Would you expect the code below to compile, because you expect the compiler to infer a transitive view f;g: A -> C from the existence of f: A->B and g:B->C ?
I don't see a view from String to TriState there, but I see two views from String to List[Char] and from List[Char] to Tristate.
Burak
On 1/29/07, Tony Morris <tmorris@xxxxxxxxxxx> wrote:
I have found what appears to be an anomaly in implicit type coercion. I
have attempted to trivialise my example, but it is still 50 or so LOC. I
have annotated the code with the relevant question in the relevant place
in the code with comments. The problem is that this code will not
compile. I expect that the code will compile successfully, or fail to
compile with 2 errors, yet it seems to fail only with 1 error. That is
to say, a contradiction exists from my perspective and I'd appreciate
any explanation that clarifies this situation for me, thanks.
Search for:
"why does this compile..."
and
"...while this does not compile?"
sealed abstract class CList[+A]
case object Nil extends CList[Nothing]
final case class Cons[+A](head: A, tail: CList[A]) extends CList[A]
sealed abstract class TriState
final case object ON extends TriState
final case object OFF extends TriState
final case object NEUTRAL extends TriState
object CList {
implicit def string2CList(xs: String): CList[Char] = {
// todo
// inefficient, write imperative code
// go on, you can do it.
if(xs isEmpty)
Nil
else
Cons(xs charAt 0, xs substring 1)
}
implicit def cList2String (xs: CList[Char]): String = {
// todo
""
}
}
object TriState {
import CList._
implicit def list2TriState(xs: CList[Char]): TriState = {
if(xs == string2CList("1"))
ON
if(xs == string2CList("0"))
OFF
else
NEUTRAL
}
}
final case class X(ts: TriState) {
}
object Y {
import CList.string2CList
import TriState.list2TriState
def f1(): X = {
// why does this compile...
val x: CList[Char] = "x"
X(x)
}
def f2(): X = {
// ...while this does not compile?
// error: type mismatch;
// found : java.lang.String("x")
// required: TriState
X("x")
}
}
--
Tony Morris
http://tmorris.net/
--
Burak Emir
Research Assistant / PhD Candidate
Programming Methods Group
EPFL, 1015 Lausanne, Switzerland
http://lamp.epfl.ch/~emir
