Alternatives

Syntax

Alternative =
     "\{" PathList "}" |
      "{" PathList "}".

PathList = { Path ";" }.

Evaluation

The evaluation of a source \{Q₁; Q₂; ... Q_n;} proceeds as follows. The paths Q₁, Q₂, ..., Q_n are evaluated from left to right until the evaluation of a path succeeds.

More specifically, consider the result of evaluating the path Q_j.

If the result is a ground expression Ge, then Ge is taken to be the result of the whole construct. If the result is $error(Ge), then $error(Ge) is the result of the whole construct. If the result is $fail(k+1), then $fail(k+1) is the result of the whole construct. And, finally, if the result is $fail(0), this failure is "caught", i.e. an attempt is made to evaluate the next path. If there exists no next path (i.e. j=n), the failure $fail(0) is the result of the whole construct.

An alternative {Q₁; Q₂; ... Q_n;} is equivalent to the alternative \{Q₁; Q₂; ... Q_n; $error(Fname "Unexpected fail");}, where Fname is the name of the function in which the construct appears.

     {Q1; Q2; ... Qn;}  =>=>
               \{Q1; Q2; ... Qn;
                    $error(Fname "Unexpected fail");}
          Fname is the name of the function in  which
          the  construct appears.


     Env,m,St |- \{} => $fail(0),St


     Env,m,St |- Q1 => Ge,St'
     ----------------------------------------
     Env,m,St |- \{Q1; Q2; ... Qn;} => Ge,St'


     Env,m,St |- Q1 => $fail(0),St'
     Env,m,St'|- \{Q2; ... Qn;} => X,St''
     ----------------------------------------
     Env,m,St |- \{Q1; Q2; ... Qn;} => X,St''


     Env,m,St |- Q1 => $fail(k+1),St'
     ------------------------------------------------
     Env,m,St |- \{Q1; Q2; ... Qn;} => $fail(k+1),St'


     Env,m,St |- Q1 => $error(Ge),St'
     ------------------------------------------------
     Env,m,St |- \{Q1; Q2; ... Qn;} => $error(Ge),St'