Re: Factor

Factor has a watching words feature that allows you to see the inputs and outputs of a word when it is called. I have wanted a way to define a watched code block that we could use to see the stack before and after some inner part of a word.

It turns out that it’s pretty simple to make this syntax using our existing watch implementation from the tools.annotations vocabulary:

DEFER: WATCH>

SYNTAX: <WATCH
    \ WATCH> parse-until >quotation dup (watch) append! ;

Using this, we can now watch the inner part of a word, for example this word that increments the input by 1:

: foo ( x -- y ) 1 <WATCH + WATCH> ;

And see it used:

IN: scratchpad 10 foo
--- Entering [ + ]
x 10
x 1
--- Leaving [ + ]
x 11

--- Data stack:
11

One thing that I’d like to improve on this someday, is making it so this watch syntax has a prettyprint implementation that allows it to be rendered as it is typed.

I have added this to the latest developer version if you’d like to update and give it a try!