Sorry for the vague subject, I'm not sure how else to describe this.
This is based on some thinking about the following blog post:
http://michid.wordpress.com/2008/07/30/meta-programming-with-scala-part-ii-multiplication/#comment-69Consider:
object Church {
abstract class Zero
abstract class Succ[T]
type _0[s[_], z] = z
type _1[s[_], z] = s[z]
type _2[s[_], z] = s[s[z]]
type _3[s[_], z] = s[s[s[z]]]
type _4[s[_], z] = s[s[s[s[z]]]]
trait apply1[t[_[_],_],u[_]] {
type It[a] = t[u,a]
}
type mul[m[s[_], z], n[s[_], z], s[_], z] = m[apply1[n,s]#It, z]
type x[m[s[_], z], n[s[_], z]] = mul[m, n, Succ, Zero]
type three = _3 x _1
//type four = _2 x _2
type six = _2 x _3
type eight = _4 x _2
}
This code compiles fine. We can even use the defined types! But if we
uncomment the definition of "four":
type four = _2 x _2
then we get the following:
church2.scala:19: error: illegal cyclic reference involving type _2
type four = _2 x _2
^
church2.scala:20: error: illegal cyclic reference involving type x
type six = _2 x _3
^
church2.scala:21: error: illegal cyclic reference involving type x
type eight = _4 x _2
^
three errors found
Notice that even the previously working definitions are now broken?
Why? What gives??
(This is with the current nightly build.)
Thanks...
Matt
--
Matt Hellige /
matt@...
http://matt.immute.net
