In addition to invoking the walker explicitly through the UI, it is possible to set breakpoints on words using words in the tools.walker vocabulary.

Annotating a word with a breakpoint (see Word annotations):

breakpoint ( word -- )

breakpoint-if ( word quot: ( ... -- ... ? ) -- )

breakpoint-after ( word n -- )

Breakpoints can be inserted directly into code:

break ( -- )

B

B:

Note that because the walker calls various core library and UI words while rendering its own user interface, setting a breakpoint on a word such as append or + will hang the UI.