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5.24.3 Debugging

Languages with a slow edit/compile/link/test development loop tend to require sophisticated tracing/stepping debuggers to facilate debugging.

A much better (faster) way in fast-compiling languages is to add printing code at well-selected places, let the program run, look at the output, see where things went wrong, add more printing code, etc., until the bug is found.

The simple debugging aids provided in debugs.fs are meant to support this style of debugging.

The word ~~ prints debugging information (by default the source location and the stack contents). It is easy to insert. If you use Emacs it is also easy to remove (C-x ~ in the Emacs Forth mode to query-replace them with nothing). The deferred words printdebugdata and .debugline control the output of ~~. The default source location output format works well with Emacs' compilation mode, so you can step through the program at the source level using C-x ` (the advantage over a stepping debugger is that you can step in any direction and you know where the crash has happened or where the strange data has occurred).

~~              gforth       “tilde-tilde”

Prints the source code location of the ~~ and the stack contents with .debugline.

printdebugdata              gforth       “print-debug-data”

.debugline       nfile nline –         gforth       “print-debug-line”

Print the source code location indicated by nfile nline, and additional debugging information; the default .debugline prints the additional information with printdebugdata.

~~ (and assertions) 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.