Error Traps

Syntax

ErrorTrap =
     "$trap" Path "$with" PatternAlternative.

Evaluation

The evaluation of a rest

    $trap Q $with Palt

proceeds as follows. An attempt is made to evaluate the path Q. If the result obtained is an error $error(Ge), then the alternative match

    Ge : Palt

is evaluated, and the result obtained is taken to be the result of the whole construct.

The path Q is evaluated at the level 0.

     Env,0,St |- Q => Ge,St'
     ----------------------------------------
     Env,m,St |- $trap Q $with Palt => Ge,St'


     Env,0,St |- Q => $fail(k),St'
     Env,m,St' |- Fname "Unexpected fail" : Palt => X,St''
     -----------------------------------------------------
     Env,m,St |- $trap Q $with Palt => X,St''
          Fname is the name of the function in which
          the  construct appears.


     Env,0,St |- Q => $error(Ge),St'
     Env,m,St' |- Ge : Palt => X,St''
     ----------------------------------------
     Env,m,St |- $trap Q $with Palt => X,St''