
% Copyright (c) 1991-2002, The Numerical ALgorithms Group Ltd.
% All rights reserved.
%
% Redistribution and use in source and binary forms, with or without
% modification, are permitted provided that the following conditions are
% met:
%
%     - Redistributions of source code must retain the above copyright
%       notice, this list of conditions and the following disclaimer.
%
%     - Redistributions in binary form must reproduce the above copyright
%       notice, this list of conditions and the following disclaimer in
%       the documentation and/or other materials provided with the
%       distribution.
%
%     - Neither the name of The Numerical ALgorithms Group Ltd. nor the
%       names of its contributors may be used to endorse or promote products
%       derived from this software without specific prior written permission.
%
% THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
% IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
% TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
% PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER
% OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
% EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
% PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES-- LOSS OF USE, DATA, OR
% PROFITS-- OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
% LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
% NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
% SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.



A function that does not return directly to its caller has
\spadtype{Exit} as its return type.
The operation \spadfun{error} is an example of one which does not return
to its caller.
Instead, it causes a return to top-level.
\xtc{
}{
\spadcommand{n := 0 \bound{n}}
}
\xtc{
The function \userfun{gasp} is given return type \spadtype{Exit} since it is
guaranteed never to return a value to its caller.
}{
\begin{spadsrc}[\bound{gasp}\free{n}]
gasp(): Exit ==
    free n
    n := n + 1
    error "Oh no!"
\end{spadsrc}
}
\xtc{
The return type of \userfun{half} is determined by resolving the types of
the two branches of the \spad{if}.
}{
\begin{spadsrc}[\bound{half}\free{gasp}]
half(k) ==
  if odd? k then gasp()
  else k quo 2
\end{spadsrc}
}
\xtc{
Because
\userfun{gasp} has the return type \spadtype{Exit},
the type of \spad{if} in \userfun{half} is resolved to be \spadtype{Integer}.
}{
\spadcommand{half 4 \free{half}\bound{app1}}
}
\begin{inputonly}
)set message test off
\end{inputonly}
\xtc{
}{
\spadcommand{half 3 \free{half app1}\bound{app2}}
}
\begin{inputonly}
)set message test on
\end{inputonly}
\xtc{
}{
\spadcommand{n \free{app2}}
}

For functions which return no value at all, use \spadtype{Void}.
See \spadref{ugUser} and \xmpref{Void} for more information.
%
\showBlurb{Exit}
