I'm learning to build interpreters for a toy relational language (in F#). So a fast intepreter appeal to me.

I look at http://keiapl.org/rhui/remember.htm#incunabulum but a)I don't know C b)Don't know J so wonder if exist a easy-to-understand interpreter of J somewhere? The ones I look (in the link above) are very dense. Full of code-golf.