Assertions

Sometimes parameterized components are only valid for certain parameter ranges. ACT includes assertions that are checked at expansion time that can be used to validate the assumptions made.

As a simple example, suppose we want to define an N-place buffer.

template<pint N>
defproc buffer(chan?(int) L; chan!(int) R)
{
    onebuf b[N];   // create one-place buffer elements
    (i:N-1: b[i+1].L = b[i].R;)  // connect internal channels
    b[0].L = L; // connect external input
    b[N-1].R = R; // connect external output
}  

Passing in parameter 0 to this buffer would yield:

buffer<0> x;

--[ERROR]->  
In expanding buffer<0> (filename.act:XXXX)
In expanding ::<Global>
Error on or near line number XXXX.
 id: b[0].L
FATAL: Identifer conditionally created, but used when it does not exist

The definition, as written, does not support zero-length buffers. Rather than getting a message at expansion time that may not be as understandable to a user (especially of a pre-defined library), we can add an assertion to provide a more meaningful error message to a user of the templated buffer.

template<pint N>
defproc buffer(chan?(int) L; chan!(int) R)
{
    { N > 0 : "buffer size has to be at least 1" };
    onebuf b[N];   // create one-place buffer elements
    (i:N-1: b[i+1].L = b[i].R;)  // connect internal channels
    b[0].L = L; // connect external input
    b[N-1].R = R; // connect external output
}  

With this change, the error now looks like the following:

--[ERROR]-> 
In expanding buffer<0> (filename.act:XXXX)
In expanding ::<Global>
Error on or near line number XXXX.
*** Assertion failed ***
 assertion: N>0
   message: buffer size has to be at least 1