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5.24.4 Assertions

It is a good idea to make your programs self-checking, especially if you make an assumption that may become invalid during maintenance (for example, that a certain field of a data structure is never zero). Gforth supports assertions for this purpose. They are used like this:

     assert( flag )

The code between assert( and ) 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 assertion is ( -- ). 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”

Debugging assertions.

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.

The variable assert-level specifies the highest assertions that are turned on. I.e., at the default assert-level of one, assert0( and assert1( assertions perform checking, while assert2( and assert3( assertions are treated as comments.

The value of assert-level is evaluated at compile-time, not at run-time. Therefore 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 different pieces of code at different 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 ABORT". If there is interest, we will introduce a special throw code. But if you intend to catch a specific condition, using throw is probably more appropriate than an assertion).

Assertions (and ~~) will usually print the wrong file name if a marker is executed in the same file after their occurance. They will print `*somewhere*' as file name if a marker is executed in the same file before their occurance.

Definitions in ANS Forth for these assertion words are provided in compat/assert.fs.