Canablize a runtime: embedding a language in Forth

After implementing Forth, I realized that transparent access to the runtime allows embeddeing another programming language.

Brainfuck is hardly a language. Consisting only of single-character tokens, each can be translated directly: Brainfuck's [, <, or + are, in Forth vocabulary, "words" that can be defined like any other. Brainfuck loops can re-write into Forth's native WHILE–REPEAT loops with meta-programming (POSTPONE and IMMEDIATE). The last component is Brainfuck's memory, modeled with an array TAPE and pointer HEAD.

It's surprising, to me, to see one programming language shaped out of another so directly:

VARIABLE TAPE 1000 CELLS ALLOT
TAPE 1000 0 FILL

VARIABLE HEAD
TAPE HEAD !

: PEEK HEAD @ @ ;
: POKE HEAD @ ! ;

: < HEAD @ 1 CELLS - HEAD ! ;
: > HEAD @ 1 CELLS + HEAD ! ;
: + PEEK 1+ POKE ;
: - PEEK 1- POKE ;
: . PEEK . ;
: , KEY POKE ;
: [ POSTPONE BEGIN POSTPONE PEEK POSTPONE WHILE ; IMMEDIATE
: ] POSTPONE REPEAT ; IMMEDIATE

\ example programs:
: echo , [ . , ] ;
: add , > , [ < + > - ]