more noodling with apply and tailcall

namespace path {::tcl::mathop ::tcl::unsupported}

# Lambda is defined as constructing an invocation of [apply]

interp alias {} lambda {} apply {args {list apply $args}}

# The I combinator simply returns its argument

interp alias {} I {} return -level 0

# The U combinator passes a function as an argument to itself.

proc U {f args} {tailcall {*}$f $f {*}$args}

# recursive factorial

puts [U [lambda {f x} {
    if {$x == 0} {
	I 1
    } else {
	* $x [U $f [- $x 1]]
    }
}] 6]

# iterative factorial

puts [U [lambda {f y x} {
    if {$x == 0} {
	I $y
    } else {
	tailcall U $f [* $y $x] [- $x 1]
    }
}] 1 6]