All of them.

To elaborate, the scheme spec requires tco.

Which scheme is embeddable and lightweight?

And what does lightweight mean? Does it mean low memory footprint or does it mean few-lines-of-code-to-introduce or does it mean zero-dependencies?

https://en.wikipedia.org/wiki/SIOD is lighter weight than almost anything. Way smaller than Lua.

Also look at Hedgehog Lisp. The bytecode compiler (runs on a PC) is separate from the interpreter, i.e. there is no REPL. But it means that the interpreter is only about 20KB of code. It's quite practical. It's not Scheme but rather is a functional Lisp (immutable data including AVL trees as the main lookup structure) and it is tail recursive. https://github.com/sbp/hedgehog

I meant which scheme implementation is "lightweight" and also meant to ask what "lightweight means".

For a functionnal language, TCO is really a must have. How would you do the equivalent of loops without it ?

For a purely functional language. Scheme is not that.

https://www.gnu.org/software/guile/manual/html_node/while-do...

And you are wrong because you can define a loop as iterating recursively over a list with just car and cdr.

Can but since other constructs exist that doesn't by itself make TCO "must have"

I wasn't asking which scheme has TCO.

I was asking which scheme if the 20-50 of them was "lightweight" and embeddable.