It is a good idea to make your programs self-checking, in particular, if you use an assumption (e.g., that a certain field of a data structure is never zero) that may become wrong during maintenance. Gforth supports assertions for this purpose. They are used like this:
assert( flag )
The code between
) should compute a flag, that
should be true if everything is alright and false otherwise. It should
not change anything else on the stack. The overall stack effect of the
( -- ). E.g.
assert( 1 1 + 2 = ) \ what we learn in school assert( dup 0<> ) \ assert that the top of stack is not zero assert( false ) \ this code should not be reached
The need for assertions is different at different times. During debugging, we want more checking, in production we sometimes care more for speed. Therefore, assertions can be turned off, i.e., the assertion becomes a comment. Depending on the importance of an assertion and the time it takes to check it, you may want to turn off some assertions and keep others turned on. Gforth provides several levels of assertions for this purpose:
assert0(-- gforth ``assert-zero''
important assertions that should always be turned on
assert1(-- gforth ``assert-one''
normal assertions; turned on by default
assert2(-- gforth ``assert-two''
assert3(-- gforth ``assert-three''
slow assertions that you may not want to turn on in normal debugging; you would turn them on mainly for thorough checking
assert(-- gforth ``assert(''
equivalent to assert1(
)-- gforth ``close-paren''
end an assertion
Assert( is the same as
assert1(. The variable
assert-level specifies the highest assertions that are turned
on. I.e., at the default
assert-level of one,
assert1( assertions perform checking, while
assert3( assertions are treated as comments.
Note that the
assert-level is evaluated at compile-time, not at
run-time. I.e., you cannot turn assertions on or off at run-time, you
have to set the
assert-level appropriately before compiling a
piece of code. You can compile several pieces of code at several
assert-levels (e.g., a trusted library at level 1 and newly
written code at level 3).
assert-level-- a-addr gforth ``assert-level''
all assertions above this level are turned off
If an assertion fails, a message compatible with Emacs' compilation mode
is produced and the execution is aborted (currently with
If there is interest, we will introduce a special throw code. But if you
catch a specific condition, using
probably more appropriate than an assertion).
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