguments are known. Otherwise, it waits until they become known. The programmer must be careful because all followup up commmands are suspended too.

%% introduce a free variable
declare X
%%  the equality test waits until the value of X is known.
%%  the following browse statement therefore waits too.
{Browse X==2}   
                              
%% feed the next line independently.
X=3
%% now the Browser should browse false

The equality test proceeds only once X gets bound to its value. The Browse can only become active once this has happened.

The above example may go wrong if you feed the above lines in a single block, since everything in the same block that succedes a blocking statement blocks. The statement that assign a value to X must thus be fed in another block.