forsh/f.sh

#!/bin/sh

alias :=_newword
alias ']=_execbody'
alias xshift='shift 2>/dev/null || printf "Stack underflow\n"'

_stack="" # top is 
_builtin_dictionary="swap dup rot over drop add sub div mod mul put" # + - / * .

_newword() {
	name="$1"
	shift
	eval "${name}_definition=\"$*\""
}

_execword() {
	eval "def=\"\$$1_definition\""
	eval "def=\"\$$1_definition\""
	[ "$def" ] || {
		printf '%s\n' "Error: no word $1"
		return
	}
	_execbody $def
}

_execbody() {
	for word in "$@"; do
		[ "$_debug" ] && {
			printf 'word %s\nstack %s\n' "$word" "$_stack"
			read
		}
		case "$_builtin_dictionary" in
			*"$word"*) "_$word" $_stack ;;
			*) case "$word" in
				*[0-9]*)   _stack="$word $_stack" ;;
				*)         _execword "$word" ;;
			esac
			;;
		esac
	done
}

_swap() {
	one="$1"
	two="$2"
	xshift
	_stack="$2 $1 $*"
}

_dup() {
	_stack="$1 $*"
}

_rot() {
	one="$1"
	two="$2"
	thr="$3"
	xshift
	xshift
	xshift
	_stack="$3 $1 $2 $*"
}

_over() {
	_stack="$2 $*"
}

_drop() {
	xshift
	_stack="$*"
}

_add() {
	one="$1"
	two="$2"
	xshift
	xshift
	_stack="$(( one + two )) $*"
}

_sub() {
	one="$1"
	two="$2"
	xshift
	xshift
	_stack="$(( two - one )) $*"
}

_div() {
	one="$1"
	two="$2"
	xshift
	xshift
	_stack="$(( two / one )) $*"
}

_mod() {
	one="$1"
	two="$2"
	xshift
	xshift
	_stack="$(( two % one )) $*"
}

_mul() {
	one="$1"
	two="$2"
	xshift
	xshift
	_stack="$(( one * two )) $*"
}

_put() {
	one="$1"
	xshift
	printf '%s \n' "$one"
	_stack="$*"
}